F1 and F2 backcrosses in the hybrids between two unnamed genetically distinct species of tropical sea urchins, Echinometra sp. A and Echinometra sp. C.

Experiments on back crosses using the gametes of the reciprocal F1 hybrids and the nonspecific controls of the two sympatric species of tropical sea urchins, Echinometra sp. A (Ea) and Echinometra sp. C (Ec) were conducted in the laboratory. The ova from the female hybrid of Ea x Ec and Ec x Ea yielded a higher percent of fertilization with Ea x Ea sperm than with Ec x Ec sperms. Conversely back crosses by hybrid males of either crosses yielded a higher percentage of fertilization with Ec x Ec ova than with Ea x Ea ova, indicating that Ea ova appeared to be more discriminating than Ec ova due to the sequence differences in their gamete recognition alleles. On the other hand, the higher fertilization rates between the same types of hybrids versus the different types of hybrids indicated the presence of a complex sorting of gamete compatibility genes. However, the higher fertilization rates as well as higher survival rates of larvae, juveniles and adults of the F2 hybrids eliminates the possibility that hybrid inviability/sterility is a postzygotic mechanism of reproductive isolation. In adults, EaEa x EaEc and EaEa x EcEa juveniles were consistently larger than those of equivalent ages of the other crosses in terms of relative test dimensions and growth performances; largest to smallest were EaEa x EcEa, EaEa x EaEc, EcEa x EaEa, EaEc x EaEa, EcEa x EcEc, EcEa x EcEc, EaEc x EcEc, EcEc x EcEa and EcEc x EaEc, respectively. These results of growth performances however, indicated that the F2 hybrids were viable in the lab-reared conditions. This experiment represents the first successful production of F2 hybrids (Progeny of back crosses) in the laboratory. Although, coloration patterns of F2 hybrids tended to be maternal, other characters such as test sizes and spine lengths, growth performances and phenotypic characteristics (such as tube foot and gonad spicules, pedicellaria valve length and gamete sizes) tended to be intermediate but closer to their maternal F1. Despite these findings, hybrids with these morphological characters were not found in the field, indicating possible prezygotic isolating mechanism(s) that either single or in combination with others creating barriers to hybridize in the field. The results of this study further indicate that considerable morphological and genetic integrity is maintained between Echinometra sp. A (Ea) and Echinometra sp. C (Ec) which strongly supports their recognition as distinct evolutionary species despite the fact that they produce F1 and F2 hybrids under laboratory conditions.