Hybridization in the sea:: gametic and developmental constraints on fertilization in sympatric species of Pseudechinus (Echinodermata: Echinoidea)

The constraints of gametic compatibility and developmental competence on hybrid production were examined in three sympatric species of the echinoid Pseudechinus in southern New Zealand. Differences were noted in embryonic temperature sensitivity between the species. Pseudechinus novaezealandiae embryos appeared to be better adapted to cooler temperatures than those of Pseudechinus huttoni, while Pseudechinus albocinctus embryos tolerated a wider range of temperatures than either of its sympatric congeners. Such differences were mirrored by the kinetics of gamete interaction between all possible reciprocal crosses of the three species. There appeared to be some physiological incompatibility between the eggs of P. novaezealandiae and the spermatozoa of the other two species. P. novaezealandiae spermatozoa were, however, capable of fertilizing both P huttoni and, to a lesser extent, R albocinctus ova. Hybrid echinoplutei resulting from the P huttoni (female) x P. novaezealandiae (male) cross did not, however, become competent to metamorphose. The probabilities of settlement of echinoplutei resulting from the other reciprocal crosses involving the gametes of P novaezealandiae were also low. It appears that a combination of physiological (gametic incompatibility) and genetic (developmental incompetence) factors contribute to maintaining the integrity of the P novaezealandiae lineage. These factors were not useful in understanding how P albocinctus and P huttoni remain distinct species. This species-pair was characterized by reciprocal gamete compatibility as well as developmental competence to settlement. Thus, gamete incompatibility, while important for some groups, cannot provide the basis for speciation in the New Zealand members of the genus Pseudechinus. (C) 2002 Elsevier Science B.V. All rights reserved.