The geometry of gender: hyper-diversification of sexual systems in Urtica L. (Urticaceae)

Urtica L. (Urticaceae) is generally reported as a genus of monoecious and dioecious taxa. However, the gender information found in the literature does not at all reflect the actual diversity of gender patterns in Urtica. Dioecy appears to be truly absent from Urtica, but otherwise there has been a major diversification in the geometry of gender and no comparable patterns exist in other plant groups. Thus, we here define technical terms for all unique architectural types of monoecy found in Urtica and closely related genera and reconstruct the ancestral gender states in a Bayesian framework. Our studies are based on a near-comprehensive sampling, including 61 of the 63 Urtica species recognized. We report polygamy, two types of gynodioecy and five different architectural types of monoecy. A total of 15 switches appear to have taken place within the genus. Although gender characteristics have diversified strongly, they are relatively conserved within clades. Monoecy is the predominant sexual system within Urtica and specifically basiandrous monoecy (i.e. basal inflorescence branches of each individual male only, apical branches female) is the most widespread type, reported for 11 different clades. In particular, it characterizes the basally branching pilulifera-clade and the sister genus Zhengyia, and may thus represent the plesiomorphic condition for Urtica. Gender distribution and gross morphology appear to evolve largely independently from each other and gender distribution is largely independent of growth habit. However, polygamous taxa are most common amongst rhizomatous perennials (one-third of the taxa).