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

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 future progress will come from a better integration of studies of floral mechanisms, genetics, and ecology, and recognition of how selective pressures vary in space and time.

Prior selfing and the selfing syndrome in animals: an experimental approach in the freshwater snailBiomphalaria pfeifferi

“…Genetic models of the evolution of the selfing rate indeed predict limited inbreeding depression (<0 . 5) in such species (Charlesworth & Charlesworth, 1999;Goodwillie et al, 2005). The review of the empirical evidence in plants confirmed this prediction (Husband & Schemske, 1996), although exceptions can be found (e.g.…”

“…The evolution of self-fertilization versus crossfertilization is a central issue in evolutionary biology, and has been the focus of a substantial amount of both empirical and theoretical work (reviewed in Jarne & Charlesworth, 1993;Husband & Schemske, 1996 ;Goodwillie et al, 2005). The distribution of selfing rates among species is typically U-shaped, although this is more marked in plants than in animals (Goodwillie et al, 2005;Jarne & Auld, 2006).…”

“…The distribution of selfing rates among species is typically U-shaped, although this is more marked in plants than in animals (Goodwillie et al, 2005;Jarne & Auld, 2006). Our focus here will be on the highly selfing side of this distribution.…”

Phylogenetic Pattern, Evolutionary Processes and Species Delimitation in the Genus Echinococcus

“…Mixed mating systems are common in both hermaphroditic plants (Goodwillie et al, 2005) and animals (Jarne and Auld, 2005) and present an evolutionary conundrum because theoretical studies predict the evolution of mating systems towards either pure selfing or pure outcrossing (Lande and Schmenske, 1985). The most common explanation for the maintenance of mixed mating systems is the reproductive assurance hypothesis; outcrossing is usually favoured by selection, but selfing can be favoured if mate availability varies, because selfing is better than not mating at all (Holsinger, 1996).…”

Spore Germination Determines Yeast Inbreeding according to Fitness in the Local Environment

“…Plant mating-system models that balance these opposing factors typically predict that heterogeneous selection pressures will select for intermediate (but constant) rates of selfing (for reviews, see Goodwillie et al 2005 andKarron et al 2012), rather than for plastic strategies such as increasing outbreeding in response to dispersal (Reuter et al 2007). While the touch-me-not apparently optimizes its mixed mating system further by dispersing outcrossed seeds more than selfed seeds (Schmitt et al 1985;Cheplick 1987;Schmitt and Gamble 1990), the effect of phenotypic plasticity in the ratio of outcrossed and selfed seeds is not considered.…”