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Theory and empirical study produce clear links between mating system evolution and inbreeding depression. The connections between mating systems and outbreeding depression, whereby fitness is reduced in crosses of less related individuals, however, are less well defined. Here we investigate inbreeding and outbreeding depression in self-fertile androdioecious nematodes, focusing on Caenorhabditis sp. 11. We quantify nucleotide polymorphism for nine nuclear loci for strains throughout its tropical range, and find some evidence of genetic differentiation despite the lowest sequence diversity observed in this genus. Controlled crosses between strains from geographically separated regions show strong outbreeding depression, with reproductive output of F1s reduced by 36% on average. Outbreeding depression is therefore common in self-fertilizing Caenorhabditis species, each of which evolved androdioecious selfing hermaphroditism independently, but appears strongest in C. sp. 11. Moreover, the poor mating efficiency of androdioecious males extends to C. sp. 11. We propose that self-fertilization is a key driver of outbreeding depression, but that it need not evolve as a direct result of local adaptation per se. Our verbal model of this process highlights the need for formal theory, and C. sp. 11 provides a convenient system for testing the genetic mechanisms that cause outbreeding depression, negative epistasis, and incipient speciation. [ABSTRACT FROM AUTHOR]

Speciation research has largely assumed that the genetic causes of reproductive isolation are the work of fixed, divergent alleles that interact to cause genetic problems in hybrids: Bateson–Dobzhansky–Muller incompatibilities. However, many recent studies demonstrate substantial heritable polymorphism within species for hybrid incompatibility, herein called variable reproductive isolation (VRI). In this review, I outline the causes and importance of this general phenomenon. I also identify the new challenges of quantifying the relative contributions to reproductive isolation of fixed alleles versus polymorphisms, and the change in these contributions over the course of speciation. Explicit integration of VRI into speciation theory will help to quantify the relative roles of genetic drift and selection in speciation, but this synthesis requires substantial new contributions from both theory and empirical studies. [Copyright &y& Elsevier]