Genetic diversity of rhizobia nodulating Trifolium ambiguum in North America

Abstract

Kura clover (Trifolium ambiguum M.B.) is a persistent rhizomatous forage legume, whose use in the U.S.A. is limited by establishment difficulties in part attributable to nodulation problems. In this study, soil was collected from established stands of Kura clover growing in 9 diverse North American environments. Rhizobia were plant-trapped using Kura clover cv. Endura as host, then rhizobia from nodules fingerprinted using BOX-PCR. The diversity of isolates from North America was then contrasted to that of rhizobia from a single Caucasian environment (Russia), the center of origin for this species. Populations were characterized using clustering methods, and genetic diversity estimated using the Shannon-Weaver diversity index. The genetic diversity of the North American populations was extremely limited, all isolates being closely related to two of the strains found in a locally available commercial inoculant. In contrast, Russian isolates formed a distinct cluster with significant internal genetic diversity. Genetic diversity indices for the North American and Russian populations were 3.5 and 10.76, respectively. The implication of this and other studies is that Kura clover is highly specific in Rhizobium requirement. If the performance of this legume in the U.S.A. is to be improved, either by modifying current establishment practices or plant breeding, it is essential that these studies be paralleled by more collections and evaluation of rhizobia from its center of origin, given the extremely limited diversity of rhizobia found in North America.Key words: genetic diversity, rhizobia, Kura clover, BOX-PCR.85 Résumé : Le trèfle Kura (Trifolium ambiguum M.B.) est une légumineuse de fourrage persistente à rhizome, dont l'usage aux États-Unis est restreint par des difficultés d'implantation attribuables en partie à des problèmes de nodulation. Dans l'étude présente, de la terre fut prélevée de peuplements établis de trèfle Kura poussant dans neuf environnement nord-américains distincts. Les rhizobiums furent capturés par le cultivar Endura du trèfle Kura en tant qu'hôte, puis les empreintes génétiques des rhizobiums des nodules furent prises par BOX-PCR. La diversité des isolats nordaméricains fut alors contrastée avec celle de rhizobiums provenant d'un environnement caucasien unique (Russie), le centre d'origine de cette espèce. Les populations furent caractérisées par des méthodes de regroupement et l'indice de diversité Shannon-Weaver fut employé pour estimer la diversité génétique. La diversité génétique des populations nordaméricaines était extrêmement limitée, tous les isolats étant fortement apparentées à deux des souches retrouvées dans des incoculums commerciaux locaux. En revanche, les isolats russes ont formé un ensemble distinct pourvu d'une diversité interne appréciable. Les indices de diversité génétique des populations nord-américaines et russes étaient de 3,5 et 10,76, respectivement. La portée de cette constatation et d'autres études est que le trèfle Kura serait hautement spé-cifique dan...

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