Perianth evolution and implications for generic delimitation in the Eucalypts (Myrtaceae): DNA sequences, morphological data

<p><em>Eucalyptus</em> was traditionally defined by the operculate perianth—hence the generic name (Latin, meaning "well-covered"). But after previous phylogenetic analysis placed <em>Angophora</em>, which has free sepals and petals, as sister to the bloodwood eucalypts, the latter were segregated into a new genus, <em>Corymbia</em>. We made a targeted capture of 101 low-copy nuclear exons from 392 samples representing 329 species-level taxa. The phylogeny was estimated using maximum likelihood (IQtree and RAxML) and the multi-species coalescent (Astral). We tested alternative relationships between four genera within Eucalypteae (<em>Arillastrum</em>, <em>Angophora</em>, <em>Eucalyptus</em>, <em>Corymbia</em>) at each of two nodes critical to generic delimitation using Shimodaira's Approximately Unbiased (AU) test. Monophyly of <em>Arillastrum</em> + (<em>Corymbia</em> + <em>Angophora</em>) relative to <em>Eucalyptus</em> sensu stricto was supported whereas monophyly of <em>Corymbia</em> relative to <em>Angophora</em> was decisively rejected. These results indicate that either <em>Eucalyptus</em> should be expanded to include all four genera or <em>Corymbia</em> should be split into two. All of the alternative relationships among the four currently recognised genera imply homoplasy in perianth evolution, specifically with respect to origins of the bud cap (operculum or calyptra), which has been traditionally used to define <em>Eucalyptus</em>. Inferred evolutionary transitions in perianth traits are generally congruent with divergences between major clades with a single exception: expression of separate sepals and petals in <em>Angophora</em>, which is nested within the operculate genus <em>Corymbia</em>, appears prima facie to be a reversal to the plesiomorphic perianth structure. Strictly, this is not a reversal because the petals of <em>Angophora</em> and <em>Corymbia</em> have a novel compound keel-and-limb structure that is absent in the outgroups. This structure is evident in early development, irrespective of whether the petals remain free or later become part of an operculum. Many of the currently recognised infrageneric taxa down to sectional level (and below in some cases) are well-supported by the sequence data and definable by morphological traits. Inclusion of <em>Angophora</em> within <em>Eucalyptus</em> was formally proposed two decades ago but did not gain acceptance. Here instead, we formally raise <em>Corymbia</em> subg. <em>Blakella</em> to genus rank and make the relevant new combinations.</p>