The origin of the Lotus corniculatus (Fabaceae) complex: A synthesis of diverse evidence

Lotus corniculatus, birdsfoot trefoil, is a Eurasian perennial legume, popular in temperate climates for pasture or hay and silage production. Its ancestry is controversial. While innumerable studies of L. corniculatus and allied species have been published, comprehensive morphological, geographical, and genetic studies are still needed. Nevertheless, there is sufficient information for a provisional analysis of the origin and relationships of this important economical species. Although sometimes defined to have diploid populations, L. corniculatus essentially appears to be tetraploid. Biochemical and genetic evidence indicates that this species is an allotetraploid. Although about a dozen diploid species have been proposed as ancestral to L. corniculatus, the evidence points strongly to four of these species. Lotus uliginosus uniquely shares a rhizomatous habit, an acyanogenic factor, and a tannin characteristic with L. corniculatus, clearly showing an especially close relationship, and reflecting the possibility that it is one of two direct parental species. Principal candidates for a second parent include Lotus alpinus, Lotus japonicus, and Lotus tenuis. A phenetic analysis presented here accords well with the hypothesis favoured in this paper that L. corniculatus arose as a hybrid of L. tenuis and L. uliginosus. Cytogenetic evidence indicates that L. japonicus is especially closely related to L. corniculatus. Flower colour in the hybrids between L. uliginosus and L. tenuis suggest maternal inheritance and that L. tenuis could have been the female parent. An evolutionary sequence is suggested in which L. uliginosus hybridized with L. tenuis followed by introgression from L. alpinus and L. japonicus. Alternatively, a prototype of L. alpinus, L. japonicus, and L. tenuis could have been the female parent of L. corniculatus, based on evidence from maternal inheritance of flower colour intensity and interactions of Rhizobium strains.