POPULATION STRUCTURE IN BLACK FLIES: ALLOZYMIC AND MORPHOLOGICAL ESTIMATES FOR PROSIMULIUM MIXTUM AND P. FUSCUM (DIPTERA: SIMULIIDAE).

Two aspects of the biology of black flies make the family Simuliidae an evolutionarily interesting group. First, every morphospecies that has been examined cytogenetically has proven to be divisible into multiple chromosomally distinct sibling species (Rothfels, 1979). Second, widespread cytospecies of a sibling species complex are usually broadly sympatric, and narrowly endemic species are embedded entirely within the ranges of the common species. In virtually all cases, the most closely related species pairs within a complex are found sympatrically (Rothfels, 1979, pers. comm.). Species within complexes show very small amounts of genetic divergence at the morphological, karyotypic and allozymic levels (Rothfels, 1979; Crosskey, 1981; Snyder, 1981, 1982; Snyder and Linton, 1983). Taken together, the data on distributions and genetic differentiation suggest that speciation in black flies does not occur through adaptive, allopatric divergence. Understanding the aspects of the genetic organization and biology of black flies that facilitate the apparently ready speciation that occurs in the family will require many diverse studies. One aspect of the genetic organization of a species that is often cited to play a key role in determining speciation ability is population structure (Wright, 1940, 1955; Carson, 1975; Templeton, 1980). The population structure of a species (that is, the distribution of genetic variation within and among its populations) can be readily analyzed. This study is an investigation of the population structure of two closely related species of black flies, Prosimulium mixtum and P. fuscum, using allozymic and morphological variability. Prosimulium mixtum and P. fuscum are the most widely distributed and abundant members of the P. mixtum species complex. These species are sympatric over most of northeastern North America (Peterson, 1970). Five additional members of the species complex are narrowly endemic or widespread but uncommon within this range (Rothfels and Freeman, 1977; K. Rothfels, pers. comm.). Prosimulium mixtum and P. fuscum are the most divergent members of the complex, differing by three fixed autosomal inversions and possessing unique sex chromosome arrangements (Rothfels and Freeman, 1977). These species also possess unique complements of electromorphs at the 6-Pgd locus (Snyder and Linton, 1983) and differ by minor morphological features (Peterson, 1970; Snyder and Linton, 1983). Prosimulium mixtum and P. fuscum have undergone some ecological and life history divergence. While they often occur together in the same stream, they are usually asynchronous (Snyder, unpubl.; K. Rothfels, pers. comm.) and some streams of an area may contain only one species. Prosimulium fuscum females are autogenous (may complete the first ovarian cycle without a blood meal) while P. mixtum females are anautogenous (require a blood meal) (Davies, 1961). Rothfels (1981a) hypothesized that the anautogeny of P. mixtum leads to dispersal among populations and results in a more cohesive gene pool with less population differentiation than is encountered in P.fuscum. This is consistent with