An Integrative Test of the Dead-End Hypothesis of Selfing Evolution in Triticeae (Poaceae)

Abstract

Self-fertilization is hypothesized to be an evolutionary dead end because reversion to outcrossing can rarely happen, and selfing lineages are thought to rapidly become extinct because of limited potential for adaptation and/or accumulation of deleterious mutations. We tested these two assumptions by combining morphological characters and molecular-evolution analyses in a tribe of hermaphroditic grasses (Triticeae). First, we determined the mating system of the 19 studied species. Then, we sequenced 27 protein-coding loci and compared base composition and substitution patterns between selfers and outcrossers. We found that the evolution of the mating system is best described by a model including outcrossing-to-selfing transitions only. At the molecular level, we showed that regions of low recombination exhibit signatures of relaxed selection. However, we did not detect any evidence of accumulation of nonsynonymous substitutions in selfers compared to outcrossers. Additionally, we tested for the potential deleterious effects of GC-biased gene conversion in outcrossing species. We found that recombination and not the mating system affected substitution patterns and base composition. We suggest that, in Triticeae, although recombination patterns have remained stable, selfing lineages are of recent origin and inbreeding may have persisted for insufficient time for differences between the two mating systems to evolve. K E Y W O R D S :Biased gene conversion, effective population size, mating system, protein evolution, recombination, selection efficiency, substitution rate.

Hitchhiking of Deleterious Alleles and the Cost of Adaptation in Partially Selfing Species

“…Such studies have been performed on Arabidopsis (Qiu et al 2011); Capsella (Qiu et al 2011;Slotte et al 2013;Brandvain et al 2013); and Collinsia . On the other hand, no evidence for relaxed selection was found when comparing divergence rates between species, such as in a separate study of Arabidopsis (Wright et al 2002), as well as in studies of Triticeae (Haudry et al 2008;Escobar et al 2010), with mixed evidence arising in selfing Caenorhabditis species (Cutter et al 2008). Together, this evidence suggests that selection has been relaxed recently, indicating that selfing is of recent origin and deleterious mutation accumulation is likely to be too weak to be the main cause of their higher extinction rates.…”

The demography and population genomics of evolutionary transitions to self-fertilization in plants

“…Our simulation results highlight that the effects of linked selection can happen rapidly following the shift to selfing, implying that the early stages of deleterious mutation accumulation may be apparent soon after the transition. Early tests of the hypothesis of a reduced efficacy of selection in selfers generally showed limited support for the prediction, but these studies were limited by the extent of the genome that was surveyed [49][50][51]. Furthermore, given the recent origin of most selfing populations, using substitution rates to test for relaxed selection may have limited power because of the difficulty in identifying the changes that have occurred since the shift to selfing.…”

The Capsella rubella genome and the genomic consequences of rapid mating system evolution

“…Our results seem to be the most consistent with this third hypothesis. Most previous work has found little or no evidence for relaxed selection on nonsynonymous sites in selfing lineages [14][15][16] ; our use of genome-wide polymorphism data to quantify current selection pressures in a recently derived selfing lineage provides a more powerful test of this hypothesis, suggesting that selfing lineages may in fact experience considerable accumulation of deleterious mutations even over short timescales.…”