Intraspecific phylogeography of Lasmigona subviridis (Bivalvia : Unionidae): conservation implications of range discontinuity

A nucleotide sequence analysis of the first internal transcribed spacer reg ion (ITS-1) between the 5.8S and 18S ribosomal DNA genes (640 bp) and cytoc hrome c oxidase subunit I (COI) of mitochondrial DNA (mtDNA) (576 bp) was c onducted for the freshwater bivalve Lasmigona subviridis and three congener s to determine the utility of these regions in identifying phylogeographic and phylogenetic structure. Sequence analysis of the ITS-1 region indicated a zone of discontinuity in the genetic population structure between a grou p of L. subviridis populations inhabiting the Susquehanna and Potomac River s and more southern populations. Moreover, haplotype patterns resulting fro m variation in the COI region suggested an absence of gene exchange between tributaries within two different river drainages, as well as between adjac ent rivers systems. The authors recommend that the northern and southern po pulations, which are reproductively isolated and constitute evolutionarily significant lineages, be managed as separate conservation units. Results fr om the COI region suggest that, in some cases, unionid relocations should b e avoided between tributaries of the same drainage because these population s may have been reproductively isolated for thousands of generations. There fore, unionid bivalves distributed among discontinuous habitats (e.g. Atlan tic slope drainages) potentially should be considered evolutionarily distin ct. The DNA sequence divergences observed in the nuclear and mtDNA regions among the Lasmigona species were congruent, although the level of divergenc e in the COI region was up to three times greater. The genus Lasmigona, as represented by the four species surveyed in this study, may not be monophyl etic.