Academic journal article Genetics

Genetic Consequences of Selection during the Evolution of Cultivated Sunflower

Academic journal article Genetics

Genetic Consequences of Selection during the Evolution of Cultivated Sunflower

Article excerpt

ABSTRACT

We mapped quantitative trait loci (QTL) controlling differences in seed oil content and composition between cultivated and wild sunflower and used the results, along with those of a previous study of domestication-related QTL, to guide a genome-wide analysis of genetic variation for evidence of past selection. The effects of the seed oil QTL were almost exclusively in the expected direction with respect to the parental phenotypes. A major, oil-related QTL cluster mapped near a cluster of domestication-related QTL on linkage group six (LG06), the majority of which have previously been shown to have effects that are inconsistent with the parental phenotypes. To test the hypothesis that this region was the target of a past selective sweep, perhaps resulting in the fixation of the antagonistic domestication-related QTL, we analyzed simple sequence repeat (SSR) diversity from 102 markers dispersed throughout the sunflower genome. Our results indicate that LG06 was most likely the target of multiple selective sweeps during the postdomestication era. Strong directional selection in concert with genetic hitchhiking therefore offers a possible explanation for the occurrence of numerous domestication-related QTL with apparently maladaptive phenotypic effects.

THE derivation of crop plants from their wild ancestors has typically involved rapid phenotypic evolution in response to strong directional selection (HARLAN 1992). Biologists dating back at least as far as DARWIN (1859) have argued that these dramatic, humanmediated transformations provide a model for studying phenotypic evolution. This is due, in part, to the fact that studies of evolution under domestication often enjoy a historical backdrop that is unavailable to many investigators studying phenotypic divergence in the wild. Not only do we know the types of traits that were likely under selection during crop domestication and improvement, but also the timescale over which this evolution occurred is often well documented. These factors, when combined with the recent development of genetic tools for the analysis of many domesticated plants and animals, translate into unique opportunities for studying the genetic and phenotypic consequences of strong directional selection.

One approach that has been widely applied to questions about the genetics underlying phenotypic divergence has been quantitative trait locus (QTL) mapping. While QTL mapping has produced a tremendous amount of information on the genetic architecture of trait differences, this approach is largely agnostic regarding the evolutionary forces causing these differences. The one exception to this is the QTL sign test of ORR (1998), which uses data on the direction of QTL effects to detect the footprint of directional selection. Unfortunately, the utility of this test is closely associated with the number of loci detected. As such, the QTL sign test is of little use for traits with relatively few detectable QTL.

A complementary approach to QTL mapping is to use population genetic data to identify regions of the genome that harbor selectively important genes. This idea, which was first proposed by CAVALLI-SFORZA (1966), was later formalized by LEWONTIN and KRAKAUER (1973), who used variation in the inbreeding coefficient across loci in an attempt to detect selection. Although this approach was subsequently criticized on a variety of grounds (e.g., NEI and MARUYAMA 1975; ROBERTSON 1975a,b), the underlying logic remains valid. While the effects of migration, inbreeding, and genetic drift are manifested throughout the genome, selection acts in a locus-specific manner. Thus, selective sweeps can reduce genetic variation at both the target locus and the linked neutral loci while leaving the remainder of the genome unaffected (MAYNARD-SMITH and HAIGH 1974; SLATKIN 1995). Since LEWONTIN and KRAKAUER'S (1973) initial publication on the subject, a number of authors have proposed improved methods for using population genetic data to detect selection (e. …

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