Interspecific Hybridization and Backcrosses between Two Sibling Species of Pacific Sea Urchins (Genus Echinometra) on Okinawan Intertidal Reefs

M. Aminur Rahman and Tsuyoshi Uehara (2004) Interspecific hybridization and backcrosses between two sibling species of Pacific sea urchins (genus Echinometra) on Okinawan Intertidal Reefs. Zoological Studies 43(1): 93-111. Four genetically divergent Pacific sea urchins belonging to the genus Echinometra occur sympatrically and abundantly along the coast of Okinawa Island. Among them, the 2 most distinct species, Echinometra mathaei (Em) and Echinometra sp. C (Ec), were examined for potential hybridization. A series of cross-fertilization experiments was conducted, and the resulting hybrids were reared. The percentage of successful heterogametic fertilization was high when egg of Ec and sperm of Em were involved, whereas it was significantly lower with eggs of Em and sperm of Ec, even at higher concentrations of sperm, suggesting that gamete incompatibility may be a prezygotic barrier that at least partially maintains reproductive isolation. Moreover, different exposure times of eggs and sperm showed that conspecific crosses reached asymptotic fertilization much sooner than did heterospecific crosses, indicating that affinity of the Em , s sperm to its egg during fertilization is higher than that of Ec. Hybrids from both crosses developed normally through the larval and juvenile stages to sexually mature adults. In adults, Em x Em crosses produced the largest values of live weight, followed by Ec (ova) x Em (sperm), Em (ova) x Ec (sperm), and Ec x Ec in that order. Other indicators of growth performance such as metamorphosis, survival, and recovery of hybrids and their conspecifics also followed the same trends as live weight. Phenotypic color patterns of the hybrids were closer to the maternal coloration, whereas other characters such as test size, spine length, morphology of tubefeet and gonad spicules, pedicellaria valve length, and gamete sizes were intermediate. Fertilization rates as well as larval and juvenile performances from backcrosses using the gametes of F1 progeny were high, eliminating the possibility that hybrid inviability is a postzygotic mechanism of reproductive isolation. On the other hand, intensive surveys by our team and other colleagues have failed to find individuals with such hybrid characteristics in the field, suggesting a lack of natural hybridization between the 2 species despite their physiological potential for hybridization. This strongly suggests that Em and Ec have been able to maintain their genetic distinctness due to the presence of some sort of prezygotic isolating mechanism(s) (e.g., habitat segregation, gametic incompatibility, gamete competition, and probably gamete aging) between them. http://www.sinica.edu.tw/zool/zoolstud/43.1/93.pdf