Inbreeding depression is high in a self-incompatible perennial herb population but absent in a self-compatible population showing mixed mating.

Marie Voillemot,John R Pannell

doi:10.1002/ece3.3354

Abstract

High inbreeding depression is thought to be one of the major factors preventing evolutionary transitions in hermaphroditic plants from self‐incompatibility (SI) and outcrossing toward self‐compatibility (SC) and selfing. However, when selfing does evolve, inbreeding depression can be quickly purged, allowing the evolution of complete self‐fertilization. In contrast, populations that show intermediate selfing rates (a mixed‐mating system) typically show levels of inbreeding depression similar to those in outcrossing species, suggesting that selection against inbreeding might be responsible for preventing the transition toward complete self‐fertilization. By implication, crosses among populations should reveal patterns of heterosis for mixed‐mating populations that are similar to those expected for outcrossing populations. Using hand‐pollination crosses, we compared levels of inbreeding depression and heterosis between populations of Linaria cavanillesii (Plantaginaceae), a perennial herb showing contrasting mating systems. The SI population showed high inbreeding depression, whereas the SC population displaying mixed mating showed no inbreeding depression. In contrast, we found that heterosis based on between‐population crosses was similar for SI and SC populations. Our results are consistent with the rapid purging of inbreeding depression in the derived SC population, despite the persistence of mixed mating. However, the maintenance of outcrossing after a transition to SC is inconsistent with the prediction that populations that have purged their inbreeding depression should evolve toward complete selfing, suggesting that the transition to SC in L. cavanillesii has been recent. SC in L. cavanillesii thus exemplifies a situation in which the mating system is likely not at an equilibrium with inbreeding depression.

Highlights

Hermaphrodites can potentially reproduce via a mix of self-fertilization and outcrossing
In contrast to the majority of studies reviewed by Winn et al (2011), in which mixed mating was associated with high inbreeding depression, we found little evidence for any inbreeding depression in the SC population of L. cavanillesii, as well as patterns of heterosis that are largely consistent with expectations for populations with a history of high rates of selfing
Theoretical and empirical work point to a change in inbreeding depression (ID) as one of the main forces influencing the transition to selfing and the apparent stability of mixed mating (Charlesworth & Charlesworth, 1987; Winn et al, 2011)

Summary

| INTRODUCTION

Hermaphrodites can potentially reproduce via a mix of self-fertilization and outcrossing. In a recent survey of the literature, Winn et al (2011) found lower values of inbreeding depression for selfing compared to outcrossing taxa, consistent with the theoretical expectation of purging and previous surveys (e.g., Husband & Schemske, 1996), but they found that inbreeding depression in mixed-m ating populations was similar to that found in fully outcrossing ones In these species with a high load of deleterious recessive mutations, mixed mating might be interpretable as the result of selection to maintain outcrossing, with selfing as an unavoidable consequence of the pollination mode (e.g., Dart & Eckert, 2013; Kalisz et al, 2012). In contrast to the majority of studies reviewed by Winn et al (2011), in which mixed mating was associated with high inbreeding depression, we found little evidence for any inbreeding depression in the SC population of L. cavanillesii, as well as patterns of heterosis that are largely consistent with expectations for populations with a history of high rates of selfing

| MATERIAL AND METHODS

Findings

| DISCUSSION

| Concluding remarks