Evolution of the Magnitude and Timing of Inbreeding Depression in Plants

Abstract

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 (δ = 0.23) was significantly less (43%) than that in predominantly outcrossing species (δ = 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 depression 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 δ = 0.05, N = 11; outcrossing δ = 0.32, N = 31), germination (selfing δ = 0.02, outcrossing δ = 0.12), and survival to reproduction (selfing δ = 0.04, outcrossing δ = 0.15), but not at growth and reproduction (selfing δ = 0.21, outcrossing δ = 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.

Research Article Population genetic structure of three species in the genus Astrocaryum G. Mey. (Arecaceae).

“…A study with Vaccinium corymbosum (Ericaceae) found that heterozygosity increases due to the abortion of most self-fertilized seeds, a post-zygotic mechanism of high inbreeding depression (Krebs and Hancock, 1990). In a review, Husband and Schemske (1996) found general support for the theoretical prediction that, for outcrossing species, inbreeding depression tends to be expressed at early life stages. Although, there is no information on the mating system of A. murumuru and A. paramaca, close relatives, including A. mexicanum (Eguiarte et al, 1992) and A. aculeatum, are outcrossing (Ramos et al, 2011), and Oliveira NP, Oliveira MSP, Davide LC, Kalisz S (unpublished results) indicate that A. vulgare is outcrossing as well.…”

Effect of elevation on sexual reproduction in alpine populations of Saxifraga oppositifolia (Saxifragaceae)

“…Over generations, sel®ng may reduce the inbreeding depression because deleterious alleles should be purged (Husband and Schemske 1996). Ramsey and Vaughton (1996) also argue that pollination limitation raises the threshold of inbreeding depression below which selffertility evolves.…”

“…As a consequence, reduced inbreeding depression should be detectable. Current theory suggests that inbreeding depression in outcrossers is mainly expressed in early stages of the life cycle such as seed formation (Husband and Schemske 1996). I therefore wanted to know whether the seed set of self-pollinated compared with cross-pollinated¯owers of S. oppositifolia increases from low-to high-elevation populations, which would eventually lead to a shift towards self-pollination in the breeding system of S. oppositifolia.…”

Reduced herbivore resistance in introduced smooth cordgrass ( Spartina alterniflora ) after a century of herbivore-free growth

“…The "trade-offs" approach to predicting the relationship between resistance and growth rate (Blossey and Notzöld 1995) does not consider effects of inbreeding (Charlesworth and Charlesworth 1987;Husband and Schemske 1996), which could act to reduce the average biomass of plants in introduced populations, especially if populations are established from a single introduction of a few founders. Inbreeding depression is likely to be an important factor affecting plant growth rates in introduced populations of smooth cordgrass since inbred (selfed) progeny have 30-90% slower growth rates than outbred progeny (Daehler 1996).…”