Hybridization and introgression between two diploid wild relatives of white clover, Trifolium nigrescens Viv. and T. occidentale Coombe

Trifolium nigrescens Viv. and T. occidentale Coombe are diploid (2n = 2x = 16) clover species with contrasting habits and geographic distributions. Trifolium nigrescens is a nonstoloniferous annual with widespread distribution in the Mediterranean countries of Europe and northern Africa, Turkey, the Middle East, and the Caucasus region. By contrast, T. occidentale is a stoloniferous perennial and is strictly coastal, occurring on the Gulf Stream coasts of Portugal, Spain, France, Ireland, and the United Kingdom. Here we report for the first time that both 2x T. nigrescens ssp. nigrescens and ssp. petrisavii can be hybridized with 2x T. occidentale. No special techniques are required, and advanced generation hybrid populations can be produced. Furthermore, backcrosses to T. nigrescens are easily obtained, and introgression of genes from T. occidentale occurs. The chromosomes of the two species show perfect pairing and regular disjunction at meiosis. The biological separation (reproductive isolation) of these species is incomplete. This incomplete isolation is paralleled by a partial breakdown of the genetic mechanism that would normally prevent endosperm development in these crosses. Observations on the life history of one hybrid and its progeny showed that two traits involved in perenniality were inherited in opposite ways. These species are close relatives of white clover and are potentially important sources of traits for plant breeding. This work raises the likelihood of simultaneous introgression of traits from both diploid species into white clover.