Cytotaxonomic studies on Indian Commelinaceae: A review*

Abstract

The review summarizes the progress achieved in the revision of the Indian species of Commelinaceae and analyses how far the synthetic approach coupled with orthodox taxonomy has helped to clarify the many ambiguities prevalent in the family. Cytotaxonomic studies have definitely contributed to a proper understanding of such genera as Commelina, Cyanotis (sensu lato) and Pollia, and also the various ‘species complexes’ in the above genera, though the evidence is not as forthright with reference to Aneilema and Murdannia. In the genus Commelina, polyploidy has played a rather prominent role in speciation, and intra-specific aneuploidy is relatively of minor significance. The main evolutionary trend follows the pattern with predominantly x= 15 for a majority of both the Indian and African species and even though a few African species do exhibit x= 14, this basic number is infrequent and at any rate not encountered in any of the Indian species analysed so far. In sharp contrast to this, in the genus Cyanotis, both inter- and intra-specific polyploidy is very much restricted, but judging from the recent trends both in Indian and African populations, intra-specific aneuploidy has to be reckoned as a factor for further speciation within the genus. All the Indian species of Cyanotis analysed so far share a common basic number x= 12, but nevertheless there is an increasing evidence of secondary basic numbers such as, x= 8, 11 and 13 in at least a few random populations involving three species of Cyanotis. Moreover, both taxonomic and cytological data favour the splitting up of Cyanotis (sensu lato) into three distinct genera, Amischophacelus Holla Rao et Kammathy, Belosynapsis Hassk. and Cyanotis D. Don (sensu stricto). There is ample data to justify the separation of Murdannia Royle from Aneilema (sensu lato). The evolutionary trend in Murdannia seems to follow two different patterns, one with a basic number of x= 10 and the other with x= 6, both the patterns probably arising from a common extinct ancestor with x= 5. Preliminary observations on the Indian species of Aneilema R.Br, (sensu stricto) suggest a basic number of x= 14 (7?) in sharp contrast to x= 13, widely prevalent in a majority of the African species of Aneilema. In Murdannia and Aneilema, polyploidy and also aneuploidy are quite common and these two genera offer a fertile field for further study. There does not seem to be any justification for the separation of Aclisia from Pollia. In Streptolirion, intraspecific aneuploidy coupled with structural differentiation of chromosomes Seems to be responsible for further evolution of new varieties within the species. Cytological data favour the retention of Floscopa in tribe Tradescantieae rather than in Commelineae where Bruckner has placed it. The various species complexes that had baffled the taxonomists especially in Commelina, Cyanotis and Murdannia have been critically analysed and their affinities clarified, on the basis of the present observations and study, 80 species under ten genera have so far been established, representing the Indian sub-continent.