Postmating/Prezygotic Isolation, Heterosis, and Outbreeding Depression in Crosses Within and Between Populations ofDiodia teres(Rubiaceae) Walt.

Abstract

Gene flow can limit local adaptation and speciation. The outcome of breeding between populations determines the effectiveness of migration and the extent of gene flow. In crosses between populations, it is possible that some traits may indicate isolation and others may not. I measured the rate of successful hand pollination and the fruit production of the resulting hybrid progeny of crosses within and between six populations of the selfing annual Diodia teres. I chose two populations from each of three habitat types to examine evidence for reproductive isolation and heterosis between populations from different habitats. There was postmating/prezygotic isolation in crosses between populations from different habitats and heterosis for fruit production in the F2 progeny of these crosses, as well as crosses between populations from the same habitat type. Specific pairs of populations showed postmating/prezygotic isolation, but there was heterosis for fruit production in some hybrids. Populations that develop postmating/prezygotic isolation may not diverge further if heterosis in the resulting offspring leads to increased gene flow. The heterosis suggests fixed genetic load within some populations, which may be common in selfing species and may limit local adaptation and speciation.