NATURAL HYBRIDIZATION IN THE SEA URCHIN GENUS PSEUDOBOLETIA BETWEEN SPECIES WITHOUT APPARENT BARRIERS TO GAMETE RECOGNITION
Article first published online: 10 APR 2012
© 2012 The Author(s). Evolution © 2012 The Society for the Study of Evolution.
Volume 66, Issue 6, pages 1695–1708, June 2012
How to Cite
Zigler, K. S., Byrne, M., Raff, E. C., Lessios, H. A. and Raff, R. A. (2012), NATURAL HYBRIDIZATION IN THE SEA URCHIN GENUS PSEUDOBOLETIA BETWEEN SPECIES WITHOUT APPARENT BARRIERS TO GAMETE RECOGNITION. Evolution, 66: 1695–1708. doi: 10.1111/j.1558-5646.2012.01609.x
- Issue published online: 1 JUN 2012
- Article first published online: 10 APR 2012
- Accepted manuscript online: 21 FEB 2012 02:51PM EST
- Received August 3, 2011 Accepted January 4, 2012
- gamete compatibility;
Marine species with high dispersal potential often have huge ranges and minimal population structure. Combined with the paucity of geographic barriers in the oceans, this pattern raises the question as to how speciation occurs in the sea. Over the past 20 years, evidence has accumulated that marine speciation is often linked to the evolution of gamete recognition proteins. Rapid evolution of gamete recognition proteins in gastropods, bivalves, and sea urchins is correlated with gamete incompatibility and contributes to the maintenance of species boundaries between sympatric congeners. Here, we present a counterexample to this general pattern. The sea urchins Pseudoboletia indiana and P. maculata have broad ranges that overlap in the Indian and Pacific oceans. Cytochrome oxidase I sequences indicated that these species are distinct, and their 7.3% divergence suggests that they diverged at least 2 mya. Despite this, we suspected hybridization between them based on the presence of morphologically intermediate individuals in sympatric populations at Sydney, Australia. We assessed the opportunity for hybridization between the two species and found that (1) individuals of the two species occur within a meter of each other in nature, (2) they have overlapping annual reproductive cycles, and (3) their gametes cross-fertilize readily in the laboratory and in the field. We genotyped individuals with intermediate morphology and confirmed that many were hybrids. Hybrids were fertile, and some female hybrids had egg sizes intermediate between the two parental species. Consistent with their high level of gamete compatibility, there is minimal divergence between P. indiana and P. maculata in the gamete recognition protein bindin, with a single fixed amino acid difference between the two species. Pseudoboletia thus provides a well-characterized exception to the idea that broadcast spawning marine species living in sympatry develop and maintain species boundaries through the divergence of gamete recognition proteins and the associated evolution of gamete incompatibility.