F₁ and F₂ Backcrosses in the Hybrids Between Two Unnamed Genetically DistinctSpecies of Tropical Sea Urchins, Echinometra sp. A and Echinometra sp. C

Experiments on backcrosses using the gametes of the reciprocal F₁ hybids and theconspecific 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 fromthe female hybrid of Ea x Ec and Ec x Ea yielded a higher percent of fertilization withEa x Ea sperm than with Ec x Ec sperms. Conversely backcrosses by hybrid males ofeither crosses yielded a higher percentage of fertilization with Ec x Ec ova than with Eax Ea ova, indicating that Ea ova appeared to be more descriminating than Ec ova due tothe sequence differences in their gamete recognition alleles. On the other hand, the higherfertilization rates between the same types of hybrids versus the different types of hybridsindicated the presence of a complex sorting of gamete compatibility genes. However, thehigher fertilization rates as well as higher survival rates of larvae, juveniles and adultsof the F₂ hybrids eliminates the possibility that hybrid inviability/sterility is a postzygoticmechanism of reproductive isolation. In adults, EaEa x EaEc and EaEa x EcEa juvenileswere consistantly larger than those of equivalent ages of the other crosses in terms ofrelative test dimesions 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 performanceshowever, indicated that the F₂ hybrids were viable in the lab-reared conditions. Thisexperiment represents the first successful production of F₂hybrids (Progeny ofbackcrosses) in the laboratory. Although, coloration patterns of F₂ hybrids tended to bematernal, other characters such as test sizes and spine lengths, growth performances andphenotypic characteristics (such as tubefoot and gonad spicules, pedicellaria valve lengthand gamete sizes) tended to be intermediate but closer to their maternal F_1. Despite thesefindings, hybrids with these morphological characters were not found in the field,indicating possible prezygotic isolating mechanism(s) that either singlely orincombination with others creating barriers to hybridize in the field. The results of thisstudy further indicate that considerable morphological and genetic integrity is maitainedbetween Echinometra sp. A (Ea) and Echinometra sp. C (Ec) which strongly supportstheir recognition as distinct evolutionary species despite the fact that they produce F₁and F₂ hybrids under laboratory conditions.