Ovary Position Diversity in Saxifragaceae: Clarifying the Homology of Epigyny

Abstract

We discuss the structural homology of the gynoecium and the developmental variations associated with ovary diversity in Saxifragaceae. Diverse ovary positions are present within most of the major clades in the family. We demonstrate that superior ovaries, although widely reported for various members of the family, are unusual and limited to a few species of Micranthes and Saxifraga and to male flowers of the dioecious Tanakaea among investigated genera. Most species reported to have superior ovaries actually have developmentally epigynous flowers in which the ovary has a small portion below the insertion of the perianth and androecium. Such ovaries of epigynous flowers that mimic superior ovaries are termed “pseudosuperior.” Labile shifts in proportioned growth of that part of the ovary above the insertion of the perianth and androecium to that below their insertion have created extensive variation in ovary position among closely related taxa, e.g., in both Saxifraga and Micranthes. The heterostylous Jepsonia has differences in ovary position among the different style‐length morphs, and similar variation in ovary position may be present in the dioecious Tanakaea. Epigynous flowers of Saxifragaceae fit the model of appendicular epigyny. During the organogenetic phase of early floral development, they exhibit peripheral upgrowth of the apical region, which forms a central concavity. The initial gynoecial primordium forms on the inner slope of the concavity, zonal growth in the concave flank subjacent to the initial gynoecial primordium elongates, and subsequently, the tissue lining the concave flank below the initial gynoecial primordium differentiates as ovary.