Population genetics and viability of genetic rescue in the restricted endemic Lepidospartum burgessii (Asteraceae)

Background/Question/Methods

The gypsum salt flat specialist Lepidospartum burgessii is a narrowly endemic species in Texas and New Mexico. The majority of the known occurrences of the species occur on opposite ends of the range (approx. 65 mi²) with populations on BLM land to the north and Guadalupe National Park (NPS) land to the south, although a large area of unsurveyed habitat occurs between the two known sites.  The populations are threatened due to low seedling recruitment; in particular, population censuses since the 1980s have shown a prevalence of mature and decadent plants and few juvenile plants in the population. Previous research suggests that the limit to sexual reproduction is neither pollinator limitation nor extensive clonal growth. In Asteraceae, one possible cause of poor seed set is self-incompatibility. We used four microsatellite primers (five loci) developed for Senecio species (Asteraceae) to examine inbreeding and population structure. If populations and/or colonies within each population are genetically distinct, pollen augmentation may be a successful conservation and restoration tool. We sampled 214 individuals from 19 colonies (separated by at least 100 m) from a BLM population and 86 from nine colonies from a Guadalupe National Park (NPS) population. For comparison, we sampled 68 individuals of the common species Lepidospartum latisquamumfrom five colonies within one population. We used GenAlEx to estimate diversity parameters and STRUCTURE to examine genetic differentiation.

Results/Conclusions

Genetic diversity across L. latisquamum colonies was higher (Na =4.4±0.4, He =0.58±0.04) than L. burgessii colonies (Na=3.5±0.2, He=0.46±0.02). Pairwise FST was also higher (0.11±0.02) than L. burgessii colonies (0.08±0.004).  In addition, more variation was within L. burgessii colonies (AMOVA, 68%) than among colonies (4%).  STRUCTURE analysis favored separating the two L. burgessii sites, and pairwise FST comparing BLM to NPS colonies was higher (0.11±0.01) compared to pairwise comparisons of colonies within each population (BLM: 0.05±0.004, NPS: 0.05±0.01). These results suggest that L. burgessii colonies are genetically depauperate compared to L. latisquamum. However there is genetic diversity within each L. burgessii colony, and the two sites are genetically different. This indicates that augmentation could increase the resilience and robustness of populations and their ability to adapt to diseases and locally changing conditions. Future work will implement cross-pollination experiments to determine if genetic rescue is a viable conservation technique for this species.