Structural homology and developmental transformations associated with ovary diversification in Lithophragma (Saxifragaceae)

Lithophragma, comprising only ten species, encompasses a remarkable diversity of ovary positions, reported to range from inferior to superior. The structural homology of the gynoecium and developmental transformations associated with ovary diversification are investigated for Lithophragma. Scanning electron and light microscopy indicate that all species of Lithophragma have epigynous flowers. Lithophragma campanulatum, L. glabrum, and L. heterophyllum have ovaries that externally appear nearly superior, but are actually shallowly inferior or "pseudosuperior." The inferior ovaries of Lithophragma species can be conceptually divided into superior and inferior regions that meet at the point of perianth and androecial insertion. Static and ontogenetic allometry reveal that across the species of Lithophragma the lengths of these two ovary regions are coordinated. Ovary regions in mature flowers display an approximately linear relationship that can be expressed through the allometric equation SL = -0.5314 IL + 2.0348 (where SL and IL are the lengths of the superior and inferior regions of the ovary, respectively; r2= 0.7683, df = 35, P = 2.45 x 10-12). Mapping ontogenetic allometries onto a recent phylogeny for Lithophragma shows that ovary position evolution is bidirectional and has shifted toward greater superiority in some species and greater inferiority in others.