Top 5 Characteristics of Chromosome |Plant Taxonomy

The following points highlight the top five characteristics of chromosome. The characteristics are: 1. Chromosome Number 2. Chromosome Size 3. Morphology 4. Behaviour at Meiosis 5. Banding Patterns. 

Characteristic # 1. Chromosome Number:

The number of chromosomes in a species is usually constant and this makes it an important taxonomic character. However there are exceptions where chromosome number varies. These changes usually occur in chromosomes during the process of division, and these changes may affect the gene sequence, their number or even there may be loss of chromosomes themselves.

The perpetuation of this slow process, results in the evolution of new chromosomal races. The chromosome number shows a wide range in vascular plants. The lowest chromosome number in flowering plants is recorded in Haplopappus gracilis (Asteraceae) [2n = 4] and the highest in Poa litorosa (Poaceae) [2n = 265].

Ophioglossum species (Pteridophyte) has the highest number of chromosomes in the plant kingdom (2n = 1240). This great diversity of chromosome numbers and their relative constancy within populations and species provide an important character for taxonomic groupings of large number of plants.

The chromosome number relationship with taxonomic groups can be broadly classified into the following three classes:

i. Constant Number:

In certain groups of vascular plants, the chromosome number is constant throughout the whole group, e.g., Quercus and other members of the Fagaceae have the same basic number, n = 12. In such cases chromosome number is not of any help in distinguishing various taxa within the group.

ii. Euploidy:

When chromosome numbers in various members of a taxon are in the proportion of exact multiples, the series is described as euploidy. For example in Malvaceae, the somatic numbers in various species range from 10, 15, 20, 25, 40; from 12, 18, 24 to 30; from 14, 28, 42, 56 to 84 and so on.

a. Basic Number:

In an euploid series, the various members may be unified by a basic number (x) which is the gametic number of a diploid species. As in the above example of Malvaceae, the basic number x = 5. The other species in the series are described as triploids (3x), tetraploids (4x), hexaploids (6x) … polyploids (nx). The basic number is usually constant for a genus or higher taxa and has proved useful in supra-specific studies.

b. Primary and Secondary Basic Numbers:

In many cases, more than one basic number can be present in a group. For example, in the living species of Chlorophytum (Liliaceae), the chromosome numbers vary from 14 to 28, 42, 56, 84, etc. and also from 16 to 32, which means that Chlorophytum has two basic numbers x = 7 and x = 8.

In such cases, the inferred base numbers ranging between 2 and 13 may be referred to as the primary basic numbers in the absence of living diploid members, while the remaining are termed secondary basic numbers.

In case of Chlorophytum however, Naik (1976 ) from his detailed analysis of the meiotic behavior of chromosomes in one of the species, C. laxum, has shown that the two base numbers 7 and 8 should be considered as secondary, most probably derived from the primary basic number x = 4.

c. Polyploid Pairs:

Closely related species in certain groups of plants may be cytologically distinct i.e., one may be diploid while the other a tetraploid. Such related pairs are termed polyploid pairs. For example, Cardamine hirsuta (2n = 16) and C. flexuosa (2n = 32) of the Cruciferae is a polyploid pair.

d. Dibasic Polyploidy:

It is now a well known fact that hybridization has a dominant role in evolution and such hybridization may involve crossing of any two genetically unlike individuals, which have different base numbers of chromosomes. Such hybrids undergo polyploidy since each chromosome is doubled as the pairing at meiosis is restored, and this type of polyploidy is termed as dibasic polyploidy.

An excellent example of this type of polyploidy is Karpechenko’s (1927,1928) artificially synthesized Raphanobrassica, which is a hybrid between Brassica oleracea (2n = 18) and Raphanus sativus (2n = 20) and has 2n = 38 chromosomes.

iii. Aneuploidy:

If the chromosome numbers in the different members within a group bear no simple numerical relationship to each other, then the series is termed as an aneuploid series or simply aneuploidy. Various species of Carex (Cyperaceae) for example show a wide range of chromosome numbers from n = 6 to 112 with multiples of 5,6, 7, and 8 thus exhibiting aneuploidy.

Aneuploidy may result due to either of the following:

(i) Change in the Basic Number:

An increase or decrease in the number of chromosomes may take place, whereby the same genetic material becomes distributed in a different number, leading to a change in the basic number. This phenomenon is important from the taxonomic and evolutionary point of view, as change in the basic number results in new variations and recombination’s, leading to the evolution of new varieties and ultimately to new species.

The changes in the basic number of chromosomes can be brought about by a process called polysomaly. This is very common and results in increased sets of genes wherein there is a duplication of one or a pair of chromosomes. This means that any one pair may undergo polyploidy. Due to this some of them can afford to lose one or two chromosomes and get stabilized in nature with this new chromosome number.

