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Phylogeny of Tribe Myrteae (Myrtaceae)

Phylogenetic tree of Myrteae and distribution map of the tribe against Rhodomyrtus tomentosa.

Myrtaceae has recently been reclassified into fifteen lineages, recognised as tribes. Molecular phylogenetic studies carried out at Kew over the last five years have focused on the largest tribe – the fleshy fruited and primarily Neotropical tribe Myrteae, the tribe in which baseline taxonomy is most lacking.  Due to high morphological variation at all taxonomic levels, Myrtaceae in the Neotropics has a reputation as being a ‘difficult’ family to identify even to the generic level. The subtribal classification of Myrteae is unstable as are generic boundaries within it, the number of undescribed species is high, their description often frustrated by the difficulty in confidently assigning a specimen to genus. Neotropical Myrtaceae are particularly speciose in some of South America’s most threatened habitats, with species diversity reaching its peak in the Brazilian Atlantic rainforest and cerrado, habitats decreasing rapidly under human pressure. Only c.5% of the Atlantic forest remains, with these fragments containing up to 90 species of Myrtaceae per ha., many of which are undescribed. Because of the importance of Myrtaceae and the degree of skill required to identify its species, a complete ecological description of the areas in which it is so diverse depends upon a taxonomic revision of the family that in turn depends on a firm phylogenetic framework on which it can be based. Such a revision must be undertaken as soon as possible as many of these habitats are disappearing at an unprecedented rate. 

The objective of the Myrteae phylogeny project is to clarify internal relationships within the tribe and to provide a phylogenetic hypothesis for evolutionary relationships between as many as possible of the 48 genera recognised within it. These studies rely on collaboration between the University of São Paulo (Brazil), the University of Otago (New Zealand), RBG Sydney (Australia), and RBG Kew. 

Between 2001-2005, c.130 Myrteae species representing 30 genera have been sequenced for the gene regions ITS, ETS, psbA-trnH and matK at the Jodrell laboratory. Preliminary results focusing on subtribal relationships were published in 2005 and have confirmed the paraphyly of subtribe Myrtiinae while revealing the additional paraphyly of the Eugeniinae, the monophlyly of the Myrciinae s.s. excluding Myrceugenia and the monophyly of a group of previously hard-to-place genera, Myrceugenia, Luma and one species of Blepharocalyx.

Morphological studies in tribe Myrteae have been carried out in parallel to molecular research, macromorphological characters have been surveyed to find suitable morphological characters with which to interpret the phylogeny of tribe Myrteae and character evolution within it. The phylogeny is to be dated using non parametric rate smoothing and subjected to biogeographical analysis using a variety of methodologies. The completed molecular-based phylogeny of tribe Myrteae has been submitted for publication. 

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