Cytogenetics of Polyploids

Abstract

Two cytological races having 2n=32 and 48 and designated as tetraploid and hexaploid, respectively were found in C. reflexa. While somatic chromosome complement of tetraploid race comprises of 28 metacentric and 4 submetacentric chromosomes, the complement of hexaploid races consists of 36 chromosomes with median and 12 chromosomes with submedian centromere. On the basis of general chromosome morphology and shape, the somatic chromosomes of the complement could be grouped into 8 sets, each set comprising of 4 chromosomes in the tetraploid and of 6 chromosomes in the hexaploid race. The differences observed in these parameters within the chromosomes of each set are statistically insignificant but those between the sets are significant. Meiosis in the tetraploid race is normal leading to the formation of viable pollen grains. In the hexaploid race, meiosis is highly abnormal and is arrested before heterotypic division is completed. Immediately after the pollen mother cells degenerate, branched fibrilous attachments projecting out from the degenerated PMCs are observed. When the young immature anthers were clipped off and the flowers bagged, parthenocarpic seeds are produced in the hexaploid material, However, the seed set is poor. A gene controlled suppression of homeologous pairing in the tetraploid race has been postulated. This type of control is not operative in the two hexaploid races of the species. The implications of karyotype morphology and chromosome pairing behaviour in polyploids are critically discussed in general and the role of polyploidy in the species in particular.