Agradecimentos: C.E.B.P. wrote the paper. Species circumscription and descriptions were jointly constructed by C.E.B.P. and A.C.T., and anatomical descriptions were led by the latter author. I.R.C. and E.R.F.-M. were major contributors to discussion on cytogenetics. Sampling in the field was done by J.E.Q.F., I.R.C., C.E.B.P. and V.G.S. Modus operandi in the lab, experimental design and analysis and interpretation of molecular results had significant input from T.N.C.V., E.J.L., P.W.I., L.R.M. and P.S.d-C. Most sequences were generated by I.R.C., with significant contributions from C.E.B.P., V.G.S. and T.N.C.V.. All authors contributed with comments and suggestions that improved the final draft of the manuscript. We thank the photographers that kindly contributed images (cited by name in the legends) to Figs 4 and 5, Maria Rosa Zanatta for assistance in improving the illustrations, and Stephen A. Harris for reviewing an earlier draft of this manuscript. This work was supported by: CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico - Brazil) field work and laboratory work by I.R.C., V.G.S. and C.E.B.P. [Project Estudos filogenéticos e macroecológicos voltados à conservação do gênero Psidium L. (Myrtaceae Juss. – 2011-2015, REFLORA Program)], PhD grants to C.E.B.P., I.R.C. and A.C.T., and Produtividade em Pesquisa-PQ2 Research grants to C.E.B.P. and E.R.F.-M.; KLARF (Kew Latin American Research Fellowship) doctoral grant to IRC; CAPES (Coordenação de Aperfeiçoamento de Pessoal do Ensino Superior – finance code 001) PhD grants to A.C.T., T.N.C.V. and V.G.S., and a post-doctoral grant to I.R.C. at the Jodrell Laboratory, Royal Botanic Gardens, Kew; FAPESP (Fundação de Amparo à Pesquisa de São Paulo – Brazil), MSc grant to I.R.C. and a Research grant to E.R.F.-M. and I.R.C.; FAPES/VALE/FAPERJ (Fundação de Apoio à Pesquisa do Espírito Santo/Vale S.A./Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro – Brazil) field work, herbarium and laboratory work to A.C.T. (Project Identificação e Caracterização de Espécies de Psidium da Mata Atlântica de Interesse Econômico); The Andrew T. Mellon Foundation, a post-doctoral grant to C.E.B.P. for herbarium-based work; The Emily Holmes Memorial Foundation, field work and laboratory work at the Jodrell Laboratory, Royal Botanic Gardens Kew to T.N.C.V.; and FAPDF (Fundação de Apoio à Pesquisa do Distrito Federal – Brazil), laboratory work at the Jodrell Laboratory, Royal Botanic Gardens Kew to C.E.B.P Ver menos

Abstract: Psidium is the fourthth largest genus of Myrtaceae in the Neotropics. Psidium guajava is widely cultivated in the tropics for its edible fruit. It is commercially under threat due to the disease guava decline. Psidium cattleyanum is one of the 100 most invasive organisms in the world. Knowledge of the phylogenetic relationships within Psidium is poor. We aim to provide a review of the biology, morphology and ecology of Psidium, a phylogenetic tree, an infrageneric classification and a list of species. Morphological and geographic data were obtained by studying Psidium in herbaria and in the field between 1988 and 2020. Forty-six herbaria were visited personally. A database of approx. 6000 specimens was constructed, and the literature was reviewed. Thirty species (about a third of the species in the genus) were sampled for molecular phylogenetic inference. Two chloroplast (psbA-trnH and ndhF) and two nuclear (external transcribed spacer and internal transcribed spacer) regions were targeted. Phylogenetic trees were constructed using maximum likelihood (ML; RaxML) and Bayesian inference (BI; MrBayes). Psidium is a monophyletic genus with four major clades recognized as sections. Section Psidium (ten species), to which P. guajava belongs, is sister to the rest of the genus; it is widespread across the Neotropics. Section Obversifolia (six species; restricted to the Brazilian Atlantic Forest), which includes P. cattleyanum, is sister to the innermost clade composed of sister sections Apertiflora (31 species; widespread but most diverse in the Brazilian Atlantic Forest) + Mitranthes (26 species; widespread in dry forests and probably diverse in the Caribbean). Characters associated with diversification within Psidium are discussed. Research on pre-foliation, colleters, leaf anatomy, leaf physiology, staminal development, placentation and germination associated with the anatomy of the opercular plug is desirable. Studies are biased towards sections Psidium and Obversifolia, with other sections poorly known Ver menos