Structure and development of female flowers in early-diverging restiids, Anarthria, Lyginia and Hopkinsia (Restionaceae s.l.): further evidence of multiple pathways of gynoecium reduction in wind-pollinated lineages of Poales

Abstract

Anarthria, Lyginia and Hopkinsia, endemic to south-western Western Australia, differ from the rest of Restionaceae sensu APG IV in the presence of dithecal rather than monothecal anthers. Dithecal and monothecal Restionaceae form two well-supported sister clades in molecular phylogenetic trees, but shared morphological features of Anarthria, Lyginia and Hopkinsia known so far are either symplesiomorphies (e.g. anther structure) or are characters that are homoplastic in monocots (e.g. absence of silica in culms). The taxonomic placement of the three genera has been controversial during the 20th century, including the idea of recognizing three monogeneric families. As comparative morphological data covering all three genera are limited, we provide a study of female flower structure and development in this group. This is the first developmental study of dithecal Restionaceae that uses scanning electron microscopy. With many more investigated characters, like earlier studies, we found no potential synapomorphies for the dithecal clade. However, we found the first morphological support for the conclusion from molecular data of a sister relationship between Hopkinsia and Lyginia. They share ovules with length not exceeding the width and the micropyle oriented towards the dorsal side of the carpel, which is probably a synapomorphy. Some other characters shared by the two genera could be plesiomorphic, such as inner whorl tepals that are shorter than the gynoecium throughout development. Contrary to some earlier reports, female flowers of all three genera are arranged in spikelets and do not have bracteoles. These characters are thus conserved across the restiid and graminid clades. The gynoecium of Hopkinsia is unicarpellate and not pseudomonomerous. Gynoecia with a single fertile ovule appeared several times and through different morphological pathways in the evolution of wind-pollinated Poales.