Reproductive Isolation Between Isozyme Groups of Glycine tomentella (Leguminosae), and Spontaneous Doubling in Their Hybrids

Abstract

Previous studies of infraspecific isozyme variation in Glycine tornentella have recognised five tetraploid (TI, T2, ... T5) and seven diploid (Dl, D2, ... D7) subspecific groups. This report analyses the meiosis in pollen mother cells of 31 new tetraploid hybrids, 10 diploid hybrids and three diploid by tetraploid combinations. A close relationship, over the range of diversity, was observed between isozyme similarity of the parents of hybrids and chromosome pairing at meiosis. In general, hybrids between parents belonging to the same isozyme group were fertile whereas hybrids between groups were sterile. In the tetraploids, the fertility of interregional hybrids when the accessions belonged to the same group confirmed the widespread distribution of groups T1 and T4. Alternatively, the sterility of intraregional hybrids between groups showed that groups can coexist and remain as separate gene pools. Each isozyme group apparently had a single chromosome number (TI and T5 had 2n = 78; T2, T3 and T4 had 2n = 80). Some otherwise sterile hybrid plants produced rare progeny with elevated chromosome num- ber, probably from the functioning of unreduced male and female gametes. Considerable genomic divergence was apparent from the hybrids between diploid groups, comparable with that found between recognised Glycine species. Again isozyme groups had a characteristic chromosome number (Dl and D2 had 2n = 38, the remainder had 2n = 40). Thus reproductive isolation through reduced formation of bivalents at meiosis operated both between and within cytotypes (2n = 38, 40, 78, 80) of G. tornentella. The pattern of hybrid sterility was as predicted from the analysis of isozyme similarity, confirming the grouping as a meaningful classification within this diverse species.