Document: ROB-3-25-1

Genetic divergence and ecological specialization: Genes, markers and traits.

LATTA, R.G.*

Dept. of Biology, Dalhousie University, Halifax, Nova Scotia, Canada ¹

Abstract:
Genes and traits which are subject to spatially varying selection pressures often reveal themselves by a discordant spatial pattern relative to neutral genes and traits. Spatial distribution of such neutral genes and traits are influenced by migration and drift, which act equally on all loci. By contrast, local selection pressures are unique to the traits which experience them. Several studies have documented the discordance between patterns of variation (F_st) of putatively neutral genetic markers and that of more ecologically important quantitative traits. However, little is known about the distribution of allelic variation at the quantitative trait loci (QTLs) underlying these traits. I theoretically model the contribution of individual loci to overall patterns of variation within and among populations. When selection acts on a quantitative trait, small allelic frequency differences among populations at several-to-many genes influencing the trait, will become correlated. The cumulative effects of this correlation produces much greater (positive correlation) or much smaller (negative correlation) differentiation among populations in the trait than in its underlying QTLs. This effectively de-couples the spatial patterns expected at traits and their QTLs. Instead, the QTLs will show spatial arrangements very similar to that of neutral marker loci. Thus, electrophoretic studies will provide information into the spatial distribution of allelic variation underlying traits, even if the distribution of trait values cannot be predicted.

This theory predicts that where genetically isolated populations are selected to the same trait value, negative correlation among QTLs will occur (i.e., alternate alleles at different loci will cancel each other out). This can be detected in crossing experiments by observing excess variation among F2's relative to the difference between the parental lines (known as transgressive segregation). Tests using flowering time and water relations in Avena barbata ecotypes are underway, although early results are equivocal.

Keywords: Local Adaptation, Quantitative Trait Locus, Transgressive Segregation, Avena barbata

This abstract is being presented at: 8:15 AM in session:
Symposium # 26: Conservation of Ecologically Important Variation: Comparing Molecular Markers and Ecological Traits.