Cytoplasmic Inheritance (With Diagram) | Cell Biology

In this article we will discuss about Cytoplasmic Inheritance:- 1. Subject-Matter of Cytoplasmic Inheritance 2. Terms and Definitions of Cytoplasmic Inheritance 3. Characteristics and Detection 4. Extra-Nuclear Inheritance in Eukaryotes.

Subject-Matter of Cytoplasmic Inheritance:

The existence of genes as segments of nu­cleic acid molecules, located in chromosome of nucleus, has been demonstrated by several experiments. The nuclear genes control the phenotypes of the organisms and are concerned with the transmission of hereditary charac­ter from one generation to next generation is known and predictable Mendelian fashion.

The inheritance of genes of nuclear chromosomes is characterised by the fact that the genes from male and female parents contribute equally to the genetic constitution of the offspring. There­fore, in it the reciprocal crosses between parents of different homozygous genotype will produce offspring’s of identical phenotypes except for sex-linked genes.

However, in certain cases, although male and female parents contribute equally their nuclear genes to the offspring’s, the results show a non-Mendelian inheritance pattern and the result of reciprocal crosses varies.

These variations suggest that the genes for the inheritance of certain characters do not occur within the nucleus, but they are present ill cytoplasm and play an important role in transmission of certain specific traits, which are not controlled by nuclear genes. Therefore, it builds up the concept of cytoplasmic inheri­tance. The genes for cytoplasmic inheritance are independent, self-replicating nucleic acids.

Evidence for cytoplasmic inheritance was first reported by Correns in Mirabilis jalapa and by Bar in Pelargonium zonule in 1908. Rhoades described cytoplasmic male sterility in maize in 1933. In 1943, Sonneborn discovered kappa particles in Paramoecium and described its cytoplasmic inheritance. Presence of DNA in chloroplasts was first demonstrated by Ris in plant cell.

In 1963, Nass and his co-workers proved the existence of DNA in mitochondria. Subsequently, from time to time, observations by several scientists have been reported the im­portant role of cytoplasm in genetics. Thus, on the basis of observations made on cytoplasmic inheritance of some specific traits, it has been suggested that cytoplasm is also genetically active.

Terms and Definitions of Cytoplasmic Inheritance:

Extra-chromosomal inheritance, extra-nuclear inheritance, somal inheritance and maternal inheritance are all synonyms. All these terms can be defined as the inheritance of characteristics of only one of the two parents, usually the female parent to the progeny. The reciprocal crosses show consistent differences as well as there is a lack of segregation in F₂ and subsequent generations.

The genes controlling cytoplasmic inheritance are present outside the nucleus and, in the cytoplasm, they are known as plasma genes, cytoplasmic genes, cytogeneses, extra nuclear genes or extra chromosomal genes.

The sum total of the genes present in cytoplasm of a cell is known as Plasmon. All the genes present in a plastid are known as plastoms. Similarly, all the genes present in a mitochondrion are known as chondrioms. The genes present in plastid and in mitochondrion are located in their own DNAs and are known as cp DNA and mt DNA, respectively. These DNAs are collectively termed organelle DNA.

Characteristics and Detection of Cytoplasmic Inheritance:

Cytoplasmic inheritances do not show Mendelian inheritance.

They show the following characteristic features:

i. Hereditary traits which are transmitted by cytoplasm do not show Mendelian segre­gation in crosses and in reciprocal crosses with respect to a particular set of charac­teristics controlled by a set of cytoplasmic genes produce dissimilar hybrids.

ii. Most of the recorded cytoplasmically in­herited characteristics would follow the maternal line, i.e., uniparental mode of transmission. In higher plants and ani­mals, ovum or egg cell is comparatively large and contains large amount of cyto­plasm. But male gametes or sperms have very little amount of cytoplasm. So, under this situation, most of cytoplasmic factors are transmitted to the progeny through the ovum of mother.

It is known as maternal inheritance or trans-ovarian trans­mission. In this mode of transmission, all the offspring’s of the parents have maternal condition and only female progeny can transmit the cytoplasmic characteristics to the succeeding generations. Hence the reciprocal crosses yield different or non-Mendelian results.

Characteristics of Mendelian Inheritance:

The inheritance pattern of characters of an organism as proposed by Mendel on the basis of monohybrid and di-hybrid crosses is referred to as Mendelian inheritance.

It shows the following characteristic features:

i. Contribution of both male and female is equal, hence results from reciprocal crosses are similar.

ii. Segregation produces the phenotypes ratio 3 : 1 and genotype ratio 1 : 2 : 1 in the F₂ generation of a monohybrid cross and a typical phenotype ratio 9 : 3 : 3 : 1 in di-hybrid crosses.

Mendelian inheritance pattern is regarded as a sufficient evidence for a gene to be located in chromosomes; such genes are called nuclear genes or simply as genes.

Extra-Nuclear Inheritance in Eukaryotes:

Various cases of extra-nuclear inheritance in different eukaryotic organisms have been stud­ied by several scientists.

Few important exam­ples of extra nuclear inheritance in eukaryotes are stated under some classified subheadings:

(i) Maternal Inheritance:

Maternal inheritance means the inheritance controlled by extra-chromosomal, i.e., cyto­plasmic, factors that are transmitted to the succeeding generation through the egg of female organism.

They show the following features:

i. reciprocal differences in F₁;

ii. which in most cases disappears in F₂;

iii. a smaller variation in F₂ as compared to that in F₃.

Maternal inheritance may be, broadly speak­ing, of two kinds:

i. If some treatments (chemical poison, heat shock etc.) are applied to the female par­ent, it may affect the egg’s cytoplasm. As a result subsequent offspring’s are modified in some way. Effects of this kind are called Dauer-modifications or persist­ing modifications.

It is observed that when protozoa are treated experimentally with chemical poisons or heat shocks, the treatments induce several morphological abnormalities in them. Such abnormalities go on decreasing generation after genera­tion and, eventually, disappear completely through cell division if the treatments are removed.

