Hybrid Swarm

The European carpet shell Ruditapes decussatus is among the most important shellfish
species cultured in estuaries and bays of the northern Iberian Peninsula. Increased demand
and overfishing of native R. decussatus, as well as irregular yields due in part to pollution and
urban sprawl, have led to overexploitation of the species and to the import of the non-indigenous
R. philippinarum in the early 1980s. In recent years, supplementation aquaculture of R. decussatus
(supportive breeding) has been implemented as a management tool to recover the exhausted
native carpet shell populations in Asturias, northern Spain. In this study, taxonomic analysis and a
suite of genetic markers (ITS2, 5SrDNA, and mtCOI) were employed to assess the genetic composition
of R. decussatus in 2 estuaries of the region, Ría del Eo and Ría de Villaviciosa. Our results
revealed species misidentification and evidenced genetic introgression from the non-indigenous
R. philippinarum into wild R. decussatus samples (4.2%), as well as in the aquaculture seed used
for population supplementation (2.0%). Furthermore, we found significant genetic differentiation
(FST = 0.03750, p < 0.0001) between wild populations and aquaculture seed, while the adults of
both estuaries were genetically homogeneous (p > 0.05). Several actions are strongly recommended
to prevent inadvertent introduction of non-indigenous genomes and other genetic erosive
processes on native species in the region: (1) banning releases of the alien species R. philippinarum,
(2) developing diagnostic tests for detection of hybrids in captive breeding programs,
and (3) assessing the conservation status of wild R. decussatus populations.

A highly variable, fully fertile population of Rhamnus that was first established in Ann Arbor, Michigan, circa 1952 was studied to identify the species present and determine whether some individuals were of hybrid origin. Specimens of four taxa hypothesized as being present in the popula tion—R. cathartica, R. davurica var. davurica, R. davurica var. nipponica, and R. utilis—were used to establish reference clusters using a canonical discriminant analysis. Results of the discriminant analysis suggest that the population is a hybrid swarm and that only R. cathartica and R. utilis were involved in the formation of the hybrids at the study site. Additional analyses with the computer program HYWIN corroborated the results of the discrim inant analysis, enabled circumscription of putative hybrids, and for each hy brid provided a pair of specimens in the data set that best matches the mor phologies of its parents. Utilization of both discriminant and HYWIN anal yses in studies of hybrid ...