Comparative Karyomorphology ofEu-Sorghums

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Abstract
Detailed analysis of chromosomes at pachytene and later stages of meiosis were made in the five species of Sorghum belonging to the Sub-genus Eu-sorghum. Pachytene chromosomes are depicted on the basis of total length, relative length, arm ratio, amount and distribution of heterochromatin and number and position of chromomeres. The characteristics of each of the ten chromosomes in the haploid complement are described in detail in different classes bringing out the similarities and differences amongst the five species. Species have been compared inter alia on their total lengths of chromosomes and the differences in lengths both within the species and between them were statistically tested. A remarkable feature noted regarding the asymmetrical chromosome is that in S. vulgare it is the fifth chromosome while in all the other species the asymmetrical one is the sixth, based on length. Comparisons based on chromomeric pattern revealed that the karyotype of each species could be divided into four groups for convenience of recognition and description. Pachytene chromosomes exhibit differentially staining regions which persist till late diakinesis. Studies on the pairing properties of the differentially stained region show that synapsis starts from the proximal to the distal end and separation of the split chromosomes starts from the distal to the proximal. The distribution of chiasmata at different stages was studied. The average frequency of chiasmata per cell between different species show no significant difference. A critical analysis of the bivalents at diakinesis revealed that chiasmata are absent or rarely present in the deeply staining region. If there such a localization persists, it would mean, recombination of the genes located in the heavily stained region would be highly restricted, to the non-staining regions. Based on the idiogram of the pachytene chromosomes, the possible role of chromosomes in speciation and the relationship between species are critically discussed.