Evolutionary trends in the Indian Iseilema

Abstract

OF the most highly evolved grasses the most useful to man are chiefly confined to the tropical and the warm temperate regions. For example the Andropogoneae are extensively cultivated for cereals (Sorghum spp.) sugar cane (Saccharum spp.) essential oils (Cymbopogon spp.) and a variety of other purposes. They have attained high adaptive peaks of evolution and species differentiation in the Indo-Malaysian region¹ where there is a high or moderately high rainfall. Three species of the Indian grass Iseilema are cultivated for fodder each one of which is a ‘species complex’ or a cluster of chromosome races not randomly united but formed by the genetic system of the taxonomic species. All three complexes show a gradually ascending series of haploid chromosome numbers from 3 to 18 (Table 1) and even the polyploid numbers form an ‘aneuploid’ series within a complex. Selection, environment, polyploidy and B chromosomes have had varying roles in the evolution of these complexes and their most notable characteristic is that they form a self-consistent system of evolutionary mechanisms. Here I demonstrate how Iseilema is indicative of the ancient dibasic polyploidy in its tribe of which it is the highest evolved member in a habitat where the tribe shows maximum diversity and closest adaptation.