Further evidences also come from fruit flies subjected to heat treatment and from bacteria treated with chemicals.

ii. Other kinds of maternal inheritance are also known which do not depend upon the repeated application of an external stimulus to the cytoplasm. In this case, maternal inheritance is truly controlled by independent cytoplasmic genes.

Maternal effects reflect the influence of the mother’s gene on developing tissues. Many important characteristics of both ani­mal and plants show maternal effects of which some examples axe described next.

(ii) Coiling of Snail Shells (Limnaea Peregra):

One of the earliest and classical examples of a maternal effect is that of the direction of coiling in shells of the water snail Limnaea peregra. In this snail, the shell is spirally coiled. Usu­ally the direction of coiling of the shell is clock­wise if viewed from the top of the shell. This type of coiling is called dextral. However, in some snails the coiling of shell is anticlockwise. This type of coiling is sinistral.

The direction of shell coiling of both types of snail is governed by genotype of the female parent and not by their own genotype. Further investigation suggests that coiling depends upon the early clearage in the zygote.

If the mitotic spindle is tilted to left (Fig. 22.1) of the median line of zygote, the successive cleavages will produce a spiral to left (sinistral) and if the orientation of spindle is tilted to the right of the median line of zygote, the successive cleavages will produce a spiral to right (dextral). The spindle orientation is controlled by the genotype of oocyte from which the egg develops.

In a cross (Fig. 22.2) between dextral 9 (fe­male) snail and sinistral (male), (follow the left side coloum of the Fig. 22.2) all the Fi progeny have dextral coils like their mother and also indicates that dextral character (RR) is dominant over sinistral coiling (rr).

However, in the F₁ x F₁ cross (i.e., inbreeding or self fertilisation) all the F₂ snails are also dextral. The F₃ progeny from F₂ individuals with the genotype RR and R_r will show dextral coiling while those from rr F₂ individual will exhibit sinistral coiling of their shell; this produces the typical 3 : 1 ratio in F₃ generation.

In reciprocal cross (right side column of the Fig. 22.2) between dextral ♂ (male) and sinistral ♀ (female), all the F_x progeny have sinistral coiling (Rr) instead of dextral coiling. In this case, F₁ x F₁ cross, all the F₂ snails are, again, dextral. This F₃ progenies from F₂ also exhibit the typical monohybrid ratio of 3 : 1.

Thus the features of inheritance of coiling of Limnaea shells may be summarised as:

i. F₁s from reciprocal crosses show differ­ences in coiling pattern.

ii. No segregation in F₂.

iii. Appearance of the typical 3: 1 ratio in F₃ in place of F₂.

The 3 : 1 ratio, although in F₃, clearly indicates that coiling of shell is governed by a single nuclear gene. But the segregation of this nuclear gene is apparently delayed by one gen­eration and is visible in F₃ instead of F₂ as in all other cases of Mendelian inheritance. This is because the direction of coiling in this snail is primarily determined by some substances already present in the cytoplasm or ooplasm of the egg cell.

Obviously these substances are produced by the female parent. As a result the offspring would produce the phenotype (in F₁) of the maternal parent since its nuclear gene product is possibly active after one generation later and shows delayed segregation in F₃.

(iii) Maternal Inheritance in Drosophila:

Several examples of maternal effects are known in drosophila:

i. Abnormal growth in the head region of Drosophila melanogaster was produced sporadically in a sample from a wild population collected at Acahuizotla, Mexico. Development of abnormal growth in the head region is called Tumorous head (Tu – h). Tu – h is governed by two major genes. But the frequency of tumour development in progeny is markedly influenced by the maternal effect.

When a cross is made between a normal female fly and a male fly with head tumour, less than 1% of the progeny exhibit head tumour. In contrast, when a reciprocal cross is made between a female having head tumour and a normal male, about 30% of the progeny show tumour development.

ii. In Drosophila, fertility and survival are oc­casionally influenced by maternal effect. A recessive nuclear gene, grandchild-less, af­fects the fertility of progeny in Drosophila subobscura. In this fly a female homozy­gous for which is fertile but all her offspring’s are sterile.

The reason of this effect arises from cytoplasmic dependent path­way for the development of many organs, the egg cytoplasm formed by a female fly is not uniform and various parts of the egg appear to be specifically assigned for the formation of different tissues. Thus the fate of Drosophila germ cells to produce either various or testes is determined early in development.

A daughterless (da) gene in Drosophila causes death of all XX zygotes derived from eggs of da da females. It is reported that the cytoplasm of eggs of da da females affects the two X-chromosomes and does not inhibit the development of the female phenotype.

It should be noted that the action of da is determined by the genotype of the female producing egg and not by the genotype of the egg cells themselves. It is also reported that interaction of da gene product is evident if the zygote is XX but there is no such action affecting the survival of XY zygote.

Some genes in Drosophila have lethal effects on male embryos. A recessive gene, sonless, leads to the absence of males in the progeny of females homozygous for this gene. Another gene, abnormal oocyte, reduces the frequency of males in the progeny.

These effects on the frequency of male progeny are produced due to effect of egg cytoplasm on the survival of XY zygote. Thus the genes for daughterless and sonless produce typical uniparental inheritance. These uniparental inheritance patterns depend on the nuclear genotype of the female parent.

(iv) Maternal Inheritance in Ambystoma:

Maternal inheritance has also been studied in Axololt of Ambystoma maxicanum. In this animal there is a lethal gene ‘O’. It is a recessive gene. But its dominant gene is ‘+’. The heterozygous individuals with genotype ‘+O’ are completely normal.

When two heterozygous individuals with genotype ‘+0′ are crossed, a homozygous offspring with genotype ’00’ may be produced. The homozygous individual (‘OO’) develop normally in the early stage but in later life show a slight retardation of growth.

