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

Warren M Williams,Isabelle M Verry,Helal A Ansari,S Wajid Hussain,Nicholas W Ellison,Ihsan Ullah

doi:10.1186/s12870-019-2030-5

Abstract

BackgroundTrifolium ambiguum occurs as a 2x, 4x, 6x polyploid series in W Asia. The 6x 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 6x T. ambiguum are available. Previously, 2x T. occidentale from W Europe has produced synthetic fertile hybrids with both 2x and 4x T. ambiguum and these were inter-fertile with white clover. Here we ask whether 2x T. occidentale can form fertile hybrids with 6x T. ambiguum and act as a genetic bridge to white clover and bring these species together as part of a common gene pool.ResultsTen verified F1 (6x T. ambiguum x 2x T. occidentale) hybrids were produced by embryo rescue and seven were studied further. All four investigated for chromosome number were 2n = 4x = 32 and FISH confirmed the expected 21 T. ambiguum and 8 T. occidentale chromosomes. Hybrid fertility was extremely low but 2n 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.ConclusionsAlthough T. occidentale and 6x 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 6x 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 6x T. ambiguum to be used in white clover breeding for the first time.

Highlights

IntroductionThe 6x form is the most agronomically desirable, having strong rhizomatous spread and drought tolerance
Trifolium ambiguum occurs as a 2x, 4x, 6x polyploid series in W Asia
The results reveal the commercial potential for T. occidentale to be used as a genetic bridge for including 6x T. ambiguum in the gene-pool to facilitate the breeding of improved cultivars of white clover

Summary

Introduction

The 6x 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. We ask whether 2x T. occidentale can form fertile hybrids with 6x T. ambiguum and act as a genetic bridge to white clover and bring these species together as part of a common gene pool. The polyploid series approximates an altitudinal gradient, with diploid populations at high altitudes, hexaploids at the lowest altitudes and tetraploids between [3]. This variation across the series suggests that the genomes of the ploidal forms carry adaptive genetic differences. T. repens is a tetraploid with a wide natural distribution, spanning Europe and W Asia, and is the most important of about 10–15 clover species used in world-wide agriculture [9]

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Conclusion