Heterosis in an Isolated, Effectively Small, and Self-Fertilizing Population of the Flowering Plant Leavenworthia Alabamica

Abstract

Abstract. Mildly deleterious mutations are thought to play a major role in the extinction of natural populations, especially those that are small, isolated, or inbred. Self-fertilization should reduce the effective size of populations and simultaneously reduce migration between populations. A history of self-fertilization should therefore cause a population to harbor a substantial ''local drift load'' caused by the fixation of mildly deleterious mutations. This hypothesis was tested in Leavenworthia alabamica, which contains large, self-incompatible populations and smaller self-compatible populations with adaptations for self-fertilization. The fitness of offspring from within-and between-population crosses was compared to quantify heterosis caused by the masking of deleterious alleles in the heterozygous state. Little heterosis was observed in crosses between five large, self-incompatible populations and two of the three small, self-fertilizing populations of L. alabamica. However, the most geographically isolated and genetically divergent self-fertilizing population (Tuscumbia) exhibited a 110.2% increase in germination and a 73.6% increase in fitness, which is consistent with a sizeable local drift load. The finding of substantial heterosis for fitness supports the idea that small effective size, reproductive isolation, and self-fertilization can make populations particularly vulnerable to mutation accumulation.

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

“…Strong heterosis has been observed in crosses between widely isolated natural populations of other taxa (Ebert et al. , 2002; Busch, 2006), but comparisons of the magnitude of heterosis for populations of different sizes are rare, and results are inconsistent. Paland & Schmid (2003) reported 16.5% greater fitness of between‐ than of within‐population crosses for six small ( n < 200) populations of the self‐compatible biennial plant Gentianella germanica and no evidence for heterosis for six populations larger than 200 individuals.…”

“…Inbreeding depression (reviewed by Husband & Schemske, 1996; Armbruster & Reed, 2005, Winn et al., 2011) and heterosis in crosses among plant populations (Fenster, 1991; van Treuren et al. , 1993; Ouborg & Van Treuren, 1994; Willi & Fischer, 2005; Busch, 2006) have been reported for numerous plant species, but relating these measures to drift requires estimates of size and rates of migration for multiple conspecific populations, which remain rare. Some of the expected effects of drift on the distribution of deleterious, partly recessive mutations are supported by comparisons of widely separated populations of different size (Paland & Schmid, 2003; Willi & Fischer, 2005) or of populations separated by different distances (Fenster 1991; Fenster & Galloway, 2000; Bailey & McCauley, 2006), but mean fitness, heterosis, and inbreeding depression have not been investigated in systems for which both population size and migration have been estimated.…”

Small effective size limits performance in a novel environment

“…; Higgins and Lynch ). Empirical demonstrations of heterosis in crosses between populations, and in particular between small populations, provides good evidence that the fixation of mildy deleterious, partly recessive mutations may be important (Paland and Schmid ; Willi and Fischer ; Busch ; Oakley and Winn ). Such mutations are stochastically fixed within populations by genetic drift and are masked in the hetrozygous state in between population crosses, resulting in heterosis.…”

MUTATIONAL MELTDOWN IN SELFINGARABIDOPSIS LYRATA

“…Indeed, a recent comparative study of plants in the family Solanaceae discovered that extinction occurs at higher rates in species that can potentially self compared to those that are obligate outcrossers, and this reduces net diversification despite a higher speciation rate (Goldberg et al 2010). However, few studies have estimated both inbreeding and drift load in the context of mating system variation (Busch 2006).…”