Molecular genetics of peripheral populations of Nova Scotian Unionidae (Mollusca: Bivalvia)

Abstract

Peripheral populations of eight species of freshwater bivalves (Unionidae) extending their geographic ranges into Nova Scotia, Canada, were examined electrophoretically to determine both the extent of genetic variability within such populations, and whether the hypothesized pathway of colonization across the Isthmus of Chignecto is reflected in patterns of genetic resemblance among these populations. The Nova Scotian species examined could be separated into two groups based on levels of observed heterozygosity and levels of variability in allele frequencies.The first group is characterized by low levels of heterozygosity and polymorphism compared with north-eastern American populations, and in the case of one species, Ellzptio complanata, considerable variability in allele frequencies among populations occurring in similar habitats in different drainagcs. Populations of E. complanata from Nova Scotia can be differentiated from conspecific populations on the southern Atlantic Slope by possession of fast alleles at two loci. Multivariate analyses define subgroups within populations of E. complanata consistent with hypothesis that the species invaded Nova Scotia by way of the Isthmus of Chignecto, and then split into two groups, one of which colonized Cape Breton to the north and the other of which colonized southern areas of the Province.The second group of Nova Scotian species is characterized by little reduction in heterozygosity and polymorphism compared with values observed among north-eastern American conspecifics or congeners, little variability in allele frequencies from population to population, and little evidence to suggest that these species were dependent on the land bridge to invade the Province.The type of dispersal is hypothesized to be responsible, in part, for these differences: larvae of species in the first group rely on a parasitic attachment to fish with territorial habits limited to fresh water, and are thus likely to invade new drainages separated by salt water by chance, in small numbers, and in stepping-stone fashion. Species in the serond group parasitize anadromous or saltwater tolerant hosts, are likely to be introduced into new habitats in greater numbers and/or rcreive greater amounts of gene flow subsequent to colonization, and seem less dependent on landbridges to colonize new habitats.

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

“…In the present study, a zone of discontinuity was identified between the Rappahannock and Potomac Rivers that suggests that the populations in the northern and southern portions of the range appear to be heading down different evolutionary paths, which may lead to allopatric speciation. As L. subviridis inhabits headwater streams, it is unlikely that the host would migrate from the upper reaches of one major river to another as restrictions of the host by saline or brackish water will limit dispersal between drainages ( Ortmann 1913; Johnson 1970; Kat & Davis 1984). Given the current levels of the Atlantic Ocean it is unlikely that gene exchange among northern and southern populations of L. subviridis could be re‐established until the next major ice age.…”

Testing hypotheses on the dispersal and evolutionary history of freshwater mussels (Mollusca: Bivalvia: Unionidae)

“…They were compared to the matrix of genetic distances between populations. In the same way hypotheses about the dispersal and evolutionary history of two North American freshwater mussels ( Davis et al ., 1981 ; Kat & Davis, 1984) and a European freshwater fish ( Riffel & Schreiber, 1995) were tested. In all analyses the matrix of shortest (air) distances between any two sampling sites served as the random spatial hypothesis ( Arter, 1990).…”

Fish species composition and host fish density in streams of the thick‐shelled river mussel (Unio crassus) – implications for conservation

“…It has long been known that the dispersal and population status of unionoid mussels is highly dependent on the availability of appropriate host fishes during the reproductive period (Bauer et al , ; Watters, ; Bogan, ; Vaughn and Taylor, ; Österling et al ., ). For instance, Kat and Davis () as well as Watters () found that extirpation of mussels from several rivers was linked to the disappearance of their hosts. In addition, mussel species have been documented to reinvade streams after reintroduction of their hosts (Smith, ).…”