Genetic diversity, reproductive success, and genetic differentiation from congeners in the narrow endemic Phlox pilosa ssp. sangamonensis

Risk factors that make rare plant taxa particularly susceptible to population declines include a self-incompatible breeding system combined with small population size, small range, and isolated populations separated by unsuitable habitat. Phlox pilosa ssp. sangamonensis is an endangered, self-incompatible, and narrowly endemic taxon with isolated population fragments in east-central Illinois. Here we combined a field and genetic study of Phlox pilosa ssp. sangamonensis as a study of its taxonomic status and conservation, especially because some of its remaining populations are small (< 30 flowering individuals). First, we used six polymorphic nuclear microsatellite loci developed for P. pilosa ssp. pilosa to characterize the genetics of 212 individuals in all 10 known populations of P. pilosa ssp. sangamonensis. We tested the taxon's genetic differentiation from congeners in east-central Illinois: two populations of Phlox pilosa ssp. fulgida, two populations of Phlox pilosa ssp. pilosa, and three populations of Phlox divaricata ssp. laphamii. We also quantified reproduction in each population. We surveyed fruit set for three years and tested correlations with flowering population size and distance to other flowering individuals. For one year, we collected data on seed set from successfully formed capsules. Phlox pilosa ssp. sangamonensis was genetically distinct from, and had putatively lower genetic variation, than Phlox divaricata ssp. laphamii, Phlox pilosa ssp. fulgida, and Phlox pilosa ssp. pilosa. Overall genetic diversity in P. pilosa ssp. sangamonensis appeared low, especially in small populations. Fruit set was positively associated with P. pilosa ssp. sangamonensis population size, with larger population (> 100 flowering individuals) fruit set being more than twice as high compared to smaller (< 30 flowering individuals) ones. Across populations, individuals with a greater distance to flowering neighbors also showed reduced fruit set, although the relationship was not as strong as for population size. In the year we studied seed set, we also found a positive association between population size and seed set, with approximately 30% more seeds per capsule being produced in the larger populations relative to small ones. Our data suggest that P. pilosa ssp. sangamonensis is a genetically distinct taxon from nearby Phlox taxa, and its smaller populations could be at risk of further decline, possibly due to mate or pollen limitation, and/or low genetic diversity. Management for this endangered taxon should facilitate recruitment in small (< 100 flowering individuals) populations and maintain habitat quality for the large populations.