A revised delimitation of the species‐rich genus Pilea (Urticaceae) supports the resurrection of Achudemia and a new infrageneric classification
Authors: Fu Long‐Fei , Wen Fang , Maurin Olivier , Rodda Michele , Gardner Elliot M. , Xin Zi‐Bing , Wei Yi‐Gang , Monro Alexandre K. 
Published: March 2022

Ensuring that genera are monophyletic should result in a stable classification that better reflects evolutionary history, and which can serve as a framework for answering broader scientific questions. It is also important that species can be assigned to genera easily and unambiguously so as to remove barriers to identification and species discovery. Pilea, with 933 published names and likely more than 700 spp. is the largest genus within the Urticaceae and was last monographed in 1869. Whilst the monophyly of the genus has been proposed by previous authors, this has been based on incomplete taxon sampling that has failed to resolve the position of key taxa. We aimed to generate a robust phylogeny for Pilea and allied genera that could test the monophyly of the genus and provide a framework for a taxonomic revision of this species‐rich yet poorly studied genus. To do so, we sought to sample taxa encompassing previous infrageneric classifications and species with anomalous inflorescences or flower morphology and to use the resulting phylogeny to evaluate the delimitation of Pilea and to establish an infrageneric classification. In addition, we included a representative of the Polynesian genus Haroldiella, which, morphologically, seemed indistinguishable from Pilea. Using two plastid and one nuclear region, we constructed phylogenies using Bayesian inference, maximum likelihood and maximum parsimony approaches. We used our BI phylogeny to evaluate the informativeness of 22 morphological characters and used both to delimit monophyletic groups and infrageneric sections. Our results recovered Pilea as paraphyletic, a consequence of the recovery of a monophyletic clade comprising Lecanthus and Pilea sect. Achudemia and sect. Smithiella, neither of which had been adequately sampled in previous studies. We also recovered Pilea as paraphyletic with respect to Haroldiella. Pilea (including Haroldiella) was recovered as sister to the clade comprising Lecanthus and Pilea sect. Achudemia and sect. Smithiella. We identified isomery between male and female flowers, flower part number, male sepal arrangement and achene ornamentation as being phylogenetically informative characters that can be used to distinguish Achudemia (Pilea sect. Achudemia and sect. Smithiella) from Pilea. We also identified a trend of decreasing merism and fruit size, correlated with an increase in species‐richness across the phylogeny which our results suggest may indicate a shift in pollen and fruit dispersal, pollen dispersal becoming more kinetic and fruit dispersal less reliant on animals. On the basis of our evaluation of both morphological characters and phylogenetic relationships we propose a new infrageneric classification for the genus comprising eight sections, five of which we propose for the first time here: Pilea sect. Angulatae, sect. Plataniflorae, sect. Tetraphyllae, sect. Trimeris and sect. Verrucosae.