Genetic diversity and population structure in the clonal Trillium recurvatum
In plants exhibiting a mixture of sexual and vegetative propagation, an understanding of the performance and ecological success of both genets and ramets can provide valuable insight into the roles of genetic variation and demography in shaping population dynamics. Ascertaining reliable population numbers in species which reproduce asexually can be difficult to determine since a simple ‘‘head count’’ may not be an appropriate measure. The use of genetic markers to differentiate individuals is a non-invasive strategy that yields high probabilities for distinguishing genetic individuals. We developed a simple sequence repeat library using microsatellite enrichment and subsequent next-generation sequencing. From this library, we optimized a set of polymorphic loci and used these markers for population genetic and clonal diversity analyses in the self-incompatible Trillium recurvatum, which reproduces both sexually and asexually. We specifically addressed the extent of clonality and spatial patterns of genetic diversity in a population of T. recurvatum from western Tennessee where ecological data has been collected for 12 years. Individuals were sampled in a five-by-five array of 2x2 meter quadrats. Plants were sampled to ensure the greatest spatial variation, i.e., from as many spatially distinct clusters as possible within the 2x2m quadrat.
Using the genetic data, we calculated the probability that each cluster was a single genet and that identical genotypes were not simply obtained by chance using a multi-locus probability for codominant genotypes. The genetic markers we developed here were able to distinguish genetic individuals with a high exclusion probability. In many cases, we found multiple genotypes per quadrat (4 genotypes per 2x2m quadrat), yet the number of genotypes varied within each quadrat and among quadrats encompassing the entire spatial extent of our sampling site. These results have implications for understanding the long-term population demography and genetic stability of populations. Juvenile and non-flowering adults have been shown to dominate this population, which may indicate that there are substantial fitness tradeoffs between clonality and self-incompatibility, since insect pollinations that occur between genetically identical individuals, i.e., ramets, may not produce viable seeds. We intend to sample these populations yearly to correlate genetic data with the large amounts of demographic and life history observations that continues to be collected for this species.