Paracentric Inversion in Chromosome (With Diagram)

In this article we will discuss about the paracentric inversion in chromosome.

Chromosome Pairing and Crossing Over:

In paracentric inversions chromosome pairing occurs by loop formation in the inverted region; the centromere remains outside the loop. Crossing over in the loop causes the formation of dicentric chromatids which produce bridges at anaphase.

Chromatid bridges may be formed at AI or All depending upon the position of crossing over. As shown in Fig. 15.6, crossing over may occur at different positions within the loop. It may occur in the interstitial region also.

The results of crossing over at the different positions are given below:

(1) A single crossing over at any position in the inversion loop will give rise to a dicentric chromatid and an acentric fragment. At AI, a bridge and a fragment will be observed (Fig. 15.6).

(2) A double crossing over involving the same two chromatids (2-strand double crossing over), for instance, at the positions I and II in Fig. 15.6, will produce no change in the chromosomes.

(3) A double crossing over involving three chromatids (3-strand double crossing over), e.g., at the positions I and III or IV, will produce a dicentric chromatid bridge and an acentric fragment at AI.

(4) A double crossing over involving all the four chromatids (4-strand double crossing over), e.g., at the positions I and V shown in Fig. 15.6, will produce two dicentric chromatids and two acentric fragments. As a result, a double bridge and two fragments will be observed at AI (Fig. 15.7).

(5) A crossing over in the interstitial region (position VI in Fig. 15.6) and a single crossing over at the position I in the inversion loop will produce a dicentric bridge plus an acentric fragment at AI.

(6) A crossing over in the interstitial region and a single crossing over at the position III or IV (Fig. 15.6) in the inversion loop (3-strands involved) will produce a fragment and a loop at AI, resulting a dicentric bridge in one cell of the dyad at All (Fig. 15.8).

(7) A crossing over in the interstitial region (position VI) and a single crossing over at position (V) (Fig. 15.6) in the inversion loop (4-strands involved) will produce a bridge and a fragment at AI.

(8) A crossing over in the interstitial region (position VI) and a double crossing over at the positions I and II (2 strands involved) will have no effect on the chromatids and their behaviour.

(9) A crossing over in the interstitial region (position VI) and a double crossing over at the I and III positions (3 strands involved) will produce a dicentric bridge and a fragment at al.

(10) A crossing over in the interstitial region and a double crossing over at the positions I and IV (3 strands involved) will produce a loop plus an acentric fragment at AI and a bridge plus a fragment at All.

(11) A crossing over in the interstitial region (position VI) and a double crossing over in the inversion loop at the positions I and V (4-strands involved) will produce a double bridge and two acentric fragments at AI.

(12) A crossing over in the interstitial region (position VI) and a double crossing over in the inversion loop at the positions III and IV shown in Fig. 15.6 (4 chromatids involved) will produce two loops and two acentric fragments at AI. (Fig. 15.9). It will result in a dicentric bridge at All in both cells of the dyad.

Chromosome configurations at AI and All resulting from all the above combinations are grouped into four classes and are summarised as follows:

(a) A single bridge (one dicentric chromatid) plus an acentric fragment at AI (BF).

(b) A double bridge (two dicentric chromatids) plus two acentric fragments at AI (BBFF),

(c) A loop plus an acentric fragment at AI, and a dicentric bridge in one cell of the dyad at AII(LF),

(d) Two loops plus two acentric fragments at AI, and one dicentric bridge in both the cells of the dyad at All (LLFF).

The frequencies of these configurations observed in paracentric inversion heterozygotes of different organisms are given in Table 15.2. Several environmental factors influence the occurrence of crossing over during meiosis and thereby affect the frequencies of bridges and fragments. One such factor is temperature.

Swanson found that in Tradescantia the frequency of bridge increased from 17.5% to 47.8% when the temperature was raised from 12-15°C to 27°C. Using the “Holm” and “Das” inversions in barley Powell and Nilan demonstrated that the frequencies of bridges and fragments could be altered by changing the temperature during meiosis.

Therefore, environmental influences on the frequencies of various configurations should be taken into account while comparing different inversions.

