GENETIC DIVERSITY AND POPULATION STRUCTURE IN PITCH PINE (PINUS RIGIDA MILL.)

Electrophoretic studies of protein polymorphisms in plants have focused upon herbaceous species, primarily inbreeding annuals, in efforts to characterize the levels and patterns of genic variation within and between populations (Clegg and Allard, 1972; Gottlieb, 1973, 1975; Levin, 1975, 1978; Levy and Levin, 1975; Schaal, 1975; Roose and Gottlieb, 1976; Brown et al., 1978; and others). These studies have indicated that predominantly outbreeding species maintain higher levels of intrapopulation variation than predominantly inbreeding species, while inbreeders exhibit a greater degree of population differentiation than outbreeders (Brown, 1979; Hamrick et al., 1979). This relationship is by no means perfect as Levin (1978) points out, because of differences in ecological requirements, breeding systems, dispersal mechanisms, evolutionary history, and other factors which affect the genetic system (Grant, 1958, 1971; Brown, 1979; Hamrick et al., 1979). Whether longlived perennials such as forest trees conform to the general pattern is still an open question. Allozyme studies of forest tree species have suggested that levels of genic variation are exceptionally high in natural populations (Tigerstedt, 1973; Rudin et al., 1974; Lundkvist and Rudin, 1977; Yang et al., 1977; Hamrick, 1979; Hamrick et al., 1979; Lundkvist, 1979), that certain populations appear to be moderately inbred (Rudin et al., 1974; Mejnartowicz and Bergmann, 1975; Phillips and Brown, 1977), and that populations have become differentiated over relatively short distances (Sakai and Park, 1971; Mitton et al., 1977). However, many inferences have been drawn from only one or a few loci, or only from loci known to be highly polymorphic. Valid estimates of mating system parameters may be obtained by examining only a few loci, but for estimates of heterozygosity, genic diversity, and the extent of differentiation, a large number of loci is preferred (Lewontin, 1974; Nei, 1975). As part of a continuing study of the genetics and ecology of pitch pine (Pinus rigida Mill.), we have surveyed 21 enzymatic loci in 11 populations across the species range. Pitch pine occurs from coastal Maine and southern Quebec to northern Georgia and from the Atlantic Coast to central Ohio, but almost always on relatively infertile sites. In spite of a history of overexploitation, it appears to have retained appreciable variation (Ledig and Fryer, 1974). Pitch pine demonstrates clinal patterns of variation in cone serotiny (Ledig and Fryer, 1972), wood properties (Ledig et al., 1975), and seedling growth (Ledig et al., 1976), as well as seed and needle characters (Ledig, unpubl.). It is uncertain whether these clinal patterns are the result of gene flow among pockets of differential fitness or reflect a continuous gradient in selection pressures (Endler, 1977; Givnish, 1981). The objectives of the present study were to determine the levels of genic diversity characteristic of pitch pine, and to examine the organization of genic variability within the species and the patterning of genic differentiation between populations. Among other comparisons, we contrasted marginal vs. central populations and the