Evol Ecol Res 11: 23-42 (2009)     Full PDF if your library subscribes.

Sex-ratio variation and spatial distribution of nuclear and cytoplasmic sex-determining genes in gynodioecious Thymus praecox across altitudinal gradients

Urs Landergott1, J. Jakob Schneller1, Rolf Holderegger2 and John D. Thompson3

1Institute of Systematic Biology, University of Zürich, Zürich, Switzerland, 2WSL Swiss Federal Research Institute, Birmensdorf, Switzerland and 3Centre d’Ecologie Fonctionelle et Evolutive, Montpellier, France

Correspondence: U. Landergott, WSL Swiss Federal Research Institute, Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland.
e-mail: urs.landergott@gmx.net


Background: Females and hermaphrodites co-exist in populations of gynodioecious plant species. Gynodioecy often depends on persistent polymorphisms for both cytoplasmic male-sterility (CMS) genes and nuclear restorers of male fertility.

Questions: How do ecological gradients affect population sex ratio? Is sex-ratio variation attributable to variation in the nuclear-cytoplasmic genetic composition of populations? Is there spatial covariation between CMS types and nuclear restorer alleles among populations?

Study system: Late-successional populations of gynodioecious Thymus praecox, a species widespread in the European Alps and distributed from subalpine to alpine altitudes.

Methods: We surveyed sex-ratio variation of adults along altitudinal gradients. We used offspring sex ratios from open pollination in natural populations and from controlled crosses within and among populations to estimate the diversity and spatial distribution of sex-determining alleles.

Results: The proportion of hermaphrodites decreased with increasing altitude. However, offspring sex ratios were constant across altitudes, indicating similar cytonuclear genetic diversity at contrasting adult sex ratios. Sex-determining alleles were geographically widespread, but significantly higher proportions of hermaphrodites produced by within-population crosses compared with those among populations indicated locally adjusted restorer frequencies. Balancing selection may act on sex-determining genes. Our findings further point to a role for environment-dependent selection via the relative maternal fitness of females and hermaphrodites in maintaining the altitudinal sex-ratio variation.

Keywords: cost of restoration, cytoplasmic male sterility, ecological gradient, gynodioecy, local adaptation, nuclear–cytoplasmic interactions, offspring sex ratio.

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