Hybrids of two closely related tropical sea urchins (Genus Echinometra): Evidence against postzygotic isolating mechanisms

A series of cross-fertilization experiments were conducted with two unnamed , sympatric species of sea urchins in the Echinometra mathaei species compl ex, Echinometra sp. A (Ea) and Echinometra sp. C (Ec). Heterogametic fertil ization success was high when eggs of Ec and sperm of Ea were involved, and low with eggs of Ea and sperm of Ec. Hybrids produced from crosses in eith er direction developed normally to sexually mature adults; Ea X Ea were lar gest in test size, followed by Ec (ova) x Ea (sperm), Ea(ova) X Ec (sperm), and Ec x Ec, respectively. Color patterns of the hybrids were closer to th e maternal coloration, whereas other characters such as relative test dimen sions and spine lengths, morphology of tubefoot and gonad spicules, and gam ete sizes were intermediate. Fertilization rates in F-1 backcrosses were hi gh, minimizing the possibility that hybrid infertility is a postzygotic mec hanism of reproductive isolation. On the other hand, intensive surveys fail ed to find individuals with hybrid characteristics in the field, suggesting that natural hybridization between the two species is rare. Prezygotic iso lating mechanisms, such as microhabitat separation and gamete incompatibili ty, at least between Ea eggs and Ec sperm, most likely maintain the genetic integrity of these two closely related species.