This naturally brings about a different basic number. For example, most of the species of Dahlia (Asteraceae), have x=8, but D. merckii is regarded as a polyploid, with n= 18, resulting by addition of two pairs of chromosomes.

Polyploidy has also been reported in Datura, Nicotiana, etc. Polysomics can be of various types:

a. Trisomies :

Plants containing one extra chromosome are known as trisomies i.e., 2n + 1. In trisomies the extra chromosome produces a certain amount of unbalance and thus limiting their taxonomic significance.

b. Tetrasomics :

Plants containing two extra chromosomes are known as tetrasomics i.e., 2n + 2, etc.

c. Monosomies:

Plants with one chromosome less are known as monosomies i.e., 2n – 1. Normally diploid monosomies are inviable.

e. Nullisomics:

Plants with two chromosomes less are known as nullisomics i.e., 2n – 2, etc. Polysomics are usually unstable and as they are not isolated genetically from their relatives under natural conditions, they would lose their identity through crossing with normal plants followed by selection for more viable, genetically balanced normal disomic types.

(ii) Basic Number Remaining Unaltered :

The basic number may remain unaltered, but the genetic material present may be changed due to the addition or loss of chromosomes. This phenomenon is less important from the evolutionary point of view as the genetic make-up of a taxon becomes unbalanced due to loss or addition, resulting in an unstable condition, which cannot be perpetuated and thus cannot give rise to well differentiated novelties.

This type of aneuploid alteration of the basic number has been reported in Crepis (Asteraceae) and its relatives. This loss of chromosome/s, fragmentation or misdivision of centromere can take place as irregularities during the cell division even in the diploid species.

(iii) B-Chromosomes :

They are one or more accessory or supernumerary chromosomes, in addition to normal chromosomes and have been detected in a large number of plants. In plants, when compared to the other members of the chromosome complement, they are generally of a much smaller size and are of unknown origin.

They reduce fertility or increase the vigour of plants. It has been found that they perpetuate in certain natural populations and may have some evolutionary significance.

Characteristic # 2. Chromosome Size:

The individual chromosomes of some taxa show marked differences in shape and size at mitotic metaphase. The size of chromosome varies greatly in different families and also amongst members of the same family. The monocotyledons usually have larger chromosomes than the dicotyledons. In general, woody plants have smaller chromosomes than their herbaceous relatives.

Nearly all the members of the families of Araceae, Commelinaceae, Cyperaceae, Dioscoreaceae and Zingiberaceae possess small chromosomes. Iridaceous members reveal small to medium sized chromosomes, while taxa of Amaryllidaceous exhibit large sized chromosomes. Members of the family Liliaceae are characterized by the presence of all types of chromosomes.

Chromosome length is usually used to characterize the size of the karyotype. However sometimes chromosome volume, expressed as a function of DNA content, is used for this purpose. The chromosome length in most plants varies from 0.5-30 µ.

Chromosome size is not related to the phylogeny of angiosperms in general but is characteristic of only certain groups and families.

Phylogenetic reduction in chromosome size, was first of all described by Delaunay in the genus Muscari of Liliaceae. In this genus, species with greater morphological specialization possessed smaller absolute chromosome size as compared to the relatively primitive species.

Reduction in chromosome size has also been reported in Crepis and in Dianthus. Similarly phylogenetic increase in chromosome size has been reported in the family Poaceae, in Galium and in Godetia.

The two genera Trillium and Paris possess the largest chromosomes and are definitely specialized in their vegetative characteristics, and are considered to have descended from Uvularieae, the members of which have also large chromosomes but nevertheless smaller in contrast to those found in Trillium and Paris.

The evolutionary significance of chromosome size was further elaborated by Stebbins.

The co-adaptive nature of genetic system in plants was first pointed out by Darlington. He suggested that chiasma frequency often depended on the length of the chromosomes, the larger chromosomes means high chiasma frequency resulting in high recombination index while small chromosomes mean low chiasma frequency resulting in low recombination index.

Therefore, there is a trend to linking of small chromosome size with multiplicity in their number as in tree species.

Characteristic # 3. Chromosome Morphology:

Apart from the number and size of the chromosomes of many genera and families of flowering plants, conspicuous differences in appearances of the karyotype of the chromosomes have also been found in species having the same chromosome number. The chromosomes are best discernible at mitotic metaphase.

The karyotype of the chromosomes can be characterized on the following basis:

a. Relative length of the arms of chromosomes

b. Position of the centromere

c. Presence of satellites

Accordingly the chromosomes can be characterized as following types:

I. Symmetrical :

A karyotype consisting of chromosomes all essentially similar to each other in size and with median or sub-median centromeres and with two equal arms are termed as a symmetrical chromosomes.

