Population divergence of genetic (co)variance matrices in a subdivided plant species, Brassica cretica

Abstract

The present study of Brassica cretica had two objectives. First, we compared estimates of population structure (Q st ) for seven phenotypic characters with the corresponding measures for allozyme markers (F st ) to evaluate the supposition that genetic drift is a major determinant of the evolutionary history of this species. Secondly, we compared the genetic (co)variance (G) matrices of five populations to examine whether a long history of population isolation is associated with large, consistent differences in the genetic (co)variance structure. Differences between estimates of F st and Q st were too small to be declared significant, indicating that stochastic processes have played a major role in the structuring of quantitative variation in this species. Comparison of populations using the common principal component (CPC) method rejected the hypothesis that the G matrices differed by a simple constant of proportionality: most of the variation involved principal component structure rather than the eigenvalues. However, there was strong evidence for proportionality in comparisons using the method of percentage reduction in mean-square error (MSE), at least when characters with unusually high (co)variance estimates were included in the analyses. Although the CPC and MSE methods provide different, but complementary, views of G matrix variation, we urge caution in the use of proportionality as an indicator of whether genetic drift is responsible for divergence in the G matrix.

Comparing Bayesian estimates of genetic differentiation of molecular markers and quantitative traits: an application to Pinus sylvestris

“…Large variation in population-specific heritability levels were also found for some traits in two rare plants, whereas some traits displayed very similar heritability levels between populations (Petit et al, 2001). A similar result was found by Widen et al (2002) for Brassica cretica. The within-population genetic variance varied between 0.111 and 11.0 for internode length, whereas node number only varied between 12.9 and 64.4 between populations of this species.…”

COMPARATIVE ANALYSES OF POPULATION STRUCTURE IN TWO SUBSPECIES OFNIGELLA DEGENII: EVIDENCE FOR DIVERSIFYING SELECTION ON POLLEN‐COLOR DIMORPHISMS

“…Low population structure in quantitative characters, with Q ST similar to, or lower than, marker-based F ST estimates, has also been observed in Silene diclinis (Andersson et al 2000), Brassica insularis (Petit et al 2001), B. cretica (Widén et al 2002), and Centaurea corymbosa (Petit et al 2001), four perennial herbs with narrow distributions in the Mediterranean area. Based on these observations and the fact that both barbro and jenny occupy very limited regions, it is possible that populations in such geographically or ecologically restricted species may experience too similar ecological conditions for diversifying selection to be of any major importance (Petit et al 2001).…”

The Evolution and Diversification ofS-Locus Haplotypes in the Brassicaceae Family

“…B. cretica is found in the Aegean region, mainly on the island of Crete, where it grows in isolated populations in ravines and gorges (Snogerup et al 1990). As a result of severely restricted gene flow, Cretan populations of B. cretica show an exceptionally high level of neutral genetic differentiation both at molecular marker loci and for quantitative traits (Widén et al 2002;Edh et al 2007).…”