The Maintenance of Outcrossing in Predominantly Selfing Species: Ideas and Evidence from Cleistogamous Species

Abstract

Cleistogamous species present strong evidence for the stability of mixed mating, but are generally not considered in this context. Individuals of cleistogamous species produce both obligately selfing cleistogamous flowers (CL) and potentially outcrossed chasmogamous flowers (CH) with distinct morphologies. Greater energetic economy and reliability of CL relative to CH suggest that forces that maintain selection for outcrossing may be stronger in these species than in mixed maters with monomorphic flowers. We reviewed data from 60 studies of cleistogamous species to evaluate proposed explanations for the evolutionary stability of mixed cleistogamous and chasmogamous reproduction and to quantify the magnitude of selection necessary to account for the maintenance of CH. We found circumstantial support for existing hypotheses for the stability of cleistogamy, and that forces that maintain CH must account for a 15-342% advantage of reproduction via CL. We suggest that heterosis and the effects of mass action pollination should be considered.

Reproductive system of a mixed-mating plant responds to climate perturbation by increased selfing

“…We observed pollen limitation, with pollen addition increasing seed-set in CH flowers by 45%, but this occurred independently of the climate treatment. This result, where selfing compensates for pollen limitation, may be not be surprising for Viola, given that low visitation rates have been reported previously for this genus [32] and for rare species in general [11]. As well, V. praemorsa is the earliest forb to flower in this system, potentially reducing visitation rates even though pollinator diversity is high [24].…”

“…It is unclear what explains the absence of linkages between flower types. Trade-off advantages are assumed to relate to some form of bet-hedging, increasing the chances of producing seed but minimizing the over-production of flowers [32]. We observed a presumably more metabolically expensive strategy, because two sets of flowers were produced regardless of circumstance.…”

Population‐Level and Family‐Level Inbreeding Depression in a Cleistogamous Perennial

“…For 19 such studies of 14 species, the average fitness of progeny produced by CL flowers was 1.13 times that of progeny from CH flowers (Oakley et al 2007), indicating that inbreeding depression would be negative, but these data do not provide an accurate assessment of inbreeding depression, for two reasons. First, CH flowers are self-compatible and are known to self at appreciable rates (Mitchell-Olds and Waller 1985;Waller and Knight 1989;Cole and Biesboer 1992;Lu 2000;Culley 2002).…”

“…This scenario suggests that very strong inbreeding depression would be required to favor the maintenance of CH flowers (Oakley et al 2007).…”

“…There is little empirical evidence to support or refute either of these models (Oakley et al 2007), but both rely on a presumed advantage of CH reproduction to balance the energetic economy of CL selfing, and avoidance of inbreeding depression may be a component of this advantage.…”