GENETIC POPULATION STRUCTURE AND LEVELS OF GENE FLOW IN THE STREAM DWELLING WATERSTRIDER, AQUARIUS (=GERRIS) REMIGIS...

Gene flow, in combination with selection and drift, determines levels of differentiation among local populations. In this study we estimate gene flow in a stream dwelling, flightless waterstrider, Aquarius remigis. Twenty-eight Aquarius remigis populations from Quebec, Ontario, New Brunswick, Iowa, North Carolina, and California were genetically characterized at 15 loci using starch gel electrophoresis. Sampling over two years was designed for a hierarchical analysis of population structure incorporating variation among sites within streams, streams within watersheds, watersheds within regions, and regions within North America. Hierarchical F statistics indicated that only sites within streams maintained enough gene flow to prevent differentiation through drift (Nm = 27.5). Above the level of sites within streams gene flow is highly restricted (Nm ≤ 0.5) and no correlation is found between genetic and geographic distances. This agrees well with direct estimates of gene flow based on mark and recapture data, yielding an N_e of approximately 170 individuals. Previous assignment of subspecific status to Californian A. remigis is not supported by genetic distances between those populations and other populations in North America. Previous suggestion of specific status for south-eastern A. remigis is supported by genetic distances between North Carolina populations and other populations in North America, and a high proportion of region specific alleles in the North Carolina populations. However, because of the high degree of morphological and genetic variability throughout the range of this species, the assignment of specific or subspecific status to parts of the range may be premature.