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. 2019 Jan 1;123(1):205-212.
doi: 10.1093/aob/mcy162.

On the Adaptive Value of Monomorphic Versus Dimorphic Enantiostyly in Solanum Rostratum

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Free PMC article

On the Adaptive Value of Monomorphic Versus Dimorphic Enantiostyly in Solanum Rostratum

Emiliano Mora-Carrera et al. Ann Bot. .
Free PMC article

Abstract

Background and aims: Enantiostyly is a reproductive system with heteromorphic flowers characterized by asymmetrical deflection of the style, either to the left or to the right of the floral axis. There are two types of enantiostyly. In monomorphic enantiostyly, plants produce the two types of flowers in the same individual. Dimorphic enantiostyly is restricted to only seven species and their populations consist of individuals producing either the right or the left flower type. It is hypothesized that the dimorphic form is derived from monomorphic ancestors because it functions as an outcrossing mechanism. We tested this latter hypothesis and investigated if monomorphic enantiostyly is resistant to invasion by individuals with dimorphic enantiostyly, because it functions as a reproductive assurance mechanism.

Methods: To determine the conditions favouring the invasion of dimorphic enantiostyly, measurements of reproductive success and outcrossing rates in 15 natural flowering patches of Solanum rostratum were made. To test if monomorphic enantiostyly provides a reproductive assurance mechanism, experimental plants with either manually created dimorphic or natural monomorphic reproductive systems were exposed to two different pollination scenarios (flower density treatments), and reproductive success and outcrossing rates were measured.

Key results: Naturally flowering patches experienced severe pollination limitation, showed marked differences in reproductive success and had relatively high outcrossing rates. Plants in the experimental patches also showed pollination limitation and high outcrossing rates. Individuals with dimorphic enantiostyly expressed higher reproductive and outcrossing advantages under high-density conditions. These advantages disappeared in the low-density treatment, where the monomorphic form attained a higher reproductive success and no differences in outcrossing rates were detected.

Conclusions: Monomorphic enantiostyly should be resistant to invasion of the dimorphic form because the prevalent ecological conditions favour the maintenance of geitonogamous individuals that are able to take advantage of ecological heterogeneity and generalized pollination limitation.

Figures

Fig. 1.
Fig. 1.
Natural variation of (A) mean relative reproductive success ± s.e. and (B) mean multilocus outcrossing rates (Tm) ± s.e. of individuals in 15 natural patches of Solanum rostratum of the REPSA population (N = 73). Reproductive success (W) is the multiplicative product of fruit-set, mean seeds per fruit and probability of germination; individual W were given relative to the mean of the population.
Fig. 2.
Fig. 2.
Boxplot showing the effects of high (HD) and low (LD) flower density treatments on pollinator visitation rate (visits per flower per hour). Wilcoxon test revealed differences between treatments (χ2(1) = 13.5, P = 0.0002), based on 24 h of observation. Only visits of Xylocopa tabaniformis subsp. azteca were included.
Fig. 3.
Fig. 3.
Estimated marginal means (± s.e.) of reproductive success (A) excluding and (B) including probability of germination. Mean multilocus outcrossing rates (± s.e.), estimated with 13 microsatellites. Dimorphic enantiostyly (DE; either L or R) or monomorphic enantiostyly (ME) with 1: 1 floral type ratio (L/R); and across high and low floral density patches (HD and LD, respectively).

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