Functional characterization of gynodioecy in Fragaria vesca ssp. bracteata (Rosaceae)

Background and Aims

Gynodioecy is a phylogenetically widespread and important sexual system where females coexist with hermaphrodites. Because dioecy can arise from gynodioecy, characterization of gynodioecy in close relatives of dioecious and sub-dioecious species can provide insight into this transition. Thus, we sought to determine whether Fragaria vesca ssp. bracteata, a close relative to F. chiloensis and F. virginiana, exhibits the functional and population genetic hallmarks of a gynodioecious species.

Methods

We compared reproductive allocation of females and hermaphrodites grown in the greenhouse and estimated genetic diversity (allelic diversity, heterozygosity) and inbreeding coefficients for field-collected adults of both sexes using simple sequence repeat (SSR) markers. We estimated mating system and early seed fitness from open-pollinated families of both sex morphs.

Key Results

Under greenhouse conditions, females and hermaphrodites allocated similarly to all reproductive traits except flower number, and, as a consequence, females produced 30 % fewer seeds per plant than hermaphrodites. Under natural conditions, hermaphrodites produce seeds by self-fertilization approx. 75 % of the time, and females produced outcrossed seeds with very little biparental inbreeding. Consistent with inbreeding depression, seeds from open-pollinated hermaphrodites were less likely to germinate than those from females, and family-level estimates of hermaphrodite selfing rates were negatively correlated with germination success and speed. Furthermore, estimates of inbreeding depression based on genetic markers and population genetic theory indicate that inbreeding depression in the field could be high.

Conclusions

The joint consideration of allocation and mating system suggests that compensation may be sufficient to maintain females given the current understanding of sex determination. Fragaria vesca ssp. bracteata exhibited similar sex morph-dependent patterns of mating system and genetic diversity, but less reproductive trait dimorphism, than its sub-dioecious and dioecious congeners.