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

Abstract

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 ¼ 0.098, P < 0.05) but not among regions (F RT ¼ 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 ¼ 0.192, P < 0.05), yet mean population outcrossing rates were high (t m ¼ 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.

Spatiotemporal structure of genetic variation of a spreading plant metapopulation on dynamic riverbanks along the Meuse River

“…In Boltonia decurrens, a threatened floodplain species that requires regular flooding for suitable habitat and seed dispersal, an F ST ¼ 0.098 was observed (DeWoody et al, 2004). In Populus nigra, a riparian pioneer tree species that was studied along the Drô me River, the F ST value was 0.047 (Imbert and Lefèvre, 2002).…”

“…Mixing of seeds from different source populations through water transport may not be uncommon in natural river systems (Kudoh and Whigham, 2001). DeWoody et al (2004), for example, showed that colonization events in a metapopulation of B. decurrens involved seeds from at least three to five source populations. Similarly, colonization of islands by the weedy Silene dioica through water transport also involved a mixture from several locations (Giles and Goudet, 1997).…”

“…Dynamic river systems are excellent model systems to study the effects of metapopulation turnover on genetic diversity and differentiation Whigham, 1997, 2001;Tero et al, 2003;DeWoody et al, 2004) and to evaluate the relative importance of short-and longdistance seed dispersal in determining long-term persistence of the metapopulation (Menges, 1990;Ouborg, 1993;Jäkäläniemi et al, 2005). Indeed, ephemeral habitat patches and recurrent local extinction and colonization are characteristic of these dynamic river systems (Lytle and Poff, 2004).…”

Regional persistence of an endemic plant, Erigeron acer subsp. decoloratus, in disturbed riparian habitats

“…In addition, the persistence of simulated populations of the riparian gravel shore plant Aster kantoensis is dependent on Xood frequency and the time until perennial invasion (Shimada and Ishihama 2000). Another riparian plant, Boltonia decurrens, also requires regular Xooding for suitable habitat creation and seed dispersal along the river (DeWoody et al 2004). In plants, local populations increase in habitats to which they are well adapted and decline in habitats to which they are poorly adapted.…”

“…Patchy population structure seems to be typical of many riparian plant species (Menges 1990;Shimada and Ishihama 2000;DeWoody et al 2004;Jäkäläniemi et al 2005). Spatial and temporal variation in stream Xow creates heterogeneous habitat structures, and small-scale disturbances remove vital parts of plants (e.g., inXorescences) or entire plants.…”