Abstract

Some of the most consistent morphological differences between organisms at higher taxonomic levels involve differences in the size and shape of organs. We have identified the genetic basis for such differences between Arabidopsis ecotypes and between species of snapdragon (Antirrhinum) that are adapted to very different habitats. In Arabidopsis we find that leaf and petal size differences are determined by a large number of genes of relatively small effects and that each ecotype carries a combination of ‘small’ and ‘large’ genes, suggestive of stabilising selection, even in the flowers of this predominantly self-pollinating species. Sampling of Arabidopsis populations from Edinburgh suggests significant migration, outcrossing and local adaptation in the wild.

In contrast, we find that organ shape and size differences between Antirrhinum species are determine by relatively few genes of more major effect, that the genes affect either cell division or cell expansion and that genes affecting organ size also affect shape – i.e. act allometrically. Some genes affect only leaves or floral organs while others act in both, raising the possibility of selection having acted independently on flower size (e.g. in co-evolution with pollinators) and on leaf size (e.g. in response to water use efficiency).

To interpret these findings in an evolutionary context, we have attempted to construct a phylogeny for the genus Antirrhinum. Although we find strong support for ~20 distinct species, we have so-far been unable to resolve the relationships between them. The results are consistent with rapid recent speciation, possibly following hybridisation within glacial refugia.

http://www.biology.ed.ac.uk/research/institutes/plant/pages/staff_pages/A_Hudson_staffpage.htm