Their regeneration capacity is reduced to great extent and the homozygous males (OO) are sterile and their testes are poorly developed and spermatogenesis does not produce beyond spermatogonial stages. On the other hand, the homozygous females with genotype ‘OO’ pro­duce eggs.

The eggs are capable of fertilisation and normal clearage but at the onset of gastrulation, further development is retarded and embryos normally die. This abnormal course of development is affected by the maternal effect, i.e., the growth is resulted by the genotype of the mother irrespective to that of the offspring.

If the maternal genes are recessive, i.e., ‘OO’, the growth of the offspring even with genotypes ‘+O’ is retarded. According to Biggs and Justus (1967), in the egg of homozygous female ‘OO’ a protein-like substance called corrective factor is absent.

This protein-like substance is necessary for normal growth and development of the offspring and is synthesised by normal genes ‘++’ of homozygous or heterozygous nor­mal females (‘++’ or ‘+O’) during oogenesis.

(v) Maternal Inheritance of Eye Pigmentation in Water Flea and Flour Moth:

The maternal inheritance has also been ob­served in case of water flea (Gammarus sp.) and the flour moth (Ephestia kuhniella). The normal colour of both the invertebrates is dark due to the dominant gene (AA or KK) in which the dominant gene A or K directs the production of a hormone-like substance called Kynurenine which is involved in the pigment synthesis.

The recessive gene (i.e., a or k) fails to direct the synthesis of kynurenine. In absence of kynurenine, colour of eye becomes light.

Therefore the recessive mutants do not possess pigment in the eye and have the genotype aa or Kk. When a cross is made between a heterozygous male (Aa or Kk) and a double recessive female (aa or kk), only half of the larvae show dark pigment in the eye. Again, when a cross is made between a heterozygous female and a double recessive male, all larva are with dark eye. But on reaching the adult stage, half of the progenies having the genotype ‘aa’ or ‘kk’ become light-eyed.

This indicates that some kynurenine molecules diffuse from the ‘Aa’ mother into all young (larvae) enabling them to produce pigments regardless of their genotype. But the aa progeny is unable to synthesize kynurenine and, consequently, devel­ops light eye as the kynurenine obtained from mother is used up. This example suggests an ephemeral type of maternal inheritance.

(vi) Maternal Inheritance in Mammals:

Maternal effects in mammals are very promi­nent because mammalian mothers may affect the development of their offspring’s not only through ovum cytoplasm but also through the uterine environment. Effects of the maternal Rh blood group on the developing foetus in humans and that of maternal genotype for hair loss gene on the survival of young embryos in mice are the examples of maternal effect.

Other examples of maternal effects in human are: the embryonic defects caused by maternal diabetes and maternal phenylketonuria, a ma­ternal influence on left-or right-handedness and a maternal effect on body weight.

(vii) Maternal Inheritance in Plants:

Cytoplasmic male sterility in cross-pollinating plants is a classic example of maternal inher­itance. Plants which are unable to produce functional pollen but possess normal fertile female structure aired known as male sterile.

Thus male sterility is characterised by non­functional pollen grains. This occurs in many flowering plants. In maize, wheat, sugar-beets, onions and some other crop plants, fertility is controlled, at least in part—by cytoplasmic factors and results in male sterility.

This type of male sterility is referred to as cytoplasmic male sterility. Nuclear gene did not control this type of sterility, rather it is transmitted from generation to generation through egg cyto­plasm. In other plants, however, male sterility is controlled entirely by nuclear genes. But over present discussion will be focused on only cytoplasmic male sterility in relation to maternal inheritance in plants.

Cytoplasmic male sterility is determined by cytoplasmic factors. Since the bulk of cyto­plasm of zygote is contributed by the egg cell and the pollen tube containing male gametes contributes negligible or no cytoplasm, sterility factors present in the cytoplasm of egg cell will be transmitted to the offspring which would always be male sterile.

A case of cytoplasmic male sterility was discovered and carefully analysed by M. M. Rhoades (1933) in maize. He observed a male sterile plant in maize. In this plant pollens are aborted in the anther. The male sterile plant is produced when an egg cell containing cytoplasmic male sterility factor is fertilised by pollen from normal male fertile plants.

It indicates that male sterility is contributed by the cytoplasm of female parent. It is also- observed that when a male sterile female plant is crossed with wide range of fertile males, all progenies axe male sterile in the subsequent generations.

In maize, three distinct male sterile source (Cms) cytoplasm’s axe known which are des­ignated as Cms-T, Cms-C and Cms-S. The normal male fertile cytoplasm is known as N- cytoplasm. Each of the three Cms cytoplasm’s shows strict maternal inheritance—even when all chromosomes axe replaced from male sterile plants by a male fertile source through repeated backcrossing.

Even then male sterility charac­teristics could not be avoided and the charac­teristics till persists. It indicates that if the character is present on chromosome as nuclear gene, then male sterility could be eliminated by repeated backcrossing.

Therefore, it is confirmed that male sterility is not controlled by nuclear gene, i.e., nuclear gene has no influence on cytoplasmic male sterility. In rare cases, male sterile plants produce a few fertile pollen grains.

When reciprocal crosses axe made between male sterile parent (with fertile pollen) and normal male fertile (female), the progeny is found to be male fertile. Such cases confirm maternal inheritance of male sterility.

Recent studies have shown that cytoplas­mic male sterility is controlled by either some unique polypeptide produced by mitochondria or by some plasmid-like elements which axe not found in the mitochondria of normal cytoplasm.

Cytoplasmic male sterility due to S-cytoplasm is different from the cytoplasmic male sterility due to T-cytoplasm in several ways. The mt DNA of S-cytoplasm contains two unique plasmid-like DNA fragments called SF (Mol. wt. 3.45 x 10⁶) and S-S (mol.wt.4.10 x 10⁶).

These plasmid-like DNAs are not found in the isolated DNA of chloroplast or nu­clei. Therefore, these plasmid-like DNAs are the characteristic of mitochondrial DNA of S-cytoplasm.

