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

Eucalyptus was traditionally defined by the operculate perianth—hence the
generic name (Latin, meaning "well-covered"). But after previous
phylogenetic analysis placed Angophora, which has free sepals and petals,
as sister to the bloodwood eucalypts, the latter were segregated into a
new genus, Corymbia. 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 (Arillastrum, Angophora,
Eucalyptus, Corymbia) at each of two nodes critical to generic
delimitation using Shimodaira's Approximately Unbiased (AU) test.
Monophyly of Arillastrum + (Corymbia + Angophora) relative to Eucalyptus
sensu stricto was supported whereas monophyly of Corymbia relative to
Angophora was decisively rejected. These results indicate that either
Eucalyptus should be expanded to include all four genera or Corymbia
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 Eucalyptus.
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 Angophora, which is nested
within the operculate genus Corymbia, appears prima facie to be a reversal
to the plesiomorphic perianth structure. Strictly, this is not a reversal
because the petals of Angophora and Corymbia 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 Angophora within Eucalyptus was
formally proposed two decades ago but did not gain acceptance. Here
instead, we formally raise Corymbia subg. Blakella to genus rank and make
the relevant new combinations.