Pachytene studies in 2n=14 species of Medicago

Abstract

Four annual species of Medicago (M. constricta Dur., M. polymorpha L., M. praccox D.C. and M. rigidula [L.] All.) were analysed at pachytene and idiograms prepared. Considerable differences in chromosome lengths and arm ratios occurred between species. In three of the species the longest chromosome of the complement was much longer than the second chromosome in the karyotype. In this respect they were similar to 2n=14 M. murex Willd., and similar types of origins by rranslocations from 2n=16 ancestors are probable. In M. constricta chromosome 1 was not exceptionally long, and different kinds of translocations may have been involved in its origin.

The M. constricta idiogram was most like the M. rigidula idiogram, and the M. praecox idiogram was most like the M. polymorpha idiogram. These similarities support present taxonomic classification. However, the great similarity of the M. polymorpha idiogram to that previously deseribed for 2n=14 M. murex was unexpected, as these two species are usually classified in different subsections of the genus. A closer relationship is suggested. Evidence was found in M. polymorpha of variations in chromosome contraction and differences in karyotype between accessions.