Major gene effects during weed evolution:: Phenotypic characters cosegregate with alleles at the ray floret locus in Senecio vulgaris L. (Asteraceae)

Ray floret length was used as a codominant marker to test for associations with phenotypic characters in two F-2 progenies (F(2)A and F2B) derived from crosses between the nonweedy, late-developing Senecio vulgaris subsp, denticulatus and its weedy, early developing derivative subsp. vulgaris var. vulgaris. Variance analytical tests between the three marker genotype classes indicated that the chromosomal region associated with the ray floret locus controlled more than 10% of the phenotypic variation of 9 of the 17 characters analyzed in F2B. Particularly pronounced associations (r(2) = 28-37%) were found between the marker alleles and leaf number, pedicel length, and number of involucral bracts. In addition, formal linkage analysis between the ray floret locus and the previously postulated dominant gene affecting early development was positive (r = 0.15 +/- 0.02). Estimates of dominance relationships at individual quantitative trait loci (QTLs) situated near the ray floret locus indicated directional dominance for a reduction of vegetative and reproductive structures, because the heterozygote marker class tended to be more similar to the weedy derivative, var. vulgaris. This may be interpreted as evidence for past directional selection acting on these characters. Taken together, it is suggested that a single chromosomal region within S. vulgaris exhibits a substantial proportion of nonadditive gene effects, contrasting strongly with the traditional additive-polygenic model of the genetic basis of evolutionary divergence in the wild.