Developmental morphology of female flowers of Gyrostemon and Tersonia and Floral evolution among Gyrostemonaceae

Abstract

Floral simplifications and specializations in the evolution of Gyrostemonaceae have confused the systematics of the family. Recent phylogenetic analyses have demonstrated their placement among Capparales. This investigation presents a phylogenetic analysis of Gyrostemonaceae, demonstrating that Codonocarpus and Gyrostemon form a clade that is the sister group of Cypselocarpus, Tersonia, and Walteranthus. These phylogenetic results and data on development of Gyrostemon and Tersonia are used to discuss the morphology and evolutionary diversification of female flowers of Gyrostemonaceae. The uniseriate perianth of Gyrostemonaceae consists of four to eight tepals with an unusual lateral to median developmental sequence. The female flowers of Gyrostemon and Tersonia display no distinctive evidence of an androecium, although the former has late forming, primordium-like structures positioned between the tepals and gynoecium that may be the vestiges of either a second perianth series or the androecium. The gynoecium of Gyrostemonaceae is syncarpous, although the two main clades in the family differ in the expression of ovarian synorganization. The Codonocarpus- Gyrostemon clade is unusual in having largely separate carpels that are only syncarpous because the ventral side of each is formed by the flank of the floral apex. All Gyrostemonaceae, however, incorporate the flank of the floral apex as the ventral side of the carpel, and this is the location of ovule development. On the basis of its placement in a clade that includes Tersonia and Walteranthus, the uniloculate and uniovulate gynoecium of Cypselocarpus may be pseudomonomerous. All Gyrostemonaceae have large stigmas that are typical of anemophilous taxa, and they differ from most other Capparales in this attribute. Among Capparales, Gyrostemonaceae may be most similar to Ochradenus (Resedaceae), which also appears to be anemophilous. It is unclear whether the similarities of Ochradenus and Gyrostemonaceae are homologies, indicative of a close relationship between the two groups, or evolutionary parallelisms associated with separate shifts to anemophily.