Genetic divergence predicts reproductive isolation in damselflies

Reproductive isolation is the defining characteristic of a biological species, and a common but often untested prediction is a positive correlation between reproductive isolation and genetic divergence. Here we test for this correlation in odonates, an order characterized by strong sexual selection. First, we measure reproductive isolation and genetic divergence in eight damselfly genera (30 species pairs) and test for a positive correlation. Second, we estimate the genetic threshold preventing hybrid formation and empirically test this threshold using wild populations of species within the Ischnura genus. Our results indicate a positive and strong correlation between reproductive isolation and genetic distance using both mitochondrial and nuclear genes (COII: r=0.781 and 18S-28S: r=0.658). Hybridization thresholds range from -0.43 – 1.78% for COII, and -0.052 – 0.71% for 18S-28S, and both F1-hybrids and backcrosses were detected in wild populations of two pairs of Ischnura species with overlapping thresholds. Our study suggests threshold values are suitable to identify species prone to hybridization, and that positive isolation-divergence relationships are taxonomically widespread.