HIGH-FREQUENCY OF GONADAL NEOPLASIA IN A HARD CLAM (MERCENARIA SPP) HYBRID ZONE

The etiology of bivalve gonadal neoplasia has eluded invertebrate path ologists for over 20 yr. In a coastal Florida (USA) lagoon, where two species of hard clams (Mercenaria mercenaria and M. campechiensis) co- occur and hybridize, they exhibit a persistent, unusually high frequen cy of gonadal neoplasia. Hybridity, rather than environmental or other biological factors, appears to determine susceptibility, implicating a genetic mechanism in the etiology of the disease. However, the incre ase in frequency of occurrence of the disease in hybrids is not accomp anied by an increase in severity beyond that experienced by pure-speci es genotypes, suggesting that only some components of the genetic mech anism are affected by hybridization. Differences between sexes in the overall and size-specific frequency of occurrence and in severity of t he disease suggest that the genetic mechanism is associated with sex. The excessive susceptibility of hybrid genotypes to gonadal neoplasia results in reduced hybrid fitness and constitutes an unambiguous examp le of a cellular disease that acts as a barrier to gene flow between s pecies. Moreover, because these species are of commercial importance, fishery practices that promote hybridization are common and, over time , may reduce the fitness of natural populations.