Gynoecium Structure and Evolution in Basal Angiosperms

Abstract

A comparative study on gynoecium structure in basal angiosperms revealed new features relevant to systematics. In addition, based on the new phylogenetic framework on basal angiosperms gained by molecular systematics, an evolutionary evaluation of gynoecial character states has become possible. Angiospermy patterns are diverse and are here tentatively grouped into four types according to the mode of carpel closure: (1) by secretion, without postgenital fusion; (2) with a continuous secretory canal but partial postgenital fusion at the periphery; (3) with an incomplete secretory canal and complete postgenital fusion at the periphery; (4) by complete postgenital fusion. In most basal angiosperms the carpels are free, and the inner space of each carpel is occluded from the outside by secretion and not by postgenital fusion (angiospermy type 1). Correlated with this feature is a pronounced ascidiate shape of the carpels. The carpels tend to be shortly stipitate. In relatively many basal clades with pluricarpellate gynoecia an extragynoecial compitum is present. Stigmas are secretory and tend to have pluricellular papillae. Ethereal oil cells at the surface of the carpels (probably intrusive) are present in Illiciales (including Trimeniaceae), Chloranthaceae, Piperales s.l. (including Aristolochiales), and Acoraceae. Ovule structure is diverse. Ovules are predominantly anatropous. Although Amborellaceae, the phylogenetically earliest branching extant angiosperm, has orthotropous ovules, it is argued that the anatropous condition is basal in angiosperms. Crassinucellar ovules are predominant in basal angiosperms. However, there are apomorphic tendencies toward ovule reduction: weakly crassinucellar (Nymphaeales, some Alismatales), pseudocrassinucellar (Acorales, some Alismatales, some Ranunculales), and tenuinucellar (some Piperales s.l., including Aristolochiales, Rafflesiales, some Alismatales). Lobation of the inner integument and semiannular shape of the outer integument are variously present or absent. They appear to be due to architectural constraints in ovule development and not to constitute archaic features. In contrast to anatropous ovules, orthotropous ovules often have an outer integument that is thinner than the inner one. This may be due to a function of the outer integument in ovule curvature as indicated by molecular developmental genetics. On the other hand, the presence of an outer integument that is thinner than the inner one characterizes the majority of the Piperales s.l. (including Aristolochiales), irrespective of presence or absence of ovule curvature.