EVOLUTION OF TRICHOME NUMBER IN A NATURALIZED POPULATION OF BRASSICA-RAPA

We tested the hypothesis that high trichome numbers provide resistance to herbivores and increase fruit production in a naturalized population of Brassica rapa and used greenhouse-grown paternal and maternal sibships to estimate the genetic variances and covariances for trichome number, days to first flower, and flower production. We performed one generation of directional selection for low and high trichome number and scored trichome number, herbivore damage, flowering start, and fruit production of progeny planted in the field. The artificial selection resulted in significant divergence in both trichome number and flowering start. Herbivory was low, and there was no difference in herbivore damage between lines at the end of the season. High-trichome plants produced significantly more fruits than low-trichome plants, apparently because of the advantage of later flowering by high-trichome plants. The greenhouse study revealed significant genetic variation for trichome number, flowering start, and total flower number. The genetic correlations between trichome number and flowering start (positive) and between trichome number and total flower number (negative) were weak. We suggest that the high genetic variation for trichome number in this population is a function of both weak direct selection on this character and the negligible genetic and phenotypic costs of trichome production.