Extraordinary AFLP fingerprint similarity despite strong assortative mating between reef fish color morphospecies

Recent theoretical models and empirical studies of fruit flies, birds, and fish indicate that assortative mating may initiate speciation when physical barriers to gene flow are absent, and before postzygotic barriers evolve. These are important results for marine animals like coral reef fish, where ocean currents can carry planktonic larvae over broad ranges, interconnecting populations and slowing genetic divergence. The Caribbean hamlets (genus Hypoplectrus) are a flock of reef fish morphospecies with highly distinct color pattern that mate like with like, but show little mitochondrial or microsatellite DNA differentiation. Here, we broadly screen genomic diversity using amplified fragment length polymorphisms (AFLP) and survey mating pair formation between two morphospecies in the Florida Keys, the butter hamlet (H. unicolor) and the blue hamlet (H. gemma). No AFLP was species-diagnostic (fixed), and neighbor-joining analyses revealed no clustering of individuals consistent with morphospecies boundaries. Assignment tests, however, placed most individuals within their morphospecies of origin. Field surveys showed that > 98% of mating pairs, including those of rare morphospecies, were of like color pattern. Spawning by a single mixed pair adds to earlier observations suggesting that infrequent hybridization may be a genetically homogenizing force in Hypoplectrus. This study provides a clear example of strong assortative mating in a system with limited genetic differentiation.