No evidence for RTA morphology affecting introgression in New Zealand fishing spiders

Abstract

Genital diversity has been a topic of debate among evolutionary biologists for decades. Recent evidence supports a role for genitalia in species isolation. Spiders are a compelling group to study the potential isolation mechanisms of genitalia. The retrolateral tibial apophysis (RTA), found in the RTA clade of spiders, is a highly diverse structure and essential to successful sperm transfer. One-way introgression occurs in the New Zealand fishing spiders Dolomedes aquaticus Goyen, 1888 and D. minor L. Koch, 1876 with genes flowing from D. aquaticus to D. minor, and is geographically limited to the extreme south of the species’ shared range (the ‘Introgression Zone’). Here, we used micro-computed tomography and 3D geometric morphometrics to analyse shape variation in the male RTAs to test the hypothesis that shape variation maintains the introgression. This would occur by shape differences isolating D. minor located outside the Introgression Zone from D. aquaticus, but within the Introgression Zone there being no substantial difference between D. aquaticus and D. minor RTA shape. We found highly variable RTA shape in both species, but there was no evidence of RTA shape or size limiting interbreeding of these species. Further work should include analysis of the female genital shape.