Effects of population size and spatial configuration on the maintenance of the sexes in a clonal organism

Background/Question/Methods

Competition between the sexes in clonal organisms can produce single sex populations unable to sexually reproduce.  Many clonal organisms contain single sex and two sex populations, but the stability of two sexes and the mechanisms for the maintenance of the sexes are not well understood. Theoretical models predict 1) populations with larger carrying capacities require longer times before resources are limiting and competitive exclusion of one sex occurs and 2) the interaction of populations through dispersal allow sexes to be maintained.  A high migration rate between physically separated populations suggests strongly coupled population dynamics while a low migration rate prevents influx of the losing sex; both migration rates lead to the loss of one sex.  Using metapopulations of the clonal plant Marchantia inflexa, we tested the effects of the size of subpopulations and distance between subpopulations (a measure of spatial configuration) on the maintenance of the sexes within subpopulations.  Male and female sex structures were counted monthly from January to May, 2007, along three streams containing metapopulations of M. inflexa on the island of Trinidad, West Indies.  Because single sex subpopulations could contain only females or only males, we transformed the sex ratio into a deviation from an equal sex ratio (ESR), allowing us to ask whether subpopulation size or distance between subpopulations affected the deviation from ESR and the maintenance of both sexes.   We used logistic regression to compare the deviation from ESR with size of the focal population and various spatial measurements.  Spatial measurements include distance to nearest neighbor and the Incidence Function Model, which combines the distances to and the sizes of neighboring subpopulations.

Results/Conclusions

After model reduction, size of subpopulation, distance to the nearest neighbor, and distance to the second nearest neighbor affected the deviation from ESR.  As subpopulation size increases, deviation from ESR tends to decrease (both sexes maintained).  As nearest neighbor distance increases, the deviation from ESR decreases (both sexes maintained).  However, there is a positive trend between distance to second nearest neighbor and the deviation from ESR, where increasing distance leads to biased and single sex subpopulations.  These results suggest that despite physical separation, a collection of subpopulations with small interpopulation distances may act as a single unit and lose one sex when separated by large distances from other subpopulations. While size is important, spatial configuration among subpopulations are also important in understanding the maintenance of the sexes.