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Abstract:
Theory and empirical data showed that two processes can boost selection against deleterious mutations, thus facilitating the
purging of the mutation load: inbreeding, by exposing recessive deleterious alleles to selection in homozygous form, and sexual
selection, by enhancing the relative reproductive success of males with small mutation loads. These processes tend to be mutually
exclusive because sexual selection is reduced under mating systems that promote inbreeding, such as self-fertilization in
hermaphrodites. We estimated the relative efficiency of inbreeding and sexual selection at purging the genetic load, using 50
generations of experimental evolution, in a hermaphroditic snail (Physa acuta). To this end, we generated lines that were exposed
to various intensities of inbreeding, sexual selection (on the male function), and nonsexual selection (on the female function). We
measured how these regimes affected the mutation load, quantified through the survival of outcrossed and selfed juveniles. We
found that juvenile survival strongly decreased in outbred lines with reduced male selection, but not when female selection was
relaxed, showing that male-specific sexual selection does purge deleterious mutations. However, in lines exposed to inbreeding,
where sexual selection was also relaxed, survival did not decrease, and even increased for self-fertilized juveniles, showing that
purging through inbreeding can compensate for the absence of sexual selection. Our results point to the further question of
whether a mixed strategy combining the advantages of both mechanisms of genetic purging could be evolutionary stable.
