Comparative floral morphometrics of distyly and homostyly in three evolutionary lineages of Amsinckia (Boraginaceae)

Using three ancestor-descendant lineages of Amsinckia (Boraginaceae), we studied changes in floral morphology associated with evolution of the breeding system. Each lineage comprised a distylous ancestor and a homostylous descendant: (i) Amsinckia furcata Suksd. - Amsinckia vernicosa Hook. & Arn.; (ii) Amsinckia douglasiana A. DC. - Amsinckia tessellata var. gloriosa (Suksd.) Hoover; and (iii) Amsinckia spectabilis Fischer & C. Meyer. Comparisons of 26 floral traits were made between pins and thrums within the distylous groups, between distyly (pins and thrums combined) and homostyly as well as among pins, thrums, and homostyles. Differences among the morphs were also compared across the three lineages. In distylous flowers, the six traits directly related to stamen height or pistil height varied as expected from their close relationship to the definition of pins and thrums, with the stamen-height-related characters greater in thrums and the pistil-height-related characters greater in pins. Thrums had larger pollen grains in all lineages. Pollen production was either similar in the two distylous morphs or lower in thrums than in pins, depending on species. Thrums also tended to have larger style cross-sectional area and style transmission tissue cross-sectional area. In two of three lineages, thrums had longer corollas, whereas pins exceeded thrums in functional anther-stigma separation and in stigmatic papilla size. The size order of a trait in pins versus thrums was consistent in all lineages for 18 of 26 traits; in seven of the eight remaining traits, A. spectabilis was the unusual lineage. Sepal length, corolla width, and stigma size did not differ significantly between the two distylous morphs. In homostyles, traits related to anther height and pistil height were intermediate between pins and thrums in all lineages; for other traits, including anther-stigma separation and overall size, homostyles generally had the smallest values. For most traits, lineages differed in the degree of differentiation among the three morphs as well as between distyly and homostyly. Thus, in Amsinckia, the evolution of homostyly involves a general reduction in flower size, but by an amount that varies both among traits and among lineages.