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Reproductive success is divided into two phases: preemergent (the number of viable seeds that enter the ambient environment) and postemergent (the percentage of progeny that survive to reproduce). We studied preemergent reproductive success (PERS) in flowering plants by measuring the fruit/flower (Fr/Fl) ratio and the seed/ovule (S/O) ratio in a number of species of outcrossing and inbreeding plants, where PERS=the product of (Fr/Fl) and (S/O). In order to determine the influence of the ambient environment (including resource availability) we studied pairs of outcrossing and inbreeding species occurring in the same habitat. Among outcrossing species PERS averaged about 22%, whereas in inbreeding species the average was approximately 90%. The progeny/zygote (P/Z) ratio was studied in hand-pollinated populations in Epilobium angustifolium (a strongly outcrossing species) from populations in Oregon and Utah, by direct observation of embryogenesis at twoday intervals throughout the course of seed development. The P/Z ratio in both populations averaged near 30%, and the developing embryos showed a surprising array of abnormalities that resulted in embryo death. During early development >95% of the ovules had normally developing globular embryos, but beginning with differentiation (cotyledon formation) about 70% of the original globular embryos aborted during the course of embryogenesis and seed development. The clustering of developmental lethals during peroids of major differentiation events parallels the animal model of development. We found little evidence that PERS was limited by the ambient environment (including resource availability), pollination, or factors associated with the inbreeding habit. Instead, PERS was found to be inextricably linked to outcrossing plants, whose breeding systems promote genetic variability. The high incidence of developmental lethals in E. angustifolium and the resulting low P/Z ratio (ca. 30%) is attributed to genetic load (any lethal mutation or allelic combination) possibly working in combination with developmental selection (interovarian competition among genetically diverse embryos). Examples of maternally controlled, fixed patterns of ovule abortion with respect to position or number are discussed. However, we found no need to employ "female choice" as a hypothesis to explain our results for the extensive, seemingly random patterns of embryo abortion in E. angustifolium and other outcrossing species. A more parsimonious, mechanistic explanation based on genetic load-developmental selection is sufficient to account for the differential survivorship of embryos. Likewise, the traditional concept of a positive growth regulator feedback system based on the number of surviving ovules in an ovary can account for subsequent fruit survivorship.

Greenhouse pollinations among 23 field-collected plants and their progeny revealed a high and variable degree of selfsterility in Epilobium obcordatum. Field-collected plants averaged 5% seed-set when selfed and 43% seed-set in crosses; greenhouse progeny II % and 49%, respectively. In two of three pollen chase tests, previously or simultaneously applied self-pollen reduced seed-set to that resulting from pure self-pollination. Based on the relative survivorship of self-and outcross embryos, the mean number oflethal equivalents was determined to be 11.0 (range 2.9 to 17.6) in the field-collected plants, and 7.1 (range 0.62 to 21.8) in the greenhouse progeny. Pollinations among 91 individuals showed a significant negative correlation between seed-set and inbreeding coefficients of the resulting zygotes. Intrapopulation crosses yielded significantly fewer seeds than interpopulation crosses. Early embryonic mutational load, in combination with some level of biparental inbreeding, appears to be responsible for the level of self-sterility in natural populations of this species. A pollen dosage test for an interpopulation cross resulted in a maximum seed set of 47% at 16 tetrads. Additional unmeasured characteristics govern the probability of ovules becoming mature seeds.

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Scic^nce Foundation for a series of j^rants to Peter II. leaven which made these investigations possible. The generous assistance afforded Ihiven by Mr. W. S. Bertand, head of the Electron Microscope Section, and Mrs. Lesley A. Donaldson, of the Ph>sics and Enpineerhi^ Laboratory, D.S.I.H., Lower Ilutt, New Zealand, in carrying out preliminary SEN! studies of the seeds of some 75 species of Epilohium in ]^J(i9-70 is also gratefully acknowledged.