These plasmid-like DNAs are also absent in the mitochondrial DNA of N- cytoplasm (Normal-fertile), T-cytoplasm (male sterile) as well as C-cytoplasm (male sterile). Hence it has been suggested that such plasmid- like DNAs in mitochondria are responsible for causing male sterility.

T and C-cytoplasm of male sterile plant is comparatively stable and irreversible. It means that they never give rise to fertile cytoplasm even by applying mutagens.

On the other hand, S-cytoplasm is stable. It is found to change into fertile condition in some cases due to one of the two kinds of change:

i. Cytoplasmic mutation from male sterile to male fertile;

ii. Nuclear mutation giving rise to a new repressor gene.

It is also shown that both these changes are involved to make it fertile.

When a fertile line derived from S-cytoplasm is crossed as male (♂) with a Cm-S tester female plant (♀), in some cases the offspring is male sterile. It indicates that the restorer gene is, possibly, absent. In other cases the offspring is semi-fertile. It suggests that the restorer gene is possibly, present. This restorer gene is different from the normal nuclear restorer gene – Rf₃ meant for S-cytoplasm.

These new restorer genes are likely to be located on different chromosomes where they are, possibly, attached like episome at different times and bring the change from sterile to fertile condi­tion. It has also been suggested that male fertility genes could be originally located on organelle DNA and were later transposed to a nuclear site giving rise to restorer gene.

The gene or DNA segment that has migrated from the organelle to the nucleus or to the other organelle is termed promiscuous DNA.

When this fertility gene is absent from both organelle and nucleus, this might have led to cytoplasmic male sterility. The restorer gene present in the nucleus as dominant gene generally nullifies the effect of cytoplasmic male sterility so that individuals having a restorer gene in homozygous or heterozygous state are fertile even in the presence of male sterile cytoplasm.

In case of Cms-T, plasmid-like event in the mitochondria is absent but some unique polypeptides are produced in the mitochondria which bring the male sterility. When restorer gene is present in the nucleus, it prevents the production of unique polypeptides in Cms-T, and the plant becomes fertile.

But when nuclear restorer gene is absent, the plant achieves male sterile cytoplasm. Cms-C has also two additional plasmid-like elements like Cms-S. These elements are associated with cytoplasmic male sterility.

The maternal inheritance mechanism that transmits male sterility in maize have also been demonstrated by Dhawan and Paliwal in 1964. In their experiment they used two strains of maize—Sikkim primitive-2 and another strain from Colorado—for reciprocal crosses.

When Sikkim primitive-2 was used as female parent in the cross, the offspring showed little vigour and poor yield, but when Colorado strain of maize was used as female parent, the hybrid were more vigorous and showed high yield potency. These differences in hybrids of recip­rocal crosses suggests that yield and vigour are governed by female cytoplasm.

Extra-nuclear Inheritance by Cellular Organelles:

Extra-nuclear inheritance is also associated with certain cytoplasmic organelles (mitochondria, plastids) that contain naked circular DNA and protein synthesizing apparatus. These extra nuclear genetic materials present in the organelles axe autonomous and code only for limited number of enzymes and polypeptides. Certain enzymes required for cellular respiration are synthesised in the mitochondria.

Similarly, chlorophyll and other pigments axe synthesised in the plastid. Besides the involvement of such biosynthetic activities, these organelles DNAs axe directly associated with the inheritance of some phenotypes which are not controlled by the nuclear genes. The genetic material of chloroplasts and mitochondria are transmitted almost exclusively via the egg.

The inheritance pattern is well-illustrated by the following ex­amples:

Plastid Inheritance in Mirabilis:

Plastid inheritance means the inheritance of plastid characteristics due to plasma genes lo­cated in plastids. Plastid inheritance was first described by C. Corens (1908) in the four o’clock plant, Mirabilis jalapa.

Leaves of Mirabilis jalapa may be green, white or varie­gated and some branches may have only green, only white or only variegated leaves. Varie­gation means the presence of white or yellow spots of variable size on the green background of leaves.

Thus it forms the mosaic pattern of coloration on a leaf. Due to certain inher­itable defects chloroplast of all cells or some cells of leaf often are unable to synthesize the chlorophyll pigments. Such cells remain non- green and form white or yellow coloured leaf, or white or yellow patches, interspersed with ar­eas containing normal green cells with healthy chloroplasts.

Variegation may be produced by:

(a) Some environmental factors,

(b) Some nuclear genes,

(c) Plasma-genes in some cases.

Since the first and second causes of leaf varie­gation do not concern cytoplasmic inheritance, the inheritance of variegation due to plasma­-genes will be discussed in this article.

Correns made reciprocal crosses (Fig. 22.3) in all combinations among the flowers produced on these three types of branches.

These results are summarised in Table 22.1.:

The results obtained from various crosses of leaf phenotypes of Mirabilis jalapa, as shown in Table 22.1, clearly indicates that leaf phenotype of the progeny is the same as that of the female parent (Fig. 22.3). The phenotype of male parent did not contribute anything to the progeny.

This phenomenon is referred to as uni­parental transmission. Again, the results of the crosses of Mirabilis jalapa cannot be explained by sex-linkage.

The inheritance of different leaf colours in Mirabilis jalapa might be explained if the plastids are somehow autonomous and are never transmitted through male parent. For an organelle to be genetically autonomous, it must be provided with its own genetic determinants that are responsible for its phenotype.

Since the bulk amount of cytoplasm con­taining many plastids is contributed by the egg and the male gametes contribute negligible amount of cytoplasm, therefore plastids present in the cytoplasm of egg is responsible for the appearance of maternal colour in the offspring and the failure of male plant to transmit its colour to offspring is reasonable.

In the offspring from variegated female par­ents, green, white and variegated progeny are recovered in variable proportions. The var­iegated parent produces three kinds of egg- some with colourless plastids, some contains only green plastids, and some are with both chloroplasts and leucoplasts.

