Outcrossing rates and inferred levels of inbreeding depression in gynodioecious Cucurbita foetidissima (Cucurbitaceae)

Sex-specific outcrossing rates and the inbreeding coefficient of adults in two populations of gynodioecious Cucurbita foetidissima were estimated using progeny array data from four allozyme loci to compare the frequencies of self-fertilization and the estimated levels of inbreeding depression to predictions from sex ratio theory. The frequencies of self-fertilization by hermaphrodites in both populations were similar and averaged 73 per cent. The outcrossing rate for females in one population was not different from unity whereas the estimated rate of outcrossing by females in the second population (t = 0.593, SE 0.178) indicated the occurrence of biparental inbreeding. Despite considerable self-fertilization by hermaphrodites, inbreeding coefficients of adult plants in both populations were not different from zero and thus inferred values of inbreeding depression were not different from one. Electrophoretically inferred levels of inbreeding depression are somewhat in excess of the value of 0.71 (SE 0.07) obtained in a previous field experiment which tested first-year survival of selfed and outcrossed seeds in this long-lived perennial. The high frequency of self-fertilization by hermaphrodites combined with severe inbreeding depression provides a strong selective force which, along with increased seed-set by females, is sufficient to maintain observed frequencies of females in natural populations of C. foetidissima .