The Effect of B-chromosomes on A-chromosome Chiasma Formation in Pearl Millet

Abstract

The effects of B-chromosomes on A-chromosome chiasma frequency and distribution were studied in Pennisetum typhoides (2n=14). Since B-containing plants of this species are mosaics, showing variation in B-numbers, both within and between tissues, the analyses have been based on pooled data derived from cells with the same B-phenotype but from different individuals. The results obtained on the effects of the three types of Bs viz., standard, deficient and iso Bs largely asgreed with those of the earlier reports. The earlier findings that there is threshold value for iso Bs and the effect of one iso B is same as that of two, three and four is presumed to be due to the presence of an upper limit for chiasma frequency per cell beyond which there cannot be further increase. It is suggested that there are a unmber of sites on the long arm of the long arm of the B-chromosomes which may be responsible for their effect on A-chiasma frequency. The differential effects of the three types of Bs are then assumed to be due to the relative number of such sites present with iso Bs having more sites than standards and deficients fewer than standards.
The effect of B-chromosomes on the distribution of A-chiasmata-between PMCs within a plant, between bivalents within PMCs and within bivalents between the two arms-was also studied. The cell variance of chiasma frequency increased within individuals carrying standard, deficient or iso Bs. The over all pattern regarding chiasma variance between bivalents within PMCs was that standard Bs and deficient Bs did not have any effect, whereas iso Bs increased the chiasma variance and the extent of this was proportional to their number. The chiasma distribution in individual bivalent arms was analysed by assessing the proportion of non-distal chiasmata and also the proportion of rod bivalents with two chiasmata in one arm. Standard Bs and deficient Bs decreased, while iso Bs increased, the proportion of non-distal chiasmata per cell. The correlation coefficients obtained, however, were not significant. Non-distal sites also become equally active in the presence of all three categories of B-chromosomes.