As a result, zy­gotes derived from these three types of egg cells will develop into green, white and variegated offspring’s, respectively.

Inheritance of Lojap Trait in Maize:

In maize plant, iojap is a trait which produces green and white stripped leaves. This trait is controlled by a recessive chromosomal gene (ij) when present in homozygous state. The name iojap was derived from ‘Iowa’ state (USA), the source of maize strain and japonica, the name of a stripped variety.

When a normal plant with green leaves used as a female parent is crossed (Fig. 22.4) with iojap parent, the offspring will be green leaved:

Again, when a reciprocal cross is made be­tween a normal green plant (used as male) and iojap plant (used as female).

The offspring will be of three different types:

In iojap plants, green and white stripped trait of leaf is inherited from the female parent due to maternal inheritance. It seems that iojap plants contain two types of plastids— normal green, and abnormal iojap plastids.

During the formation of egg cells plastids are randomly distributed in the egg cells. If the egg cell receives normal green plastids it will pro­duce green leaved plants irrespective of which plant acted as pollen parent. If the egg cell receives abnormal colourless plastids, it will give rise to white leaved plants. If the egg cell receives both green and abnormal plastids it will give rise to plants with green and white stripped leaves.

If stripped leaved F_x iojap (I_j,i_j) as female parent is crossed with normal green leaved (Ij Ij) as male parent the following types of offspring are obtained:

This backcross experiment shows that green males have no effect upon progeny. The appearance of iojap trait has been ex­plained by two hypotheses. One hypothesis states that frequent mutation in the chloroplast genome produces the abnormal plastids.

An­other hypothesis suggests that certain cyto­plasmic elements other than chloroplast muta­tion bring about the bleaching of chloroplasts. It is also suggested that a nuclear gene controls the development of abnormal plastids in the cytoplasm. So this type of inheritance is a case of inaction between nuclear and cyto­plasmic inheritance.

Extra-Nuclear Inheritance by Mitochondria of Yeast:

Yeast, Saccharomyces cerevisiae, are unicellu­lar ascomycetes fungi. In this fungi, sexual reproduction takes place by the fusion of two somatic cells to form a diploid zygote nucleus. Next follows two successive nuclear divisions forming four haploid daughter nuclei, all of which take part in ascospore formation. Now the mother cell, i.e., zygote cell, is called ascus.

The diploid zygote can also be grown vegetatively as a diploid strain that will later sporulate. Respiration of yeast cell takes place both aerobically and anaerobically (fermenta­tion). Certain mutant yeast cells are unable to utilise oxygen and are comparatively small- sized and slow growing producing small colonies on agar medium. These small colonies forming mutant strains of yeast are known as petites.

In petite strains, the necessary components (cytochrome b, c₁) and some enzymes (cy­tochrome oxidase a, a₃) for aerobic terminal respiration activity are absent. But these components are present in the cell of normal strain where they are associated with the inner membrane of mitochondria.

Petite strain can be maintained indefinitely in the vegetative state and can be mated with normal yeast cells. When such mating are car­ried out, three petite varieties can be classified:

Nuclear (segregational) Petites:

When a normal haploid strain of yeast is crossed with a haploid petite strain, a normal diploid zygote is produced. The haploid ascospores produced from zygote by sporulation are segregated in the ratio 1 : 1 (petite : normal). Hence the result of such cross follows ordinary nuclear mendelian inheritance (Fig. 22.5).

Neutral Petites:

In this type, only normal wild ascospores are produced from mattings between petite and normal strain of yeast. The petite character­istics is absent in the product of segregation. So it shows the non-Mendelian inheritance. This non-Mendelian behaviour is very difficult to explain on the basis of nuclear genes and indicates that such petite characteristics are caused by extra-nuclear inheritance.

Suppressive Petites:

In this type, all ascospores produced from mat­ing between normal and petite strain are petite type. Such petites seem to suppress normal respiratory behaviour and the suppressive pe­tite factor acts as a dominant. Fig. 22.5 shows diagrammatic scheme for explaining some dif­ferences between neutral and suppressive petite in terms of DNA.

Therefore, there are two different genetic causes for respiratory deficiency in yeast. One is nuclear and the other is extra nuclear. On this basis a neutral petite having the nuclear gene for normally functioning mitochondria is crossed with a segregational petite (Fig. 22,6). The diploid zygote produced from such cross can use the normal nuclear genes from neutral petite and respire normally.

When such diploid zygotes are grown vegetatively as a diploid strain, they produce diploid colonies that are of normal size and respire normally. But when such diploid zygotes are allowed to sporulate, they undergo meiosis and produce four haploid ascospores of which two are petites and other two are normal.

It indicates that normal and petite characteristics segregate in the 1 : 1 ratios expected from mendelian segregation.

It is noted that the neu­tral petite contains the normal nuclear gene for the respiratory enzymes but the segregational petite does not contain respiratory enzymes, so it is obvious that the cytoplasmic factor of the neutral petite appears in the cytoplasm of diploid zygotes where the factor is possibly independent of nuclear control.

It is also noted that neutral petite strains are readily produced by subjecting normal strain in low doses of acriflavines dyes as well as ethidium bromide. But the treatment of such doses of dye does not induce any nuclear changes. Thus it strongly indicates the involvement of extra-nuclear change of gene controlling petite characteristics. Such changes ultimately shows the extra-nuclear inheritance of petite charac­teristics.

(viii) Extra-Nuclear Inheritance by Mitochondria of Porky Strain of Neurospora:

There are many examples for mitochondrial enzyme deficiency which are cases of extra chromosomal inheritance—petite yeast came from the studies of Neurospora crassa, a member of ascomycetes group of fungi. In this fungus, there is a slow-growing mutant strain called porky.

