Historical habitat connectivity affects current genetic structure in a grassland species.

Many recent studies have explored the effects of present and past landscape structure on species distribution and diversity. However, we know little about the effects of past landscape structure on distribution of genetic diversity within and between populations of a single species. Here we describe the relationship between present and past landscape structure (landscape connectivity and habitat size estimated from historical maps) and current genetic structure in a perennial herb, Succisa pratensis. We used allozymes as co-dominant markers to estimate genetic diversity and deviation from Hardy-Weinberg equilibrium in 31 populations distributed within a 5 km(2) agricultural landscape. The results showed that current genetic diversity of populations was related to habitat suitability, habitat age, habitat size and habitat connectivity in the past. The effects of habitat age and past connectivity on genetic diversity were in most cases also significant after taking the current landscape structure into account. Moreover, current genetic similarity between populations was affected by past connectivity after accounting for current landscape structure. In both cases, the oldest time layer (1850) was the most informative. Most populations showed heterozygote excess, indicating disequilibrium due to recent gene flow or selection against homozygotes. These results suggest that habitat age and past connectivity are important determinants of distribution of genetic diversity between populations at a scale of a few kilometres. Landscape history may significantly contribute to our understanding of distribution of current genetic structure within species and the genetic structure may be used to better understand landscape history, even at a small scale.

How much genetic variation is stored in the seed bank? A study of Atriplex tatarica (Chenopodiaceae).

We investigated to what extent the soil seed bank differed genetically and spatially in comparison to three consecutive life history stages (seedlings, mature plants, and fruiting plants) in a natural population of Atriplex tatarica. Representatives of particular life history stages from twenty subunits within a large population were randomly collected and subjected to allozyme analysis. Comparison of population polymorphism among various life history stages showed significant differences in observed heterozygosity (H(O)) and F statistics (F(IS) and F(ST)), but nonsignificant ones in the cases of number of alleles per polymorphic locus (A) and gene diversity (H(S)). These results indicate an increasing number of heterozygotes, a decreasing level of inbreeding and an increase of the partitioning genetic diversity among populations with increasing population age. Spatial autocorrelation was used to calculate f, the average co-ancestry coefficient between individuals within distance intervals of two meters along a 39 m long transect. Significant positive fine scale genetic structure was detected in mature and fruiting plants but not in soil seeds and seedlings stages. The results of the presented study on A. tatarica indicated that significant differences exist in genetic differentiation, differentiation in allele frequencies and spatial autocorrelation among early (soil seeds and seedlings) and late (mature and fruiting plants) life history stages but not within early and late ones. This pattern suggests that, rather than storing genetic variability in the soil or germination and establishment success, self-thinning might be the major microselective force in populations of A. tatarica.