Depending on the position of the centromere they may be of further two types (Fig. 8.12):

(i) Metacentric or V-shaped — Chromosomes with median centromere.

(ii) Sub-metacentric or L-shaped — Chromosomes with sub-median centromere.

Karyotypes of this nature are the most common ones and Levitzky considered them as generalized types. For example, the chromosomes in the karyotype of the primitive genus Helleborus (Ranunculaceae), differ little from each other in size and most of them are V-shaped with median or sub median centromeres.

II. Asymmetrical:

This type of karyotype possess many chromosomes with sub-terminal or terminal centromeres, or great differences in size between the largest and the smallest chromosomes, or both. Depending on the position of the centromere they may be further of two types (Fig. 8.13):

(i) Acrocentric or J-shaped — Chromosomes with sub-terminal centromere.

(ii) Telocentric or I-shaped — Chromosomes with terminal centromere.

Karyotypes of this nature are considered as specialized types. For example, in the advanced genera Aconitum and Delphinium (Ranunculaceae), the flowers have the largest number of J-shaped chromosomes.

III. Secondary Constrictions and Satellites:

The karyotypes can also be differentiated on the basis of secondary constrictions, which are small bead-like appendages. Occasionally they occur at the terminal ends of one or more pairs of chromosomes in many species and are known as satellites (Fig. 8.14).

These structures are widely distributed in the plant kingdom, which shows that they are a valuable, if not essential, part of the chromosomal complement. However, very little is known about the evolutionary changes in the satellites and nucleoli.

Generally, the asymmetrical karyotypes are most common in plants, which are usually specialized morphologically, while symmetrical ones are found in more or less generalized plants, but also occur in morphologically specialized ones too.

The reason for this increasing asymmetry in karyotype evolution is not known. However, Stebbins (1950) suggested that unequal translocations and inversions involving the centromere seem to be responsible for this.

Generally symmetrical karyotypes are taken to reflect primitive status of taxa, while various specialized types (asymmetrical karyotypes), are considered to have been derived, from the symmetrical karyotypes.

However, Jones has challenged this view by postulating that symmetrical karyotypes originally evolved from asymmetrical ones by end-to-end fusion of telocentric chromosomes. This reverse trend of specialization, i.e. from telocentric chromosomes to metacentric ones, has been demonstrated in certain members of the Commelinaceae.

Characteristic # 4. Chromosome Behaviour at Meiosis:

A study of chromosome behaviour at meiosis can provide some valuable information about the relationship of populations and species. Pairing behaviour at meiosis is mostly determined by chromosome number and chromosome homology.

The kind and degree of pairing can:

1. Show whether hybridization has occurred – The degree of chromosome homology, in hybrids is an indication of the degree of relationship of the parental species.

2. Indicate structural differences in the parental chromosomes – Meiotic pairing behaviour in hybrids can also point to structural differences in the chromosomes of its parents.

Such differences may be the result of (Fig. 8.15):

a.Translocations — a chromosomal segment is removed from one place and reinserted somewhere else in the genome, either in the same or in some other chromosome.

b. Inversion of segments — a segment of a chromosome becomes reinserted in the same chromosome but the opposite way around.

It can be further of two types:

(i) Paracentric — inversions involving only one chromosomal arm.

(ii) Pericentric — inversion incorporating the centromere i.e., involving both chromosomal arms.

a.Deletions — an interstitial or terminal chromosomal segment is lost.

b.Duplications — when a segment of the chromosome is represented two or more times in a chromosome of a homologous pair.

In each case, there results a characteristic configuration of chromosomes at the time of pairing. For example, let us consider two diploid species, which differ in the structure of two chromosomes of the set, due to exchange of equal segments. The diploid complement of the hybrid between these species will have two pairs with unlike gene arrangements.

During meiotic pairing, at the time of matching process, a characteristic configuration results, which indicates the involvement of translocations.

Similarly, characteristic configurations result, when structural differences in the chromosomes of two species are a consequence of deletions, duplications or inversions. Such differences in chromosome structure have been observed in different races of certain species (e.g. Datura stramonium).

c. Patterns of variations of populations.

d. Explain the causes of sterility — A study of the process of meiosis in hybrids yields information of great evolutionary significance. The fertility of a hybrid depends upon the degree of homology between the chromosomes of its parents. When this process is irregular in hybrids, it results in disharmony between the genetic systems of the parents involved.

The more the number of non-homologous segments in the hybrid, the more is the incidence of irregularities, the greater the degree of sterility and greater the magnitude of evolutionary distance between the parents.

