Chromosome Studies in Different Species and Varieties of Sida with Special Reference to Accessory Chromosomes

Abstract

Detailed cytological studies have been carried out on nine species and varieties under the genus Sida. Some varieties of S. rhombifolia showed the presence of accessory chromosomes and so populations of these varieties from different ecological habitats were collected. Paradichlorobenzene and aesculin were found to be the most effective pretreating chemicals in studying their somatic chromosomes. The chromosome numbers reveal that polyploidy, both inter- and intraspecific, has played a role in evolution in the genus, in addition to aneuploidy. Of the different basic numbers (n=7, 8, 9) 7 is found to be deep seated.
In addition to polyploidy, structural rearrangements of chromosomes have been observed in all cases showing their prominent role in speciation within this genus. Moreover, regular bivalent formation even in intraspecific tetraploids suggests structural changes during evolution. However homogeneity of the genus as a whole is indicated in the general pattern of the karyotypes of different species and varieties and also in the similarity in their total chromatin length. Within this genus at least polyploidy in speciation has not been associated with diminution in chromosome size.
The constant presence of accessory chromosomes in a phenotypically distinct population of S. rhombifolia, indicates some selective advantage. Regarding the origin of accessory chromosomes, it has been suggested that some of the extra chromosomes of high polyploid cells which are often found in the individuals bearing accessories, may lose their function and undergo heterochromatinization and as such are transformed into accessory, ones.