Genetic variation of inbreeding depression among floral and fitness traits in Silene nutans

Abstract

The magnitude and variation of inbreeding depression (ID) within populations is important for the evolution and maintenance of mixed mating systems. We studied ID and its genetic variation in a range of floral and fitness traits in a small and large population of the perennial herb Silene nutans, using controlled pollinations in a fully factorial North Carolina II design. Floral traits and early fitness traits, that is seed mass and germination rate, were not much affected by inbreeding (do0.2). In contrast, 'late' fitness traits and multiplicative fitness suffered severely from inbreeding (d40.4). Lack of genetic correlations indicated that ID in floral, early and late traits is genetically decoupled. There was a trend that the smaller population was less affected by ID than the large one, although the differences were not significant for most traits. Hence, evidence for purging of deleterious alleles remains inconclusive in this study. Genetic variation in ID among paternal families was statistically significant in most floral and all seed traits, but not in late fitness traits. However, some paternal families had do0.5, even in the multiplicative fitness measure that suffered most from ID (d ¼ 0.74), suggesting that the mixed mating system of S. nutans might be evolutionary stable.

Inbreeding depression and the cost of inbreeding on seed germination

“…d within populations can be affected by current and past (bottlenecks, founder effects and genetic drift) population size and genetic variation. It is expected to be lower in small than in large populations, and in populations with low than with high genetic variation (Mustajärvi et al, 2005;Hirayama et al, 2007;Thiele et al, 2010;Angeloni et al, 2011). In general, lower levels of d are expected in small populations with a long history of inbreeding because deleterious genes presumably have been purged from populations of selfers.…”

Contrasting inbreeding depression in early and late stages of the life cycle of a Mediterranean shrub, Anagyris foetida (Leguminosae)

“…In general, parameters not involved in fitness seem to be less affected by ID (DeRose and Roff, 1999). Thiele et al (2010) proposed the lower effect of ID in floral traits than in fitness traits could be because selection acts more strongly on flowers than on fitness or because selection acting on floral traits may be stabilizing, in contrast to selection on fitness traits that likely is directional. Indeed, previous studies showed ID has hardly any effect on flower size or on the floral pieces such as sepals or petals (Ellmer and Andersson, 2004;Thiele et al, 2010).…”

Evolutionary Assurancevs. Mixed Mating

“…They suggested that mixed mating (MM) in many species may represent transitional states or incidental by-products of other adaptive mechanisms Porcher et al, 2009;Busch et al, 2010). In contrast, increasing evidence suggests that MM is evolutionarily stable in seed plants (Goodwillie et al, 2005;Davis and Turner-Jones, 2008;Winn and Moriuchi, 2009;Thiele et al, 2010), even if they possess strong inbreeding depression (Goodwillie et al, 2005) or in obligately outcrossing species (Crawford et al, 2010).…”

“…In natural populations, inbreeding depression often varies among different populations (Weber and Goodwillie, 2009), isolated patches within a population (Kolehmainen et al, 2010), individuals (Busch et al, 2010), life-cycle stages (Gonzalez-Varo and Traveset, 2010;Hirao, 2010), generations (Glaettli and Goudet, 2006;Ferriol et al, 2011), and stressful environmental conditions (Cheptou, 2006). This variation is important for the evolution and maintenance of MM systems (Thiele et al, 2010), and the relative composition of inbred and outbred plants, which in a natural population is determined directly by population selfing rate, substantially influences the magnitude of inbreeding depression (Cheptou and Schoen, 2007). For example, in the perennial herb Silene nutans (Caryophyllaceae), high inbreeding depression (δ > 0.5), representing multiplicative fitness at the population level, suggests that the MM system is not stable, but should evolve towards outcrossing; however, some paternal families had δ < 0.5, even in the cumulative relative fitness measure that suffered most from inbreeding depression (δ = 0.74).…”

“…For example, in the perennial herb Silene nutans (Caryophyllaceae), high inbreeding depression (δ > 0.5), representing multiplicative fitness at the population level, suggests that the MM system is not stable, but should evolve towards outcrossing; however, some paternal families had δ < 0.5, even in the cumulative relative fitness measure that suffered most from inbreeding depression (δ = 0.74). Hence, complete outcrossing may be prevented by recurring evolution of prevalently selfing lines that suffer only moderately from inbreeding depression, whereas evolution towards increased selfing will be prevented by generally high inbreeding depression in late fitness traits that are apparently difficult to purge (Thiele et al, 2010), suggesting that the MM system of S. nutans may have been evolutionary stable over a long period of time.…”