The altitudinal distribution, morphology, phenology and allozyme polymorphism at 12 loci were studied in diploid and tetraploid populations of Lotus growing at over 1800 m in the French Alps to clarify relationships between these cytotypes. In general, diploids occurred at higher elevation than tetraploids, although some sites at intermediate elevation contained both cytotypes, diploids predominating in the upper part and tetraploids in the lower part of the contact area. Evidence for an autopolyploid origin of the tetraploids was provided by tetrasomic inheritance at two enzyme loci, although no tetravalents were observed at meiosis. Diploid and tetraploid plants shared morphological traits distinct from those of other Lotus species and showed differences in size, which may be attributable to chromosome doubling. The diploid cytotype, L. alpinus, may thus be the ancestor of the Alpine tetraploids. Both cytotypes showed nearly identical suites of alleles at all loci and very similar genetic parameters, except for heterozygosity, which was higher in the tetraploid plants. However, the occurrence of few alleles specific to each ploidy level indicated limited gene flow between cytotypes, probably as a result of spatial segregation and variation in flowering time. Of the individuals in a tetraploid population, 25% showed morphological traits similar to those observed in L. corniculatus, suggesting genetic introgression between the two tetraploid species.

RFLP of chloroplast DNA was studied in two diploid (2x) and five tetraploid (4x) populations of Lotus alpinus (D.C.) Schleicher in the French Alps. This was compared with four non-Alpine populations and two cultivars of Lotus corniculatus L. (4x) introduced to the Alpine valleys. Based on the 13 site and 11 length mutations detected, seven distinct haplotypes could be identified, which clustered in two groups, corresponding to the two Lotus species. With only one exception, the diploid and the tetraploid L. alpinus plants had the same haplotype. This is to be expected, because morphological variation and tetrasomic inheritance suggest that the tetraploids were derived from the diploids by autopolyploidy. In L. corniculatus, two very similar haplotypes were found in the natural very distant populations. In contrast, the chloroplast DNA (cpDNA) haplotypes of the two cultivars differed considerably from each other and were more related to those observed in L. alpinus, suggesting that these cultivars may be derived maternally from L. alpinus individuals. In one tetraploid L. alpinus population, a single cpDNA haplotype was identified. As this haplotype was related to those observed in the L. corniculatus group, local genetic introgression may have occurred between 4x L. alpinus plants and cultivated L. corniculatus.