Variation in population sex ratio is particularly pronounced in gynodioecious angiosperms. Extremely high female frequencies in gynodioecious populations cannot be readily explained by selective forces alone. To assess the contributions of drift and gene flow to extreme sex-ratio variation, we documented sex ratio and population size in 92 populations of Lobelia siphilitica across its range and genotyped plants using plastid and nuclear genetic markers. Similarity in spatial patterns of genetic and demographic variables may suggest that drift and/or gene flow have contributed to population sex-ratio variation in L. siphilitica. We found strong spatial structuring of extremely high female frequencies: populations with >50% female plants are restricted to the south–central portion of the range. However, we did not detect any spatial structuring in population size nor metrics of genetic diversity, suggesting that extreme variation in female frequency is not strongly affected by drift or gene flow. Extreme sex-ratio variation is frequently observed in gynodioecious plants, but its causes are difficult to identify. Further investigation into mechanisms that create or maintain spatial structure of sex ratios in gynodioecious species will provide much needed insight.

Plastid haplogroups and nuclear microsatellites for sampled Lobelia siphlitica

Plastid haplogroups for 580 individuals of Lobelia siphilitica. Individual plants are listed with their year of collection, population location (US state or Canadian province), population of origin, identifier, and sex (female, F, hermaphrodite, H, or partially sterile, PS). Plastid haplogroups are based on intergenic sequences of psbK–rps16, excluding an imperfect minisatellite locus (13 unique haplogroups labeled 'C#').

Nuclear microsatellite data for 1713 individuals of Lobelia siphilitica. Individual plants are listed with their year of collection, population location (US state or Canadian province), and population of origin. Allelles are displayed for three nuclear microsatellite loci: Lob6, Lob9, and Lobtri1. Zeros indicate missing data.