Gene diversity in a fragmented population of Briza media: grassland continuity in a landscape context

Abstract

Summary1 We investigated patterns of allozyme variation in demes of the grass Briza media in semi-natural grassland fragments within a mosaic agricultural landscape on the Baltic island of Öland. In the study area, Briza is both a characteristic species of old pastures and an early colonizer of young grasslands developing on previously forested or arable sites. 2 Generalized linear models revealed that descriptors of both present landscape structure and past grassland history are significant determinants of genetic variation in the Briza demes. Genetic structure and levels of within-deme diversity are influenced by the size of grassland fragments, the type of habitat surrounding the grasslands, the size/ spatial extent of the demes, the geographic position of the demes and the historical continuity of the grassland fragments. 3 Gene diversity ( H ) was higher in demes from grassland polygons with a high proportion of adjacent grassland, higher in the more extensive demes, and decreased northwards within the study area. 4 The negative association between the inbreeding coefficient ( F IS ) and grassland continuity is interpreted in terms of a two-stage colonization process: recruitment into young grasslands leads initially to spatial patchiness, but subsequent selection in maturing pastures occurs within an increasingly uniform and dense sward. 5 Despite a weak overall genetic structure (as indicated by Bayesian cluster analysis) the between-deme F ST was significant. Linear discriminant analysis of within-deme allele frequencies grouped the demes according to the age and previous land-use history of their grassland polygons. The convergence of the allele frequency profiles in the younger grasslands towards those of the old grasslands is consistent with convergence of selective regimes as pastures mature towards an increasingly uniform, dense sward and characteristic species assemblage. 6 The genetic composition of demes of a grassland species appears to be influenced by the process of plant community convergence during grassland development -complementing the recent finding that convergence of species composition in experimental assemblages of grassland plants is dependent on the genotypic composition of the component species.

Genetic diversity and differentiation in six species of the genus Rhinanthus (Orobanchaceae)

“…Several studies have shown that habitat variation and management intensity can impact genetic variation. Prentice and coworkers described significant associations between allozyme frequencies and grassland habitat variation in the grasses Festuca ovina (Prentice et al 1995) and Briza media (Prentice et al 2006). In addition, Magda et al (2004) showed that mowing juvenile R. minor can drastically reduce population density the following year and the population may even become extinct within 3 years.…”

Linking landscape history and dispersal traits in grassland plant communities

“…The Jordtorp area (56°33 0 58 00 N, 16°33 0 58E) is located on the Baltic Island of Ö land (Sweden) and covers an area of 4.5 9 4.5 km with an overall flat topography (Prentice et al 2006;Johansson et al 2008;Reitalu et al 2008). The present landscape is characterized by a mosaic of arable fields, deciduous forest and grasslands.…”

The structuring of quantitative genetic variation in a fragmented population of Briza media (Poaceae)

“…Studies using marker genes reveal that some grassland species show clear evidence of genetic drift as a result of population fragmentation (e.g., van Treuren et al 1994;Luijten et al 2000;van Rossum et al 2004;Helm et al 2009), while other studies show low levels of differentiation between fragmented populations (e.g., Weidema et al 2000;Honnay et al 2006). Prentice et al (2006) used allozyme data from the outcrossing, perennial grass Briza media to explore the association between genetic diversity and various descriptors of landscape structure and land-use history in a fragmented grassland landscape on the Baltic island of Ö land (SE Sweden). The present-day habitat connectivity was significantly associated with the level of gene diversity within demes (i.e.…”