Inferring demographic processes from the genetic structure of a metapopulation of Boltonia decurrens (Asteraceae)

Boltonia decurrens (Asteraceae), a federally listed, threatened floodplain species, requires regular flooding for suitable habitat and seed dispersal. Flood suppression and habitat destruction have resulted in fewer than 25 populations remaining throughout its 400 km range. Because individual populations are widely interspaced ( > 10 km) and subject to frequent extinction and colonization, seed dispersal along the river, not pollen flow, is likely the primary determinant of population genetic structure. We used neutral genetic markers (isozymes) assayed for fourteen populations to determine which demographic processes contribute to the genetic structure of B. decurrens. Significant genetic differentiation was detected among populations (F-ST = 0.098, P < 0.05) but not among regions (F-RT = 0.013, P > 0.05), suggesting that long-distance dispersal events occur and involve seed from a small number of populations. Correspondingly, we found no evidence of isolation by distance, and admixture analyses indicate that colonization events involve seed from 3 to 5 source populations. Individual populations exhibited high levels of fixation (mean F-IS = 0.192, P < 0.05), yet mean population outcrossing rates were high (t(m) = 0.87-0.95) and spatial autocorrelation analyses revealed no fine-scale within population structure, indicating that inbreeding alone cannot explain the observed fixation. Rather, genetic bottlenecks, detected for 12 of 14 populations, and admixture at population founding may be important sources of fixation. These observations are consistent with a metapopulation model and confirm the importance of regular flooding events, capable of producing suitable habitat and dispersing seed long distances, to the long-term persistence of B. decurrens.