INBREEDING DEPRESSION AND PROXIMITY-DEPENDENT CROSSING SUCCESS IN PHLOX DRUMMONDII.

Viability depression is a typical consequence of inbreeding in cross-fertilizing species. In some populations of Homo sapiens (Schull and Neel, 1965; FreireMaia and Azevedo, 1971; Schull et al., 1970) and Drosophila (Dobzhansky et al., 1963; Stone et al., 1963; MalogolowkinCohen et al., 1964; Mettler et al., 1966), the level of viability depression is linearly dependent on the inbreeding coefficient of an individual. If extrapolated to the maximum level (F = 1) from sib and cousin values, mortality due to homozygosity for detrimental genes would exceed 50% in most Homo and Drosophila populations. In most populations of Homo and Drosophila, the mean number of lethal equivalents per zygote is between 1 and 4. Populations of Tribolium also fall within this range (Levene et al., 1965). Viability depression following inbreeding in angiosperms is similar to that in the aforementioned animals. The mean numbers of lethal equivalents per zygote are as follows: Secale cereale, 2.7 (Landes, 1939); Medicago sativa, 1.2-4.5 (Cooper and Brink, 1940; Sayers and Murphy, 1966); Fagopyrum esculentum, 1.3-5.2 (Komaki, 1982); and Stylidium spathulatum, 3.4 (James, 1979). Among conifer species, the mean number of lethal equivalents per zygote varies from 1 to 10, values exceeding 8 in Pinus and Pseudotsuga (Sorensen, 1969; Franklin, 1972; Koski, 1973). Ferns are similar to conifers in their range of lethal equivalents (Klekowski, 1970; Ganders, 1972; Lloyd, 1974; Saus and Lloyd, 1976). The magnitude of viability depression from inbreeding in plants is dependent upon genetic and environmental variables. The frequency of lethal and detrimental genes in populations is a prime factor. It exceeds .15 in several species (Apirion and Zohary, 1961; Crumpacker, 1967; Kiang and Libby, 1972; Ohnishi, 1979; Komaki, 1982). The level of viability depression also is governed by the tolerance of species to higher levels of homozygosity (Mayo, 1980). Species whose genetic systems are adapted to relatively high levels of homozygosity are likely to show the least viability depression with inbreeding. Finally, the viability differential between outcross and inbred progeny is influenced by environmental quality. Under conditions of drought, disease and other stresses, the relative viability of inbreds is much poorer than under favorable conditions (Allard and Hansche, 1964; Pawsey, 1964; Koski, 1973; Libby et al., 1981). Viability depression in plants is most pronounced during seed development. For example, in Pseudotsuga menziesii, about 95% of the lethality following inbreeding occurs prior to germination (Orr-Ewing, 1957). Seed abortion usually is 2 to 10 times greater following selffertilization than cross-fertilization in conifers (Sorensen, 1969; Koski, 1971, 1973; Franklin, 1972; Birshir and Pepper, 1977) and angiosperms (Brink and Cooper, 1947; Linck, 1961; Rowlands, 1960; Sayers and Murphy, 1966; James, 1979). If the products of self-fertilization or crosses among sibs have reduced viability relative to outcrosses, we may expect progeny from neighboring plants in natural populations to have lower viability than those from distant plants when plant relatedness declines with distance. This relationship between proximity and relatedness would be a consequence of restricted pollen and seed dispersal, and is