The harmful effects of close inbreeding have been noticed for many centuries (34, 35, 165). With the rise of Mendelian genetics, it was realized that the main genetic consequence of inbreeding is homozygosis (165, Ch. 2). Two main theories were early proposed to account for inbreeding depression and its converse, heterosis (the increase in vigor observed in an F1 between two inbred lines). These are the overdominance and partial dominance hypotheses, discussed in more detail below. Research into this question has continued up to the present, and this is one of the topics that we discuss. Darwin (35, 36) was the first to point out that the evident adaptations of many plants for ensuring outcrossing could be understood in terms of the selective advantage of avoiding inbreeding depression. We review the evidence that the evolution of breeding systems of animals and plants has been significantly influenced by the occurrence of inbreeding depression. In order to do this, we consider the contemporary genetic theory of inbreeding depression and heterosis, and the experimental data concerning the strength of inbreeding depression. We emphasize data and theory relevant to natural, rather than domesticated, populations as we are chiefly concerned to evaluate the evolutionary significance of inbreeding depression. We do not attempt to give a complete bibliography of this very extensive field but try to concentrate on what seem to be the most significant findings in relation to this aim.

https://www.jstor.org/stable/pdf/2097132.pdf

Inbreeding depression and its evolutionary consequences

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.

1. Introductory remarks 2. Convolvulacaea 2. Scrophulariaceae, Gesneriaceae, Labiatae, etc. 4. Cruciferae, Papaveraceae, Resedaceae, etc. 5. Geraniaceae, Leguminosae, Onagraceae, etc. 6. Solanaceae, Primulaceae, Polygoneae, etc. 7. Summary of the heights and weights of the crossed and self-fertilised plants 8. Difference between crossed and self-fertilised plants in constitutional vigour and in other respects 9. The effects of cross-fertilisation and self-fertilisation on the production of seeds 10. Means of fertilisation 11. The habits of insects in relation to the fertilisation of flowers 12. General results Index.

The Effects of Cross and Self Fertilisation in the Vegetable Kingdom: CONVOLVULACEÆ

▪ Abstract Mixed mating, in which hermaphrodite plant species reproduce by both self- and cross-fertilization, presents a challenging problem for evolutionary biologists. Theory suggests that inbreeding depression, the main selective factor opposing the evolution of selfing, can be purged with self-fertilization, a process that is expected to yield pure strategies of either outcrossing or selfing. Here we present updated evidence suggesting that mixed mating systems are frequent in seed plants. We outline the floral and pollination mechanisms that can lead to intermediate outcrossing, review the theoretical models that address the stability of intermediate outcrossing, and examine relevant empirical evidence. A comparative analysis of estimated inbreeding coefficients and outcrossing rates suggests that mixed mating often evolves despite strong inbreeding depression. The adaptive significance of mixed mating has yet to be fully explained for any species. Recent theoretical and empirical work suggests that...

/pdf/the-evolutionary-enigma-of-mixed-mating-systems-in-plants-4ai6ful81i.pdf

The Evolutionary Enigma of Mixed Mating Systems in Plants: Occurrence, Theoretical Explanations, and Empirical Evidence

Selfing, the fusion of male and female gametes from a single genetic individual or colony, is possible in many plants and also in hermaphrodite animals. We review the occurrence of selfing and mechanisms for its avoidance, in functionally hermaphrodite animal and plants. We discuss means by which selfing can be detected and briefly review techniques for estimation of selfing frequencies in natural populations. Although many functionally hcmaphrodite species are probably almost complete outcrossers or inbreeders, mixed mating systems are also found in both plant and animal populations. We review theories for the advantages and disadvantages of selfing, and for the maintenance of mixed mating systems, together

The Evolution of the Selfing Rate in Functionally Hermaphrodite Plants and Animals

In most self-incompatible (SI) plants, pollen tube growth in self-pollinated flowers is inhibited on the stigma or in the style. SI systems that operate in the ovary have been assumed to be extremely rare. Evidence from many plant species is presented to show that the SI barriers in the ovary, described here as late-acting SI systems, are quite common. The late-acting SI systems are divided into four categories: (1) ovarian inhibition of incompatible pollen tubes before the ovule is reached; (2) prefertilization inhibition in the ovule; (3) post-zygotic rejection of the embryo, and (4) ovular inhibition for which the cytological details have not been established.

Late-acting self-incompatibility in angiosperms

Self-sterility in Epilobium obcordatum (Onagraceae)

Evolution of Seed Size, Shape, and Surface Architecture in the Tribe Epilobieae (Onagraceae)

WithinEpilobium sectEpilobium, a cytological analysis of 121 experimental hybrids, involving 40 species, indicates the presence of a widespread BB chromosome arrangement in Eurasia, Africa, and Australasia, as well as in North and South America less commonly The AA chromosome arrangement, which differs from BB by one reciprocal translocation, occurs in North America, South America, and in at least three European species The CC arrangement, which differs from AA by two reciprocal translocations, characterizes theAlpinae, a circumboreal group Distinctive or only partly worked out chromosome arrangements occur in the EuropeanE duriaei andE nutans and in the North AmericanE luteum, E obcordatum, E oregonense, andE rigidum With earlier results, the chromosome arrangements of some 65 of the estimated 185 species of the section have been established fully or partly

Chromosomal evolution inEpilobium sect.Epilobium (Onagraceae)

An analysis of chromosome pairing in thirty-four artificially produced hybrids involving seven South American species of Epilobium (Onagraceae) and three from elsewhere has shown that the approximately thirty-five species of this worldwide genus on that continent have been derived from at least two separate introductions from North America, two from Australasia, and one or more additional introductions that could have been from either source. The species derived from North America have the AA chromosome arrangement, and most of the others the BB arrangement, which differs from AA by one reciprocal translocation. The South American population of E. hirtigerum examined had a chromosome arrangement that differed from BB by one reciprocal translocation, from AA by two. In addition to these species, there are at least two others introduced into South America by man, one from North America and one from Europe.

Chromosomal Differentiation and the Sources of the South American Species of Epilobium (Onagraceae)

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