Cytotaxonomic Investigations in the Panicum maximum Complex III

Cytological features, chromosome diminution and evolution of the karyotype

Abstract

Meiosis in 20 accessions of Panicum maximum, classifiable into 5 distinct types on the basis of morphological and epidermal features (Jauhar and Joshi 1966a, 1967), is described. Though they all had predominantly 2n=32 chromosomes, chromosome mosaicism was also encountered. Pooled data on chromosome association at diakinesis and metaphase I are presented. Out of a total of 210 cells analysed, the number of quadrivalents varied from 0-6 (mean per cell=2.04), of trivalents from 0-2 (mean=0.15), of bivalents from 4-18 (mean=11.5) and that of univalents from 0-6 (mean=0.55). On the basis of data on meiosis and on morphological features gathered earlier (Jauhar and Joshi 1966a) the nature of ploidy in this species is discussed. The most appropriate term to describe polyploidy in this species seems to be “intraspecific autopolyploid”.
The number 2n=32 seems to have been derived through a multiplication of basic number of x=9 chromosomes followed by the elimination of 4 chromosomes during the course of evolution and differentiation of the species, rather than from a multiplication of the basic number of x=8 as suggested by earlier workers.
Various mechanisms which may have been responsible for bringing about phylogenetic diminution of chromosome number and repatterning of karyotype are considered. The possible role of unequal translocations in bringing about chromosome reduction is discussed. It has been considered that at least 2 pairs of chromosomes could have undergone unequal reciprocal translocations followed by an elimination of 2 pairs of mutant, small chromosomes. The evolutionary implication of the occurrence of accessory chromosomes in this species is also discussed.