Effects of population size and isolation on heterosis, mean fitness, and inbreeding depression in a perennial plant

Abstract

Summary• In small isolated populations, genetic drift is expected to increase chance fixation of partly recessive, mildly deleterious mutations, reducing mean fitness and inbreeding depression within populations and increasing heterosis in outcrosses between populations.• We estimated relative effective sizes and migration among populations and compared mean fitness, heterosis, and inbreeding depression for eight large and eight small populations of a perennial plant on the basis of fitness of progeny produced by hand pollinations within and between populations.• Migration was limited, and, consistent with expectations for drift, mean fitness was 68% lower in small populations; heterosis was significantly greater for small (mean = 70%, SE = 14) than for large populations (mean = 7%, SE = 27); and inbreeding depression was lower, although not significantly so, in small (mean = )0.29%, SE = 28) than in large (mean = 0.28%, SE = 23) populations.• Genetic drift promotes fixation of deleterious mutations in small populations, which could threaten their persistence. Limited migration will exacerbate drift, but data on migration and effective population sizes in natural populations are scarce. Theory incorporating realistic variation in population size and patterns of migration could better predict genetic threats to small population persistence.

Population size, center–periphery, and seed dispersers’ effects on the genetic diversity and population structure of the Mediterranean relict shrub Cneorum tricoccon

“…Both populations are small and are geographically isolated in islands, where genetic drift can increase the fixation of deleterious alleles and outcrosses may be carried out between related parents. Therefore, reduced individual fitness and increased inbreeding depression can be expected in both populations (Kimura, Maruyama, & Crow, 1963; Lynch & Gabriel, 1990; Oakley & Winn, 2012). However, these populations are located at the dry edge of the species’ distribution range, and strong selection pressures related to water availability may be selecting drought‐adapted phenotypes, as has been suggested for marginal populations of other species (Kirkpatrick & Barton, 1997; Lesica & Allendorf, 1995), including Tertiary relicts such as Ramonda myconi (Muller et al., 1997).…”

Heterosis and outbreeding depression in crosses between natural populations of Arabidopsis thaliana

“…Many mildly deleterious recessive or nearly recessive mutations are predicted to become fixed when effective population sizes are modest and gene flow is limited (Whitlock et al, 2000). Heterosis in F 1 crosses between natural populations is ubiquitous (for example, Fenster, 1991;Armbruster et al, 1997;Edmands, 1999;Oakley and Winn, 2012).…”

The Influence of Natural Variation in Population Size on Ecological and Quantitative Genetics of the Endangered Endemic PlantHypericum cumulicola

“…Reduced pollinator visitation has been observed at low density in this species (Boyle and Menges 2001), which could select for increased autogamous selfing as a means of reproductive assurance. Although the short-term selective advantage of autogamy could be diminished by inbreeding depression (reviewed in Goodwillie et al 2005), inbreeding depression in this species has been shown to be not significantly different from zero and averages less than 28% (Oakley and Winn 2012).…”

“…Previous work strongly implicates a role of genetic drift in shaping genetic variation related to fitness in small populations. Heterosis in crosses between small populations (Oakley and Winn 2012) suggests the fixation of partly recessive deleterious mutations within populations, which cannot be attributed to natural selection. The habitat requirements and demography of this species have also been well studied (Quintana-Ascencio and Morales-Hernandez 1997; QuintanaAscencio et al 1998QuintanaAscencio et al , 2003QuintanaAscencio et al , 2007, permitting identification of phenological and morphological traits likely to affect fitness in this species.…”

“…Adams (Hypericaceae) is uniquely suitable for studying the influence of population size on patterns of genetic diversity and differentiation for ecologically important traits. Census population sizes (ranging from 11 to 11000), relative effective population sizes, and migration between populations have been quantified for 16 populations (Oakley and Winn 2012). Previous work strongly implicates a role of genetic drift in shaping genetic variation related to fitness in small populations.…”