Such strain exhibits poorly differenti­ated mitochondria which are deficient in the membrane-bound cytochromes b, a₁ and a₃, essential proteins of the respiratory electron transport chain and also possess greatly re­duced numbers of the small ribosomal units. As in yeast, this trait—inherited via the female parent In some strains in non-Mendelian fash­ion, indicate its extra chromosomal nature.

When porky as female parent (proto perithecial parent) is crossed with a normal strain as a male parent (mitochondrial parent), the progeny axe found to be porky. In reciprocal cross, the progeny are normal. This non-Mendelian uniparental inheritance suggests that the cytoplasm of female parent is important because the only difference between reciprocal crosses is the contribution of cytoplasm.

Thus nuclear genotype has no effect on this particular phenotype.

(ix) Extra-Nuclear Inheritance by Symbionts:

There are many cases of cytoplasmic inheri­tance which are actually due to the presence of certain intra-cellular parasites like bacteria, virus particles etc. that make a symbiotic relationship with the host cell. They are able to reproduce within the host cell and look like the cytoplasmic inclusions.

These cytoplasmic symbionts provide some evidences regarding the cytoplasmic inheritance of the host cell. These symbionts are variously designated by Greek alphabets as a (sigma), k (kappa), etc. The various types of extra-nuclear inheritances due to parasites or symbionts are discussed next.

Kappa Particles in Paramoecium:

One of the most stricking and spectacular example of cytoplasmic inheritance due to symbiont bacteria is noted in the most common ciblate protozoam Paramoecium aurelia. In 1943, T. M. Sonneborn reported that some strains of P. aurelia contain kappa particles and are known as killer strain.

Kappa particles are the symbiont bacteria called Caedobacter taeniospiralis. The diameter of kappa particles are about 0.2µ. They are bounded by a membrane and contain a little bit of cyto­plasm with DNA. The strain of Parameocium in which the kappa particles are absent are called sensitive strain. The sensitive strains are killed by the killer strain.

The destruction of sensitive strain occurs through secretion of a toxic substance called paramecin. This toxic substance is believed to breakdown the food vacuole membrane of the sensitive strain. Paramecin is diffusible in the liquid medium (Fig. 22.8).

When killers are allowed to remain in a medium for a time, they are not killed. It means that paramecin has no effect on killers. Paramecin is associated with a particular kind of kappa that occurs in about 20 percent of a kappa population.

These kappa bacteria possess a refractile protein containing ‘R’ body and are called brights because they are infected with a virus that controls the synthesis of a viral protein as well as R protein body in kappa bacterium.

The virus may act as the toxin in the killing response and R body facilitates the penetration of the toxin. The non-bright kappa bacteria may also contain virus but the virus may be in provirus state in them.

The killer character of Paramoecium has a nuclear as well as cytoplasmic basis. The existence of kappa particles is determined by presence of a nuclear dominant gene K. Kappa particles, like other bacteria, multiply through fission.

But their multiplication in the cyto­plasm of Paramoecium depends on the presence of a dominant nuclear gene K which helps to make an environment necessary for the bacteria to reproduce.

When killer strain of Paramoecium conju­gates with sensitive strain under appropriate condition for brief period and no cytoplasm exchange occurs, two kinds of clones result- one from the original killer cell which contains allel K (Kk) and kappa particles and the other from the original sensitive cell which carries the allel K (Kk) and lacks kappa particles.

It indicates that homozygous (either KK or kk) strains become heterozygous following an exchange of K and k genes without cytoplasmic exchange.

Following autogamy (a process of self- fertilisation within one undivided cell resulting in homozygosity), half the progency (50%) are sensitive Paramecia. But all progenies of sensitives following autogamy will be sensitive’s.

In this conjugation, following autogamy of killers, 50% progeny will receive Kk genotype with cytoplasmic kappa particles other 50% progeny will receive kk genotype with cytoplasmic kappa particles. But it will be sensitive, because kappa cannot reproduce in the cells unless a K allele is present in the nucleus and, as a consequence the kappa are eliminated.

On the other hand, in this conjugation the product of autogamy of sensitive strain obtained after conjugation are all sensitive. All through, 50% progeny of autogamy have KK genotype without cytoplasmic kappa particles because no cytoplasm has been transferred in this conjugation. Remaining 50% progeny of autogamy of sensitive’s have kk genotype and no cytoplasmic kappa particles.

Under some conditions of conjugation per­sists much longer; a long connection is estab­lished between conjugants (killer and sensitive). In this conjugation, cytoplasm as well as nu­clear genes are exchanged (Fig. 22.9). As a consequence both ex-conjugants will receive the genotype Kk and the cytoplasm with kappa particles.

Therefore, conjugation for longer period with cytoplasmic exchange will produce all killer strains. Autogamy of both ex-conjugants pro­duces homozygotes KK (killer) and kk (sensi­tive) cell in the 1 : 1 ratios, respectively, as expected from Mendelian segregation.

Therefore, conjugation for shorter period without cytoplasmic exchange does not follow the Mendelian pattern of inheritance. Hence it confirms the cytoplasmic basis of inheritance of killer trait.

Mu Particles in Paramoecium:

There is another type of killer trait found in certain strain of Paramoecium due to presence of ‘mu’ particles in the cytoplasm. A Paramoecium with a ‘mu’ particle is called mate killer. On the other hand, a Paramoecium having no ‘mu’ particles is called mate sensitive.

It is so named because when a Paramoecium with ‘mu’ par­ticle conjugates with a partner Paramoecium without ‘mu’ particle then the former kills the latter. The ‘mu’ particles exist only in those cells whose micronucleus contains at least one dominant of either of the two pairs of unlinked chromosomal genes such as M₁ and M₂. The ‘mu’ particles are symbionts which are made of DNA, RNA and other substances.

The maintenance of the ‘mu’ symbiont in a Paramoecium is dependent upon the genotype of the Paramoecium. In fact, the mate-killers of few genotypes maintain their normal number of particles for about seven generations. From the eighth generation, the particles suddenly and completely disappear from the a small fraction of the cell.

