The effect of B-chromosomes on A-chromosome chiasma formation in pearl millet.

Abstract

Pantulu and Manga (1975) studied the influence of standard B-chromosomes while Subba Rao and Pantulu (1978) studied the effect of the derived B-chromosomes on mean cell chiasma frequency in pearl millet, Pennisetum typhoides (Burm.) S. & H. (2n=14). No critical assessment of the effects of Bs on A-chromosome chiasma distribution is, however, available. This information is of interest because when there is a distal localization of chiasma formation as is the case in pearl millet, the occurrence of interstitial chiasmata may bring about a recombination of otherwise tigtly linked genes (Rees and Jones 1977). Particular attention has therefore been paid in this paper to the effect of B-chromosomes on chiasma distribution.In an earlier paper, Subba Rao and Pantulu (1978) speculated that the differenti al effects of the three types of Bs present in pearl millet on A-chiasma frequency were due to relative amounts of euchromatin and heterochromatin present in them. This conclusion was arrived at particularly from the effects observed when the three types of Bs were present in different combinations and ratios.The B-chromosomes of pearl millet are, however, unstable and exhibit numeri cal variation between cells of the same tissue and between tissues of the same plant (Jayalakshmi and Pantulu 1982). The earlier workers carried out observations and analyses at the individual plant level. For this purpose, they classified the B-carry ing plants on the basis of 'modal' number, i.e., that number which is most frequent. How far the Bs present in numbers deviating from the modal number are liable to influence the results is not known. In the present study, therefore, analyses were carried out after pooling the cells with the same B-chromosome type and number from different plants.

Influence of B-chromosomes on Genetic Recombination and Fertility in Pearl Millet

“…Still others like Jones and Rees (1967) found no influence of B-chromosomes on A-chromosome chiasma frequency in Rye. In the present study, a clear cut increase in chiasma frequency of carrier plants over non-carrier plants could be observed which again is partial support earlier reports of Rao and Pantulu (1978) and Jayalakshmi and Pantulu (1984).…”

“…Regarding their function, there are overwhelming evidences on their role in modification of A-chromosome chiasma frequency and distribution as well as in causing sterility. Their effect on genetic recombination and sterility has been demonstrated to differential results among Rye (Jones and Rees 1967), maize (Hanson1969), Trigonella (Pant and Raghuvanshi 1980), Plantago (Paliwal andHyde 1959, Raghuvanshi andKumar 1983), Pennisetum (Rao andPantulu 1978, Jayalakshmi andPantulu 1984), Impatiens Mahajan 1982, Kumar andDikshit 2002).…”

B-chromosome Paradigms in Genetic System of Poppy (<i>Papaver somniferum</i> L.)

“…Various researchers have worked to investigate the mechanism by which B-chromosomes modify the chiasma frequency of A-chromosomes, and contrasting reports on the behaviour of B-chromosomes effecting chiasma frequency have been presented by various researchers: Jones and Rees (1967) found no influence of B-chromosome on chiasma frequency in rye, Sheidai et al (2002) observed significant variations in the chiasma frequency in Aegilops, while Hason (1969) in the case of maize has a concording view with the present observation along with the supporting studies of Jayalakshmi and Pantulu (1984) on pearl millet and Kumar and Dixit (2002) on Impatiens balsamina. The increase in chiasma frequency could be attributed to an increase in double and triple exchange classes.…”

Crossover effects of B chromosomes may be 'selfish'

“…This idea has been mentioned by some authors (Bell & Burt, 1990;Jayalakshmi & Pantulu, 1984). Such a crossover effect might occur if the addition of an extra chromosome(s) could evoke a general response in the crossover system of the organism.…”