Division of Genetics, Indian Agricultural Research Institute, Delhi-12
Meiosis in 20 aneuploid accessions (2n = 32) and one diploid collection (2n = 18) of Panicum maximum was studied; nature of chromosome pairing at diaki-nesis and metaphase I is considered in this paper. On the basis of these and some more information gathered earlier the evolutionary origin of the aneuploid forms of this grass is discussed. The number 2n = 32 appears to have been derived through the multiplication of basic number of x=9 chromosomes followed by the loss of 4 chromosomes during the course of evolution, rather than from the multiplication of x=8 as suggested by the earlier workers. Various mechanisms which may have brought about this phylogenetic diminution of chromosome number and repatterning of the karyotype are described and the possible role of unequal translocations in chromosome reduction discussed. A plausible evolutionary course by which the 32-chromosome forms might have originated is indicated. The evolutionary implication of the occurrence of accessory chromosomes in this species is also discussed.