Gibson and Beale (1962) suggested that the maintenance of ‘mu’ particle in Paramoecium was due to the presence of another cytoplasmic particle called metagon. It is possibly a long-lived messenger RNA or informosome and may be a product of M₁ and M₂ gene. One metagon may be necessary for the maintenance of hundred ‘mu’ particles.

(x) Sigma Virus in Drosophila:

Some strains of Drosophila melanogaster are sensitive to C0₂ as they die when briefly exposed to C0₂, while normal flies can be exposed for long periods to pure C0₂ without permanent damage. The high degree of C0₂– sensitivity is associated with the presence of a DNA virus called sigma factor found in the cytoplasm of C0₂ sensitive Drosophila.

Sigma factor is transmitted through the egg cyto­plasm. When a cross is made between C0₂– sensitive female with normal male, all offspring’s are C0₂ sensitive. Again, in reciprocal cross, i.e., a cross between normal female and C0₂ sensitive male, most of the offspring’s are normal except for a small proportion of progeny which are C0₂ sensitive. Therefore, the inheritance pattern of C0₂ sensitivity is non-Mendelian and confirms the cytoplasmic basis of inheri­tance.

(xi) Spirochaetes and Maternal Sex Ratios in Drosophila:

Spirochaetes sometimes enter into the female body cell of Drosophila and live there as endoparasites. When spirochaetes enter the egg cell and these infected egg cells are fertilised, the zygotes having XY sex. chromosome are killed early in embryonic development and XX zygotes survive.

Therefore, the presence of spirochaete in the female body gives rise to exclusively female progeny—this condition is known as maternal sex ratio. It is evident that XY embryos are killed by a toxic substance which may derive from a DNA virus present within spirochetes that live as endoparasite in the female body of Drosophila. Maternal sex ratios in Drosophila is also considered as an example of extra nuclear inheritance.

Milk Factor in Mice:

This is an interesting example of extra nuclear inheritance. It is found that certain types of mice are very susceptible to mammary cancer and this characteristic is found to be transmitted maternally.The results of reciprocal cross between susceptible mice and low-incidence mice depend on the trait of female parent.

When the young mice of low cancer incidence parent are allowed to feed milk by a susceptible foster mother, it produce, a high rate of cancer in them. Hence this is a case of infective agent transmitted in the milk. The milk factor responsible for causing cancer is possibly a virus. The presence of milk factor depends on nuclear gene.

(xii) RNA Viruses in Fungi:

Like Paramoecium, there are two strains of yeast (Saccharomyces cerevisial). One strain is killer and other one is sensitive. The hiller strain secretes a proteinaceous toxic substance that kills the sensitive strain of yeast cell.

When a cross is made between killer and sensi­tive strain of yeast, only killer offspring’s are produced—indicating uniparental inheritance. There are some other strains of yeast which are called neutral strains.

Neutral strains are neither killed by killer nor do they kill the sensitive strain. But the cytoplasm of both killer and neutral strains contain two types of double-stranded RNA in the form of isometric virus-like particles (about 39 nm in diameter).

The existence and maintenance of virus par­ticles in the yeast cytoplasm are controlled by some dominant nuclear genes called MAK genes (maintenance of killer). Some other nuclear genes—e.g., KEXx (killer expression) and KEX₂—convert killers into neutrals.

A similar situation is noted is case of Ustilago maydis, a maize smut fungus. Here the cyto­plasm of killer strain also contains maycovirus like particle containing double-stranded RNA. Killer strain secretes a toxin which kills sensi­tive strains but it has no lethal effect on resis­tant strains. Resistant strains are particularly resistant to one of the killer strains designated as p₁, p₄ and p₆. Some nuclear genes denoted as P^r₁,P^r₄ and p^r₆ convert sensitive strain into resistant ones.

In all such cases mentioned above, the virus like particles are not the integral part of the normal cellular organisation but their existence and transmission indirectly provides some evi­dences in favour of cytoplasmic inheritance.

(xiii) Uniparental Inheritance in Chlamydomonas Reinhardi:

R. Sager (1970) and N. Gilham (1968) have re­ported some cases of extra-chromosomal inheri­tance in green alga Chlamydomonas reinhardi. The alga reproduces by asexual as well as sexual means. The sexual reproduction takes place by fusion between two morphologically similar but physiologically dissimilar haploid gametes com­ing from different haploid parents designated as ‘+’ and ‘-‘.

The gametic fusion produces the zygote. The sex is determined by a single chromosomal gene. When meiosis occurs in the zygote, four haploid daughter protoplasts are formed which give rise to a new plant body. Although both the sexes contribute equally to the zygote, there is maternal transmission of certain cytoplasmic traits.

Chlamydomonas is a haploid unicellulate green alga. It has two mating types—’+’ and ‘-‘.The two mating types are governed by two alleles of a nuclear single gene. The alleles axe named as mt⁺ and mt^–. The + mating type is considered as female, while the – mating type is regarded as male. During sexual reproduction one mt⁺ and one mt^– cell pair and fuse together to form a zygote where there is mixture of cytoplasm coming from both mt+ and mt^– gametes.

The zygote undergoes meiosis to produce 4 haploid meiozoospores of which two zoospores contain ‘+’ alleles and other two contain ‘-‘ alleles, i.e., it shows typical 1 : 1 segregation for nuclear genes. But for their plasma genes all zoospores are identical and contain only mt⁺ type plasma genes by mt⁺ plasma genes. The inactivation is not clear but it may involve an enzymatic process.

R. Sager isolated two strains of Chlamy­domonas: one strain was resistant (S_r) to 500/xg of streptomycin per ml of culture so­lution and the other one is sensitive. The trait of streptomycin resistance is believed to be located in its cp-DNA (chloroplast DNA).

