Chromosome evolution and affinity of certain genera of Orchidaceae.

Abstract

The present paper deals with a detailed chromosome study of 35 species including cytotypes of the tribe Kerosphaereae of Orchidaceae. There are 17 new reports from Eastern Himalayas and the Khasia hills. On the basis of the chromosome behaviour and karyotype analysis it has been concluded that in the subtribe Coelogyneae, the genus Pholidota is allied to Coelogyne and those two represent homogeneous assemblages. In the subtribe Dendrobeae, the genus Dendrobium is quite homogeneous in which polyploidy and aneuploidy along with chromosome changes have played an important role in evolution. Intraspecific chromosome races are common and a clear case has been demonstrated in D. chrysanthum where such cytotypes are correlated with distinct environmental conditions. Dressler (1961) described the northern epiphytes of Mexico and also recorded the severe frost-damage in them excepting Epidendrum conopseum R. Br. (Dressler 1964). In the subtribe Sarcanthineae, the affinity of Aerides with Sarcachilus has been revealed and presence of supernumerary constriction indicates the extent to which structural changes of chromosomes are operating in evolution. The cytological affinity of Stauropsis with Vanda has been shown. The position of the species Hexisea reflexa, newly recorded here from India, has been assigned under Sarcanthineae. In Vanda, polyploidy and aneuploidy occurring at intra and interspecific level as well as cryptic structural alterations have been considered to be the factors which have contributed to the wide geographical distribution and adaptation of the genus. In general, genera cannot be categorised on the basis of their karyotypes. In broad features of the karyotype, the family as a whole represents a normal grouping, as karyotype alterations, though playing a role in evolution, are equally effective at an intra and interspecific level. An illustrated work was published on Orchid pollination and evolution (Dodson 1961).