Experimental hybridization between two recently diverged species of tropical sea urchins, Echinometra mathaei and Echinometra oblonga

Summary

Two species of tropical sea urchins, Echinometra mathaei and Echinometra oblonga, occur abundantly on Okinawan intertidal reefs; E. mathaei is found mainly on inshore reef flats while E. oblonga burrows in the offshore reef margins. The two species spawn during the same season, and hybrid embryos can be formed in the laboratory. We confirmed earlier experiments showing that fertilization rates are high between eggs of E. oblonga and sperm of E. mathaei, but significantly lower than those in homogametic crosses, whereas few eggs of E. mathaei are fertilized by sperm of E. oblonga, even at very high sperm concentrations. If they spawn together in the field, most or all ova of E. oblonga would be fertilized by conspecific sperm before encountering sperm of E. mathaei on the reef flat, so hybridization is partially minimized by habitat separation. However, many viable sperm of E. oblonga would still be available to fertilize ova of E. mathaei. Species-specific differences in sperm chemotaxis or gamete compatibility may provide prezygotic barriers that also would prevent hybridization, especially between ova of E. mathaei and sperm of E. oblonga. We found that homogametic crosses reach peak fertilization levels much sooner than heterogametic crosses, even between ova of E. oblonga and sperm of E. mathaei, indicating the presence of prezygotic barriers to hybridization in the gametes. Nevertheless, laboratory crosses between E. mathaei and E. oblonga in both directions produced viable hybrids that developed through larval and juvenile stages to sexually mature adults. Phenotypic color pattern of hybrids tended to be maternal while other characteristics were intermediate (test size, weight, spine length, growth, tubefoot and gonad spicule morphology, pedicellaria valve length, gamete size). Adult F₁ hybrids from both combinations were completely fertile and exhibited high fertilization rates in backcrosses. On the other hand, intensive field surveys failed to find hybrids, showing the rarity or absence of natural hybridization. These results mirror those found with hybridization studies on other species of Echinometra on Okinawa. They further support the conclusion that prezygotic isolating mechanisms such as habitat segregation as well as species-specific sperm chemotaxis and/or gamete compatibility have a role in maintaining the genetic integrity of these species.

Key words:

• Echinometra
• hybridization
• reproductive isolation
• sea urchins
• speciation