Academic journal article Genetics

Genetic Differentiation, Clinal Variation and Phenotypic Associations with Growth Cessation across the Populus Tremula Photoperiodic Pathway

Academic journal article Genetics

Genetic Differentiation, Clinal Variation and Phenotypic Associations with Growth Cessation across the Populus Tremula Photoperiodic Pathway

Article excerpt

ABSTRACT

Perennial plants monitor seasonal changes through changes in environmental conditions such as the quantity and quality of light. To ensure a correct initiation of critical developmental processes, such as the initiation and cessation of growth, plants have adapted to a spatially variable light regime and genes in the photoperiodic pathway have been implicated as likely sources for these adaptations. Here we examine genetic variation in genes from the photoperiodic pathway in Populus tremula (Salicaceae) for signatures diversifying selection in response to varying light regimes across a latitudinal gradient. We fail to identify any loci with unusually high levels of genetic differentiation among populations despite identifying four SNPs that show significant allele frequency clines with latitude. We do, however, observe large covariance in allelic effects across populations for growth cessation, a highly adaptive trait in P.tremula. High covariance in allelic effects is a signature compatible with diversifying selection along an environmental gradient. We also observe significantly higher heterogeneity in genetic differentiation among SNPs from the photoperiod genes than among SNPs from randomly chosen genes. This suggests that spatially variable selection could be affecting genes from the photoperiod pathway even if selection is not strong enough to cause individual loci to be identified as outliers. SNPs from three genes in the photoperiod pathway (PHYB2, LHY1, and LHY2) show significant associations with natural variation in growth cessation. Collectively these SNPs explain 10-15% of the phenotypic variation in growth cessation. Covariances in allelic effects across populations help explain an additional 5-7% of the phenotypic variation in growth cessation.

(ProQuest: ... denotes formulae omitted.)

SPECIES occupying heterogeneous environments are often subjected to spatially variable natural selection, that is, natural selection that differs among populations that a species inhabits. Such spatially variable selection is expected to have important consequences for how phenotypic variation is partitioned within and among populations and also for genetic differentiation at the underlying loci controlling these traits (Latta 1998; Le Corre and Kremer 2003; Whitlock 2008). A major motivation for studying the genetic consequences of local adaptation is to identify loci that control local adaptation. The identification of loci (and traits) subjected to spatially variable selection is complicated by the fact that genetic drift introduces heterogeneity among loci. However, it should be possible to identify putative targets of spatially variable selection by searching for loci with unusually high levels of genetic differentiation (Lewontin and Krakauer 1973; Beaumont 2005). Although the initial idea championed by Lewontin and Krakauer (1973) has attracted much criticism, the general principle of searching for loci with unusually high levels of genetic differentiation has proved to be quite successful in identifying loci subject to spatially variable selection, especially when selection is strong relative to migration (e.g., Beaumont and Balding 2004; Beaumont 2005).

However, even though spatially variable selection acting on a quantitative trait conferring local adaptation may be strong, selection on the underlying causal loci can often be quite weak because individual loci contribute only a small proportion to the total phenotypic variation (Latta 1998; Le Corre and Kremer 2003). In such cases migration may overwhelm local selection acting on the underlying QTL (that is, m > s) and genetic differentiation at these loci is actually well approximated by genetic differentiation at neutral loci, even though genetic differentiation at the quantitative traits themselves can be high (Latta 1998; Whitlock 2002; Le Corre and Kremer 2003). The reason for this is that selection generates linkage disequilibrium between loci and the combined effect of multiple loci changing in parallel is much greater than what can be predicted from the effects of individual loci alone (Latta 1998; Le Corre and Kremer 2003). …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.