Reproductive structure and development in Griselinia littoralis (Griseliniaceae) with a focus on its pseudomonomerous gynoecium: implications for early floral evolution in Apiales

The asterid order Apiales currently includes seven families, of which Pennantiaceae, Torricelliaceae, and Griseliniaceae are successively branching lineages forming a basal grade of the order. All members of these families are characterized by pseudomonomerous gynoecia with one fertile and mostly two sterile carpels. Recent studies have shown that two of these families, Pennantiaceae and Torricelliaceae, have fundamentally similar gynoecia, with the ovule being attached at the border between the ascidiate and plicate zones (in the so-called cross zone) of the fertile carpel. Data on the gynoecium structure of the monogeneric Griseliniaceae remained insufficient for accurate comparisons with the other two families. Here we provide the first detailed description of inflorescence and flowers, including floral vasculature and development, in a member of Griseliniaceae (Griselinia littoralis). We focused on the functionally female flowers of this dioecious species. We observed that its gynoecium is usually initiated as three primordia, with the primordium in the petal radius often being the smallest. In flower vasculature, two of the three groups of ventral bundles reach the ovule by bending around the ovary locule inwardly from the other bundles in the ovary wall. We conclude that these bundles belong to the septa of the ancestrally trilocular ovary, in which two locules have been completely lost. Therefore, the single locule of G. littoralis is formed by the ascidiate zone of the fertile carpel, with the single ovule attached in its cross zone. We show that all the examined members of the basal grade of Apiales share the same gynoecium ground plan, including the location of the fertile carpel in a petal radius. Our data support pseudomonomery as an ancestral condition for the gynoecium in Apiales, which implies a subsequent reversal to carpel monomorphism in the species-rich core group of Apiales. We also found that Griselinia may be unusual among angiosperms in possessing two contrasting types of deviations from axillary branching (concaulescence and recaulescence).