Hybridization and Introgression between Two Diploid Wild Relatives of White Clover, Trifolium nigrescens Viv. and T. occidentale Coombe

Abstract

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.

Experimental evidence for the ancestry of allotetraploid Trifolium repens and creation of synthetic forms with value for plant breeding

“…The identity of the ancestors has remained elusive and, despite many attempts [e.g. [3,5-7] there has been no successful re-synthesis. A phylogenetic analysis of Trifolium based on the nuclear internal transcribed spacer region of 18 S–26S rDNA (ITS) and chloroplast trnL intron DNA (cpDNA) sequences [8] suggested that the closest extant diploid ancestors were T. pallescens (2n=2x=16) and T. occidentale (2n=2x=16) .…”

Development of Genomic Resources in the Species of Trifolium L. and Its Application in Forage Legume Breeding

“…Supporting this hypothesis is also the fact that, unlike T. repens and T. nigrescens, all the T. occidentale and T. pallescens accessions tested to date are acyanogenic [67,68]. Furthermore, cross-hybridisation of diploid T. nigrescens with artificially doubled [63,69,70] and diploid T. occidentale [71] can be achieved without the aid of special techniques, while hybridisation of T. pallescens and T. occidentale could only be achieved by embryo rescue (Williams, personal communication).…”

Genetic diversity in white clover and its progenitors as revealed by DNA fingerprinting

“…It is likely that T. nigrescens and T. occidentale are close relatives on one side of the ancestry of white clover. In the light of the recent evidence as reported by Ansari et al (2004), Ellison et al (2006) and Williams et al (2008) it seems that the ancestry of the allopolyploid white clover remains elusive.…”

“…Genomic Southern hybridization and/or fluorescence in situ hybridization (FISH) performed on 17 Trifolium species/subspecies by Ansari et al (2004) revealed that it is a conserved lineage-specific repeat confined to several species within the section Lotoidea originating in the Mediterranean region. Williams et al (2008) reported evidence suggesting that T. nigrescens is a close relative of T. occidentale but not a direct ancestor of white clover. It is likely that T. nigrescens and T. occidentale are close relatives on one side of the ancestry of white clover.…”