Mating between mt+ streptomycin resistant (S_r) and mt sensitive (S_s) cells produce only resistant progeny but the nuclear genes for mat­ing type segregate as expected (Fig. 22.10). But the reciprocal cross between mt+ susceptible and mt^– resistant shows again the expected segregation for mating type but all progenies are sensitive type. Therefore, it clearly provides an example for extra-nuclear inheritance. It is also observed that in less than 0.1% of zygotes plasma genes from mt” parent are not inacti­vated and produce cytohets, i.e., heterozygotes for cytoplasmic genes.

(xiv) Inheritance of Preformed Structures:

In some lower group of animals like ciliated protozoa there are some characteristics which show cytoplasmic inheritance. These traits are not controlled by any plasma gene. In Paramoecium, the cytoplasm is differentiated into cortex or ectoplasm and medulla or endoplasm.

The mouth and the contractile vacuole are the prominent preformed traits that are present in the cortical region of Paramoecium. These preformed structures can be transmit­ted independent of the transmission of nuclear genes and plasma genes.

For normal sexual reproduction two individ­uals of Paramoecium called conjugants come close together, exchange their nuclear material, and then separate as ex-conjugants with zygote nucleus. In some rare cases, conjugants do not separate and remain as doublet animal with two sets of mouth and contractile vacuole structures.

When this doublet animal repro­duces asexually by binary fission, they also give rise to doublet ex-conjugants. When doublets are mated with normal singlet’s, the progeny of doublets ex-conjugants are doublets and the progeny of singlet’s are singlet’s. This type of inheritance is also found when the animals reproduces through autogamy.

In these crosses it is noted that the nuclear genes of doublets and singlet’s are inherited in normal Mendelian fashion. But the mode of duplicated mouth and contractile vacuole is independent of the mode of inheritance of nuclear genes as well as cytoplasmic hereditary factors.

Furthermore, the transplantation ex­periments reveal that such cortical structure (mouth, contractile vacuole) are autonomous and are maintained by cell division.

Some born has suggested that different parts of the cortex might serve as sites for the specific absorption and orientation of molecules derived from the milieu and genetic action. Besides this, preformed cortical structures could act by determining where , some gene products go in the cell, how these combine and orient and what they do.

(xv) Extra-Nuclear Inheritance in Bacteria:

In many cases, extra-nuclear inheritance has also been reported in bacteria. In addition to main chromosome, bacterial cell has got a unique extra-nuclear genetic system in their cytoplasm. Such extra-nuclear genetic system plays an important role for cytoplasmic basis of inheritance.

In bacteria, the cytoplasmic inheritance is performed by:

(i) plasmids, and

(ii) episome.

In addition to main chromosome, subsidiary DNA is also present in the bacterial cell in the form of plasmid. Plasmids are mini-circular DNA duplex and are capable of independent replication and transmission. By definition, a plasmid is a relicon that is stably inherited in an extra-chromosomal state.

Plasmids are not essential for the survival of bacterial cells, i.e., disposable-except under certain environmental conditions. Plasmids vary in size and contain three to several hundred genes. A bacterial cell may contain more than one plasmid.

There are several types of bacterial plasmids of which three general types have been studied extensively such as:

(a) F plasmid;

(b) R plas­mids, and

(c) Col plasmids.

F plasmids carry genes for the development of F pili and are responsible for conjugation.

R plasmids carry genes which are responsible for resistance to antibiotics or other antibacte­rial drugs.

Col plasmids carry genes which code for colicins. Colicins are proteins that kill sensitive E.coli cells.

Plasmids may again be divided into two types on the basis of whether or not they mediate conjugative self-transfer.

They are:

(a) Conjugative and

(b) Non-conjugative.

Conjugative plasmids are also known as transmissible plasmids that mediate the transfer of DNA through conjugation. All F plasmids, R plasmids and some col plasmids are the examples of conjugative plasmids.

These plasmids spread rapidly among the bacterial cells of a population. Transmission of R plasmids is responsible for many pathogenic bacteria to become resistant to many of the widely used antibiotics such as penicillin, tetracycline, streptomycin, kanamycin, chloramphenicol etc.

The transmission of these plasmids are not only restricted among the population of the same species but are also known to transfer to others like Proteus, Salmonella, Hemophilus, Pasteurella, Shigella etc.

Non-conjugative or non-transmissible plas­mids do not mediate DNA transfer through conjugation. Some R and Col plasmids are of this type.

Some plasmids are capable of becoming at­tached and integrated into the bacterial chro­mosome. Then they are named episomes.

Plasmid and episomes contain insertion se­quences which are also present in bacterial chromosomes. Insertion sequences are transposable in that they can move about within and between chromosomes and mediate genetic recombination between otherwise non-homolo­gous genetic elements within which they are located.

Insertion sequence are also responsible for the transfer of genetic controlling resistance to antibiotics from one genetic element to the other.

Considering the mode of transmission, location and the presence of genes controlling certain characteristics, it is clear that the phenomenon of extra-nuclear inheritance still exists even in most simple, prokaryotic unicel­lular organism like bacteria.

Considering the discussion of this article we can summarise the characteristic features of cytoplasmic inheritance as:

i. In case of cytoplasmic inheritance, recip­rocal crosses show marked difference for characteristics governed by plasma genes.

ii. In most cases female parent contributes the plasma genes, i.e., uniparental inheritance or maternal inheritance.

iii. In general, F₂, F₃ and so on generations do not show segregation for cytoplasmically inherited characteristics. It is a non-Mendelian inheritance.

iv. In case of bi-parental inheritance, irregular segregation takes place in F₁.

v. Several plasma genes are associated with cp-DNA or mt DNA in higher eukaryotic organisms and with plasmids or eRisomes in prokaryotes.

vi. In many cases, a cytoplasmically inher­ited characteristic is associated with an endosymbiont or parasite or virus present in the cytoplasm of the organism.

In some specific cases the cytoplasmic in­heritance of some preformed characteristics is not affected by exchange of cytoplasm and is not controlled by nuclear genes. They are au­tonomous and are maintained by cell division.