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Molecular phylogeny and morphological perianth evolution in Corymbia (Myrtaceae), and the implications for generic delimitation: data and tree files

Abstract

Premise: Eucalypts (Myrtaceae tribe Eucalypteae) are currently placed in seven genera. Traditionally, Eucalyptus was defined by its operculum but when phylogenies placed Angophora, with free sepals and petals, as sister to the operculate bloodwood eucalypts, the latter were segregated into a new genus, Corymbia. Yet generic delimitation in the tribe Eucalypteae remains uncertain. Here we address these problems using phylogenetic analysis with the largest molecular dataset to date.

Methods: We captured 101 low-copy nuclear exons from 392 samples representing 266 species. Our phylogenetic analysis used maximum likelihood (IQtree) and multi-species coalescent (Astral). At two nodes critical to generic delimitation, we tested alternative relationships among Arillastrum, Angophora, Eucalyptus and Corymbia using Shimodaira's AU test. Phylogenetic mapping was used to explore the evolution of perianth traits.

Results: Monophyly of Corymbia relative to Angophora was decisively rejected. All alternative relationships among the seven currently recognised Eucalypteae genera imply homoplasy in evolutionary origins of the operculum. Inferred evolutionary transitions in perianth traits are congruent with divergences between major clades except that expression of separate sepals and petals in Angophora, which is nested within the operculate genus Corymbia, appears to be a reversal to the plesiomorphic perianth structure.

Conclusions: Here we formally raise Corymbia subg. Blakella to genus rank and make the relevant new combinations. We also define and name three sections within Blakella (B. sect. Blakella, B. sect. Naviculares and B. sect. Maculatae), and two series within Blakella sect. Maculatae (B. ser. Maculatae and B. ser. Torellianae). Corymbia is reduced to the red bloodwoods.