Pachytene chromosomes of perennial Medicago species

Abstract

The pachytene chromosomes of two groups of perennial Medicago species, not usually considered to be related to M. sativa, were analysed. M. daghestanica Ruprecht., M. pironae De Visiani, and their sterile hybrid had similar idiograms, and the two species are probably closely related. The hybrid sterility has causes other than gross chromosomal differences. The idiograms differed from those of the M. sativa-falcata-glutinosa complex only in proportional lengths of chromosomes, hence the two groups may share a common ancestor. M. rhodopea Velen, and M. rhodopea X M. rupestris M. B. had almost identical idiograms, leading to the conclusion that M. rhodopea and M. rupestris are closely related. The idiogram of M. rhodopea was very similar to that of M. sativa, and in a triploid hybrid between diploid M. sativa and autotetraploid M. rhodopea, trivalent pairing was observed, confirming some homology between chromosomes of the two species. The M. sativa group and M. rhodopea group may have diverged only relatively recently. In pachytene cells of M. rhodopea and two of the M. rhodopea X M. rupestris hybrids, a large darkly staining body was observed in the nucleoplasm. This was shown by pyronine/methyl green staining to contain RNA, and in behaviour resembled an accessory nucleolus.