Sex inheritance in gynodioecious species: a polygenic view

Abstract

Gynodioecy is defined as the coexistence of two different sexual morphs in a population: females and hermaphrodites. This breeding system is found among many different families of angiosperms and is usually under nucleo-cytoplasmic inheritance, with maternally inherited genes causing male sterility and nuclear factors restoring male fertility. Numerous theoretical models have investigated the conditions for the stable coexistence of females and hermaphrodites. To date, all models rest on the assumption that restoration of a given male sterile genotype is controlled by a single Mendelian factor. Here, we review data bearing on the genetic determinism of sex inheritance in three gynodiecious plant species. We suggest that restoration of male fertility is probably best viewed as a quantitative trait controlled by many loci. We develop a threshold model that accommodates an underlying polygenic trait, which is resolved at the phenotypic level in discrete sexual morphs. We use this model to reanalyse data in Thymus vulgaris, Silene vulgaris and Plantago coronopus. A simple Mendelian inheritance of sex determinism is unlikely in all three species. We discuss how our model can shed additional light on the genetics of restoration and point towards future efforts in the modelling of gynodioecy.

Figure 1

Mean proportion of hermaphrodites (±s.e.) in offspring of b-, c- and d-females (see text for further explanation) of Thymus vulgaris. Proportions are based on pooled offspring from 10 b-females (n=344 offspring), 17 c-females (n=764 offspring) and 18 d-females (n=704 offspring). Several pollen donors were used for these crosses, but each female was only crossed to a single pollen donor. Data extracted from Assouad (1972).

Figure 2

Graphical representation of the threshold model. (a) A two-morph threshold model with females and hermaphrodites. A single threshold T (dotted line) is used (b). A three-morph threshold model featuring females, partially male sterile (PMS) and hermaphrodites. Two thresholds, T_PMS and T_H, are used.