Gamete incompatibility is a prezygotic barrier: evidence from experimental hybridization between two recently diverged species of Pacific sea urchins

Four recently diverged species of Indo Pacific sea urchins, belonging to the genus Echinometra, occur most abundantly and sympatrically along the coast of Okinawa Island, southern Japan. Among them, the two most distinct species- Echinometra sp. A (Ea) and Echinometra mathaei (Blainville, 1825) (Em)-were examined for potential hybridization through a series of cross-fertilization and hybrid raising experiments. Compared to conspecific crosses, fertilization rates in both heterospecific crosses were very low, even at a very high concentration of sperm. At limited sperm concentration, where conspecific crosses achieved near 100% fertilization, heterospecific crosses had <3% fertilization success. This strong block to fertilization in either heterospecific cross indicates the presence of a protein-binding system for gamete recognition. A significantly lower survival rate in larvae, juveniles, and adults does indicate a postzygotic isolating mechanism that is strong enough to limit hybrid survival, but is not complete, and did in fact cause some amount of developmental incompatibility or hybrid inviability. In adults, Em (ova) x Ea (sperm) was largest in live weight, followed by Ea (ova) x Em (sperm), Ea x Ea, and Em x Em, in that order. Other growth performances, such as test sizes, gonad weight, and gonad index of hybrids and their conspecifics, also followed the same trend as live weight. Phenotypic color patterns of the hybrids were maternally inherited, while other characters (spine length, morphology of tubefoot and gonad spicules, pedicellaria valve length, and gamete sizes) were intermediate. Ova and sperm of the F, hybrids were fertile in backcrosses, minimizing the possibility that hybrid infertility is a postzygotic isolating mechanism. Nevertheless, intensive surveys failed to find individuals with such hybrid characteristics, suggesting a very low presence or complete absence of natural hybrids in the field. This strongly suggests that pre- and postzygotic isolating mechanism(s) are isolating these species. Among these mechanisms, strong gametic incompatibility and probably microhabitat separation, differences in spawning times, gamete competition, or other factors appear to be important for maintaining genetic integrity of these two species.