Premating, Not Postmating, Barriers Drive Genetic Dynamics in Experimental Hybrid Populations of the Endangered Sonoran Topminnow

Abstract

The timing and pattern of reproductive barrier formation in allopatric populations has received much less attention than the accumulation of reproductive barriers in sympatry. The theory of allopatric speciation suggests that reproductive barriers evolve simply as by-products of overall genetic divergence. However, observations of enhanced premating barriers in allopatric populations suggest that sexual selection driven by intraspecific competition for mates may enhance species-specific signals and accelerate the speciation process. In a previous series of laboratory trials, we examined the strength of premating and postmating barriers in an allopatric species pair of the endangered Sonoran topminnow, Poeciliopsis occidentalis and P. sonoriensis. Behavioral observations provided evidence of asymmetrical assortative mating, while reduced brood sizes and male-biased F 1 sex ratios suggest postmating incompatibilities. Here we examine the combined effects of premating and postmating barriers on the genetic makeup of mixed populations, using cytonuclear genotype frequencies of first-and second-generation offspring. Observed genotype frequencies strongly reflect the directional assortative mating observed in behavioral trials, illustrating how isolating barriers that act earlier in the reproductive cycle will have a greater effect on total reproductive isolation and may be more important to speciation than subsequent postmating reproductive barriers.

Difference in diel mating time contributes to assortative mating between host plant-associated populations of Chilo suppressalis

“…This indicates an asymmetry of reproductive isolation between the two populations. Similar asymmetry in patterns of premating isolation has been reported in many insects1523425152 and in fish53. In the R population, mating activity occurred throughout the entire scotophase but was more frequent in the first half of the scotophase, whereas in the W population almost all mating activity was confined to the second half of the scotophase.…”

Tests of reproductive isolation among species in the Fundulus notatus (Cyprinodontiformes: Fundulidae) species complex

“…Reproductive barriers are typically classified based on the stage in the life cycle that they occur (prezygotic or postzygotic) or the factors that act to reduce hybrid fitness (endogenous or exogenous selection) (Jiggins et al 1997;Coyne and Orr 1998;Jiggins and Mallet 2000;Bronson et al 2003;Hurt et al 2005). Some have suggested strong mate selection (Mendelson 2003) or body size differences (McKinnon et al 2004) will facilitate development of prezygotic barriers.…”

The Genetic Architecture of Reproductive Isolation in Louisiana Irises: Flowering Phenology

“…Recent, detailed experiments in which the relative contributions of numerous prezygotic and postzygotic barriers were quantified consistently revealed prezygotic barriers to be the strongest impediments to interspecific gene flow (Ramsey et al 2003;Husband and Sabara 2004;Hurt et al 2005;Whiteman and Semlitsch 2005;Martin and Willis 2007). If divergence between organisms often occurs simultaneously with gene flow (see Arnold 2006 for a review), the strength of such prezygotic barriers may be fundamentally important for the establishment of new evolutionary lineages.…”