Origin and maintenance of androgenesis : male asexual reproduction in the clam genus Corbicula

Asexual species which never incorporate novel genetic material from other lineages will go extinct faster than sexually reproducing species, because adaptive variability may be lower and a larger number of harmful mutations may accumulate. One form of asexuality, androgenesis, results in offspring that are clones of the father. Both androgenetic and sexual species are found in the clam genus Corbicula. I used genetic data to explore why there are multiple species of androgenetic Corbicula, and whether genetic exchange occurs between species. I found that in North American locations where two invasive, androgenetic species co-occur, restriction digest mapping of rDNA failed to detect recent nuclear exchange. However, in these same locations, mitochondrial markers were shared between species. In places where only one species was found, mitochondrial markers were unique to that species. This suggests androgenetic clams are able to parasitize eggs of closely related species. Whereas maternal mitochondria are retained in the fertilized egg, maternal nuclear chromosomes are expelled, and the mother incubates male clones of another species. To look at possible gene exchange over the long term, I compared phylogenetic tree topologies of one mitochondrial and two nuclear markers from multiple sexual and androgenetic species. Since several androgenetic species share similar or identical alleles, androgenesis seems to have evolved relatively recently in Corbicula. However, since different androgenetic species also have divergent alleles not shared between species, genetic capture of maternal nuclear DNA from other species may rarely occur. This rare capture of genetic material from other species may permit the long-term persistence of androgenesis in Corbicula.