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

Using three ancestor-descendant lineages of Amsinckia (Boraginaceae), we st udied changes in floral morphology associated with evolution of the breedin g system. Each lineage comprised a distylous ancestor and a homostylous des cendant: (i) Amsinckia furcata Suksd. - Amsinckia vernicosa Hook. & Arn.; ( ii) Amsinckia douglasiana A. DC. - Amsinckia tessellata var. gloriosa (Suks d.) 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 a s among pins, thrums, and homostyles. Differences among the morphs were als o 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 sta men-height-related characters greater in thrums and the pistil-height-relat ed characters greater in pins. Thrums had larger pollen grains in all linea ges. Pollen production was either similar in the two distylous morphs or lo wer in thrums than in pins, depending on species. Thrums also tended to hav e larger style cross-sectional area and style transmission tissue cross-sec tional area. In two of three lineages, thrums had longer corollas, whereas pins exceeded thrums in functional anther-stigma separation and in stigmati c papilla size. The size order of a trait in pins versus thrums was consist ent in all lineages for 18 of 26 traits; in seven of the eight remaining tr aits, A. spectabilis was the unusual lineage. Sepal length, corolla width, and stigma size did not differ significantly between the two distylous morp hs. In homostyles, traits related to anther height and pistil height were i ntermediate between pins and thrums in all lineages; for other traits, incl uding anther-stigma separation and overall size, homostyles generally had t he smallest values. For most traits, lineages differed in the degree of dif ferentiation among the three morphs as well as between distyly and homostyl y. Thus, in Amsinckia, the evolution of homostyly involves a general reduct ion in flower size, but by an amount that varies both among traits and amon g lineages.