Occurrence:

Paracentric inversion is easily detected due to the presence of bridge plus fragment during meiosis. But in pericentric inversions, bridge is never formed; hence their detection is relatively more difficult. However, bridges and fragments also appear due to other abnormalities, such as, breakage and fusion of chromosomes, breakage and reunion of sister chromatids, duplications and stickiness.

Paracentric inversions have been studied in several plant species, e.g., maize barley, Trillium, Liliumtestaceum, L. formosanum, Viciafaba and others. A number of paracentric inversions have been described in animals like Drosophila, mosquito (Anophilesmesseae), yellow fever mosquito (Aedesaegypti), grass hopper Cammulapellucida, Sciara, human and others. Inversions may occur spontaneously or they may be induced through mutagens.

(1) McClintock, B. 1938. Missouri Agr. Exp. Sta. Res. Bull. 290 : 1-48.

(2) Russel, W.A. and Burnham, C.R. 1950. Sci. Agr. 30 : 93-111.

(3) Rhoades, M.M. and Dempsey, E. 1953. Amer. J. Bot. 40 : 405-424.

(4) Brandham, R.E. 1969. Chromosoma26 : 270-286.

(5) Sjodin, J. 1971. Hereditas67 : 39-54.

(6) Brown, S.W. and Zohary D. 1955. Genetics 40 : 850-873.

(7) Smith, L. 1941. J. Genet. 30 : 227-232.

(8) Das. K. 1955. Indian J. Genet.15 : 99-111.

(9) Ekberg, I. 1967. Ph.D. Thesis, Univ. Stockholm.

(10) Prasad. G. 1975. Barley Genet.Ill Proc. 3rd Int. Barley Genet.Symp.Garching.pp. 282- 288.

(11) Nur. U. 1968. Chromosoma25 : 198- 214.

Bridge Elimination Mechanism:

It has been observed in maize that dicentric bridges are not included in the megaspore which forms the embryo sac, as a consequence of which they are never present in the egg. In Drosophila also, the bridge is not included in the ovum. The mechanism which prevents the bridge from inclusion in the female gamete is similar in both the organisms.

In Drosophila, the dicentric bridge is formed at AI due to a single crossing over within the inversion loop. The second meiotic division occurs in such a way that the two inner nuclei are linked together by the chromatid bridge, while the two outer most nuclei receive the non-cross over chromatids, i.e., one nucleus receives the normal chromatid, while the inverted chromatid is passed on to the other (Fig. 15.10).

One of these two outer nuclei forms the ovum, while the remaining three form polar bodies. In maize, a linear tetrad is formed, and one of the outer megaspores develops into an embryo sac (Fig. 15.11). This megaspore contains either normal or the inverted non-crossover chromatid which is included in the egg nucleus. Thus paracentric inversions are maintained in the populations.

Fertility of Paracentric Inversion Heterozygote:

In cases of preferential segregation described in the previous section, fertility of paracentric inversion heterozygote females or ovule fertility is not affected. But no such mechanism is operative in pollen mother cells; as a result, the microspores contain all the products of crossing over within the inversion loop. PMC’s without a crossing over in the inversion loop produce 100% balanced and functional gametes.

But crossing over within the inversion loop leads to the formation of deficiency-duplication chromatids, and the spores carrying such chromatids are unbalanced and nonfunctional. The BF and LF cells (Fig. 15.6, 15.8) produce 50% balanced and 50% unbalanced gametes, whereas BBFF and LLFF cells produce 100% unbalanced gametes (Fig. 15.7, 15.9).

Therefore, on the basis of the various configurations observed during AI, pollen fertility/sterility can be predicted. The observed and expected pollen fertility in two paracentric inversion heterozygotes of barely are presented in Table 15.3.

The average frequencies of BF, BBFF, LF and LLFF configurations at AI in one inversion were 19.0%, 7.8%, 17.2% and 5.6% respectively. Therefore, the predicted pollen sterility would be 31.5% [= (19.0) /2 + 7.8 + (17.2) 12 + 5.6 per cent] which is quite close to the observed value of 33.6%.

On the other hand, the estimated pollen fertility is 68.5% which is comparable to the observed value of 66.4%. Close similarities between the observed and predicted pollen fertility have been observed in different plants like maize, barley, Viciafaba and others.