Many a slip: dissecting the causes of reproductive isolation in two species ofTegenariaspiders (Agelenidae)

Abstract

Key to our understanding of the mechanisms underlying the process of speciation is the determination of the nature of the barriers to gene flow between related taxa. Species that show zero gene flow in sympatry or parapatry are of little use in this respect. In the present study, we used two closely-related species of large house spider, Tegenaria saeva and Tegenaria gigantea, which hybridize to a limited extent along a natural contact zone in southern Britain. The species are apparently indistinguishable with respect to habitat utilization and phenology. Laboratory crosses using individuals from both allopatric and parapatric populations suggest that, although male and female courtship, as well as web and cuticular-borne pheromones, are conserved between the species, mechanical difficulties are experienced during interspecific copulation. Copulation bouts are, on average, significantly shorter during interspecific matings because of these difficulties, and are probably not sufficiently long for effective sperm transfer to take place. In the two cases of successful interspecific crossing, and in subsequent F1 and backcross generations, there are few indications of differential fertility, fecundity or viability, suggesting little post-zygotic incompatibility. The high success rate of crosses between F1 hybrids and both parental species underlines the principally mechanical barrier to gene flow between these taxa. Once this is breached, there appears to be little impediment to continuing introgression, which could, in some geographical areas at least, ultimately lead to the fusion of the two species. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113, 355–367.