Do Genetic Drift and Gene Flow Affect the Geographic Distribution of Female Plants in Gynodioecious Lobelia siphilitica?

Abstract

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 the spatial structure of sex ratios in gynodioecious species will provide much needed insight.

Simulated pollinator decline has similar effects on seed production of female and hermaphrodite Lobelia siphilitica, but different effects on selection on floral traits

“…The relative frequency of female plants in our experiment (31%) reflected the relative frequency of female plants in the population that we collected seeds from (36%; Caruso and Case, 2013). Females occur at a lower relative frequency than hermaphrodites in many gynodioecious populations: the mean relative frequency of female plants was 22% in 92 L. siphilitica populations (Appiah‐Madson et al, 2022) and 25% in populations of 63 gynodioecious species (Varga and Soulsbury, 2020). Consequently, less power to detect selection on traits in females than in hermaphrodites is a common feature of studies of gynodioecious species (e.g., Eckhart, 1993; Caruso and Yakobowski, 2008).…”

“…It can also reproduce asexually by producing a clonal rosette that can overwinter and flower in the following year (Beaudoin Yetter, 1989). Among L. siphilitica populations, the mean frequency of females is ~22% ( N = 92 populations; Appiah‐Madson et al, 2022), and is higher in populations in the southern portion of the species range (i.e., southern Indiana and Ohio; Caruso and Case, 2007, 2013; Case and Caruso, 2010; Appiah‐Madson et al, 2022). Female L. siphilitica flowers are consistently smaller than hermaphroditic flowers (Caruso et al, 2003a; Caruso and Yakobowski, 2008; Caruso, 2012).…”