Thus, whether the hybrid is vigorous or weak, whether it produces viable or in-viable pollen, and whether it is capable of producing greater or lesser amount of seed, helps in estimating the degree of homology of the genomes involved, which ultimately reflects a measure of species relationships i.e., degree of pairing is proportional to the degree of homology of the genomes.

Characteristic # 5. Banding Patterns of Chromosome:

In recent years new staining techniques have developed, using .Giemsa and fluorochrome dyes, which stain chromosomes in a consistent banding pattern instead of with uniform intensity as in the case of previous usual techniques of staining with basic fuchsin (Fuelgen reagent).

DNA is inevitably non-homogeneous along the arms of a chromosome. During mitosis and meiosis, local variations in the degree of condensation of the chromatin to form heterochromatin may be apparent in metaphase preparations.

The new techniques have thus, allowed to distinguish morphologically between chromosomes with enhanced efficiency. These techniques, for example, have been utilized by Ehrendorfer and Ehrendorfer et al.in studying the systematic position of the genus Anacyclus, which confirms with the results from other fields of studies, particularly morphological and phytochemical approaches.

Systematic value of cytotaxonomy:

Cytological evidence is of immense importance in interpreting a classification and establishing relationships. The information collected from chromosome characteristics has been found to be valuable at all levels of the taxonomic hierarchy and the usefulness of cytological data varies from group to group.

A few examples are under mentioned:

Cytological Variation at Family Level:

Cytological data have been fruitfully employed to unravel affinities and inter-relationships at the family level.

Following are a few examples:

(i) Cytological data has provided logical basis for improved arrangement of tribes and genera in some families such as the Ranunculaceae, Brassicaceae and Poaceae. For example in Ranunculaceae, all the genera with n = 7, 8 and 9, as well as genera with long chromosomes and short chromosomes have been placed under Anemoneae and Helleboreae.

Based on karyological similarity i.e., with same base number n = 7 and small-sized chromosomes, Thalictrum and Anemonella of Anemoneae, and Isophyrum and Aquilegia of Helleboreae have been grouped in tribe Thalictreae. Similarly two other genera of the Helleboreae, Coptis and Zanthorhiza with very small chromosomes and the base number 9 have been placed under an additional tribe, Coptideae.

(ii) The number and size of the chromosomes are the basis of the major sub-divisions of the family Poaceae as recognized currently.

(iii) Some earlier workers have placed the genera Agave and Yucca under separate families, Amaryllidaceous and Liliaceae respectively. Cytological studies reveal that both have 5 long and 25 short chromosomes. The karyotype similarity, thus, justify their inclusion in the family Agavaceae as has been done by Hutchinson.

(iv) Delimitation of the tribes in Asteraceae has been done on the basis of chromosome numbers.

Cytological Variation at Generic Level:

There are several examples where chromosome numbers support generic status.

A few are cited below:

(i) The genus Cistus (Cistaceae) was formerly included in Helianthemum. However, Cistus has base chromosome number 8 and Helianthemum has 9. This supports the recognition of Cistus as a separate genus.

(ii) Several taxonomists treated the genera Physaria and Lesquerella of Brassicaceae as a single genus. However cytological details suggest that they should be treated as two different genera.

(iii) Based on cytological studies, a new classification of the genus Narcissus of Amaryllidaceae has been proposed by Fernandes.

(iv) The two genera Physaria and Lesquerella, under the family Brassicaceae, were recognized by many workers, as a single genus. Cytological evidence, however, suggests that these two genera should remain separated.

Cytological Variation at Specific and Infra-Specific Level:

There are several examples of cytological variation occurring at specific and infra-specific level.

Following are a few examples:

(i) All the species under the genus Tephrosia (Fabaceae) are with 2n = 22 except for T. constricta, which has 2n = 16. Thus, this species has been separated as a distinct genus, Sphinctospermum and the chromosome number supports this separation.

(ii) Cheenaveeraiah, on the basis of karyotypic studies suggested that the section Sitopsis of the species Aegilops, should be shifted to Triticum from Aegilops, or it should be given the rank of a new genus.

(iii) Based on cytological studies, Brandt recognized two races in Veronica prostrata of the family Scrophulariaceae, and suggested that they should be treated as subspecies prostrata (n = 8 and with small pale blue flowers) and subspecies scheereri (n = 16 and with larger dark blue flowers).

(iv) Previously Monotropahypopitys of the family Monotropaceae, was treated as a single species with two varieties, var. hirsuta and var. glabra. However, cytological examination revealed the former to be a hexaploid, with 2n = 48, and the latter a diploid, with 2n = 16. Thus the hexaploid was retained as the species M. hypopitys and the var. glabra was raised to the specific rank as M. hypophegea.