Estimates of inbreeding depression obtained from the literature were used to evaluate the association between inbreeding depression and the degree of self-fertilization in natural plant populations. Theoretical models predict that the magnitude of inbreeding depression will decrease with inbreeding as deleterious recessive alleles are expressed and purged through selection. If selection acts differentially among life history stages and deleterious effects are uncorrelated among stages, then the timing of inbreeding depression may also evolve with inbreeding. Estimates of cumulative inbreeding depression and stage-specific inbreeding depression (four stages: seed production of parent, germination, juvenile survival, and growth/reproduction) were compiled for 79 populations (using means of replicates, N = 62) comprising 54 species from 23 families of vascular plants. Where available, data on the mating system also were collected and used as a measure of inbreeding history. A significant negative correlation was found between cumulative inbreeding depression and the primary selfing rate for the combined sample of angiosperms (N = 35) and gymnosperms (N = 9); the correlation was significant for angiosperms but not gymnosperms examined separately. The average inbreeding depression in predominantly selfing species (8 = 0.23) was significantly less (43%) than that in predominantly outcrossing species (8 = 0.53). These results support the theoretical prediction that selfing reduces the magnitude of inbreeding depression. Most self-fertilizing species expressed the majority of their inbreeding de- pression late in the life cycle, at the stage of growth/reproduction (14 of 18 species), whereas outcrossing species expressed much of their inbreeding depression either early, at seed production (17 of 40 species), or late (19 species). For species with four life stages examined, selfing and outcrossing species differed in the magnitude of inbreeding depression at the stage of seed production (selfing 8 = 0.05, N = 11; outcrossing 8 = 0.32, N = 31), germination (selfing 8 = 0.02, outcrossing 8 = 0.12), and survival to reproduction (selfing 8 = 0.04, outcrossing 8 = 0.15), but not at growth and reproduction (selfing 8 = 0.21, outcrossing 8 = 0.27); inbreeding depression in selfers relative to outcrossers increased from early to late life stages. These results support the hypothesis that most early acting inbreeding depression is due to recessive lethals and can be purged through inbreeding, whereas much of the late-acting inbreeding depression is due to weakly deleterious mutations and is very difficult to purge, even under extreme inbreeding.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1558-5646.1996.tb04472.x

Evolution of the magnitude and timing of inbreeding depression in plants.

The ecological significance of spacing among plants in contributing to the maintenance of species richness, particularly in tropical forests, has received considerable attention that has largely focussed on distance- and density-dependent seed and seedling mortality. More recently it has become apparent that plant spacing is also relevant to pollination, which often constrains seed production. While seed and seedling survival is reduced at high conspecific densities, pollination success, by contrast, is positively correlated to local conspecific density. Distance-dependent mechanisms acting on pollination and seed production have now been described for a variety of plants, with relatively isolated plants or fragmented populations generally suffering reduced fecundity due to pollen limitation. Yet there is considerable variability in the vulnerability of plant species to pollination failure, which may be a function of breeding system, life history, the pollination vector, the degree of specialisation among plants and their pollinators, and other indirect effects of habitat change acting on plants or pollinators. As reduced tree densities and population fragmentation are common outcomes of anthropogenically altered landscapes, understanding how pollination processes are affected in such degraded landscapes can inform effective conservation and management of remaining natural areas.

/pdf/pollen-and-seed-dispersal-among-dispersed-plants-5egudw9wrw.pdf

Pollen and seed dispersal among dispersed plants.

The Different Forms of Flowers on Plants of the Same Species: POLYGAMOUS, DIŒCTOUS, AND GYNO-DIŒCIOUS PLANTS

Models for the maintenance of females in gynodioecious species are reviewed. Some new results are given concerning digenic models and models of cytoplasmic inheritance with restorer genes, when the cytoplasmic and nuclear factors may have fitness effects in addition to their effects on male fertility. It is suggested that many gynodioecious species may be interpreted as having a cytoplasmic-genic inheritance of male-sterility, and that this interpretation would resolve a number of difficulties. However, much of the evidence is circumstantial, and complete details of the genetics are not available for any case of natural polymorphism for male-sterility. This makes it impossible to test the population genetics theories in a more precise way. It is clear, however, that both the digenic and the cytoplasmic-genic model require large fitness differences if a polymorphism for females is to be maintained. Some evidence exists in maize for the type of fitness effect required by the cytoplasmic-genic model.

/pdf/a-further-study-of-the-problem-of-the-maintenance-of-females-1mq58kv4g5.pdf

A further study of the problem of the maintenance of females in Gynodioecious species

Hermaphroditic individuals can produce both selfed and outcrossed progeny, termed mixed mating. General theory predicts that mixed-mating populations should evolve quickly toward high rates of selfing, driven by rapid purging of genetic load and loss of inbreeding depression (ID), but the substantial number of mixed-mating species observed in nature calls this prediction into question. Lower average ID reported for selfing than for outcrossing populations is consistent with purging and suggests that mixed-mating taxa in evolutionary transition will have intermediate ID. We compared the magnitude of ID from published estimates for highly selfing (r > 0.8), mixed-mating (0.2 ≤r≥ 0.8), and highly outcrossing (r < 0.2) plant populations across 58 species. We found that mixed-mating and outcrossing taxa have equally high average lifetime ID (δ= 0.58 and 0.54, respectively) and similar ID at each of four life-cycle stages. These results are not consistent with evolution toward selfing in most mixed-mating taxa. We suggest that prevention of purging by selective interference could explain stable mixed mating in many natural populations. We identify critical gaps in the empirical data on ID and outline key approaches to filling them.

/pdf/analysis-of-inbreeding-depression-in-mixed-mating-plants-232ow6liy4.pdf

Analysis of inbreeding depression in mixed-mating plants provides evidence for selective interference and stable mixed mating

To determine the rate of self-fertilisation in male-fertile plants of thyme, four populations, homozygous for dominant chemical marker alleles, were chosen. Male-sterile plants and male-fertile plants with the corresponding recessive marker alleles were transplanted into these populations and the seedlings obtained by growing the seeds gathered in 1974 and 1975 from these test plants were analysed. An appreciable rate of self-fertilisation has been found in the four populations (0·36, 0·49, 0·35 and 0·10) but the individual test plants give highly variable results (from 0 to 0·79). The figures were rather consistent from one year to the next. An important part of the variation in self-fertilisation may be ascribed to differences in the densities of local male-fertile plants in the first few metres around the test plants.

/pdf/self-fertilisation-in-male-fertile-plants-of-a-gynodioecious-347654n253.pdf

B Dommée

Papers

Self-fertilisation in male-fertile plants of a gynodioecious species: Thymus vulgaris L.