A Eurasia-wide polyploid species complex involving 6 x Trifolium ambiguum, 2 x T. occidentale and 4 x T. repens produces interspecific hybrids with significance for clover breeding

Abstract

Background

Trifolium ambiguum occurs as a 2 x, 4 x, 6 x polyploid series in W Asia. The 6 x form is the most agronomically desirable, having strong rhizomatous spread and drought tolerance. These traits would be potentially very valuable if they could be transferred to white clover ( T. repens) which is the most important agronomic clover species. However, to-date, no fertile interspecific hybrids with 6 x T. ambiguum are available. Previously, 2 x T. occidentale from W Europe has produced synthetic fertile hybrids with both 2 x and 4 x T. ambiguum and these were inter-fertile with white clover. Here we ask whether 2 x T. occidentale can form fertile hybrids with 6 x T. ambiguum and act as a genetic bridge to white clover and bring these species together as part of a common gene pool.

Results

Ten verified F ₁ (6 x T. ambiguum x 2 x T. occidentale) hybrids were produced by embryo rescue and seven were studied further. All four investigated for chromosome number were 2 n = 4 x = 32 and FISH confirmed the expected 21  T. ambiguum and 8  T. occidentale chromosomes. Hybrid fertility was extremely low but 2 n female gametes functioned with white clover pollen to produce seeds. Derived plants were confirmed using FISH and were successfully backcrossed to white clover to produce partially fertile breeding populations.

Conclusions

Although T. occidentale and 6 x T. ambiguum are widely separated by geography and ecological adaptation they have maintained enough genomic affinity to produce partially fertile hybrids. Inter-fertility of the hybrids with allotetraploid T. repens showed that T. occidentale can provide a genetic bridge between 6 x T. ambiguum and white clover to produce plants with new phenotypes combining the traits of all three species. Use of this information should enable potentially valuable stress tolerance traits from 6 x T. ambiguum to be used in white clover breeding for the first time.