a1 UMR Diversité et Adaptation des Plantes Cultivées, Montpellier SupAgro – INRA – IRD – UMII, 2, Place Pierre Viala, 34060 Montpellier Cedex 1, France
a2 Institut des Sciences de l'Evolution, Université Montpellier 2, place Eugène Bataillon, Montpellier, France
a3 UMR ASP 1095, Université Clermont Ferrand, INRA, F-63100 Clermont Ferrand, France
Mating systems and recombination are thought to have a deep impact on the organization and evolution of genomes. Because of the decline in effective population size and the interference between linked loci, the efficacy of selection is expected to be reduced in regions with low recombination rates and in the whole genome of self-fertilizing species. At the molecular level, relaxed selection is expected to result in changes in the rate of protein evolution and the pattern of codon bias. It is increasingly recognized that recombination also affects non-selective processes such as the biased gene conversion towards GC alleles (bGC). Like selection, this kind of meiotic drive in favour of GC over AT alleles is expected to be reduced in weakly recombining regions and genomes. Here, we investigated the effect of mating system and recombination on molecular evolution in four Triticeae species: two outcrossers (Secale cereale and Aegilops speltoides) and two selfers (Triticum urartu and Triticum monococcum). We found that GC content, possibly driven by bGC, is affected by mating system and recombination as theoretically predicted. Selection efficacy, however, is only weakly affected by mating system and recombination. We investigated the possible reasons for this discrepancy. A surprising one is that, in outcrossing lineages, selection efficacy could be reduced because of high substitution rates in favour of GC alleles. Outcrossers, but not selfers, would thus suffer from a ‘GC-induced’ genetic load. This result sheds new light on the evolution of mating systems.
(Received August 26 2007)
(Revised October 20 2007)