Plant Mating Systems Often Vary Widely Among Populations

Michael R Whitehead,Randall J Mitchell,Robert Lanfear,Jeffrey D Karron

doi:10.3389/fevo.2018.00038

Abstract

Most flowering plants are hermaphroditic, yet the proportion of seeds fertilized by self and outcross pollen varies widely among species, ranging from predominant self-fertilization to exclusive outcrossing. A population’s rate of outcrossing has important evolutionary outcomes as it influences genetic structure, effective population size, and offspring fitness. Because most mating system studies have quantified outcrossing rates for just one or two populations, past reviews of mating system diversity have not been able to characterize the extent of variation among populations. Here we present a new database of more than 30 years of mating system studies that report outcrossing rates for three or more populations per species. This survey, which includes 741 populations from 105 species, illustrates substantial and prevalent among-population variation in the mating system. Intermediate outcrossing rates (mixed mating) are common; 63% of species had at least one mixed mating population. The variance among populations and within species was not significantly correlated with pollination mode or phylogeny. Our review underscores the need for studies exploring variation in the relative influence of ecological and genetic factors on the mating system, and how this varies among populations. We conclude that estimates of outcrossing rates from single populations are often highly unreliable indicators of the mating system of an entire species.

Highlights

IntroductionMost flowering plants are hermaphroditic, yet the proportion of seeds fertilized by self and outcross pollen varies widely among species, ranging from predominant self-fertilization to exclusive outcrossing (Schemske and Lande, 1985; Barrett and Eckert, 1990; Vogler and Kalisz, 2001; Goodwillie et al, 2005)
We focused exclusively on multilocus estimates of outcrossing rate because these estimates best distinguish true selfing from biparental inbreeding (Ritland, 1985), and are less influenced by selection than are single locus estimates (Ritland, 2002)
As expected for a 0– 1 bounded variable, among-population variation in tm estimates was lowest for species with extremely high or extremely low mean outcrossing rates (Figure S2)

Summary

Introduction

Most flowering plants are hermaphroditic, yet the proportion of seeds fertilized by self and outcross pollen varies widely among species, ranging from predominant self-fertilization to exclusive outcrossing (Schemske and Lande, 1985; Barrett and Eckert, 1990; Vogler and Kalisz, 2001; Goodwillie et al, 2005). High rates of outcrossing are thought to be favored when the cost of inbreeding depression exceeds the transmission advantage of selfing These predictions led to the hypothesis that flowering plant populations should be subject to disruptive selection on the rate of outcrossing, Population Variation in Plant Mating Systems causing mixed mating to be rare and transitory (Lande and Schemske, 1985; Barrett, 2003).

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