Philotheca (sensu Wilson, 1971)

Orel, Harvey K., McLay, Todd G. B., Neal, Will C., Forster, Paul I. & Bayly, Michael J., 2023, Plastid phylogenomics of the Eriostemon group (Rutaceae; Zanthoxyloideae): support for major clades and investigation of a backbone polytomy, Australian Systematic Botany 36 (5), pp. 355-385 : 381

Future classification of Philotheca

The recovery of Philotheca as polyphyletic in our analyses is consistent with previous work ( Bayly et al. 2013; Appelhans et al. 2021; Duretto et al. 2023), and here we have provided further clarification of this taxonomic issue. Prior to the current study, the best estimate of relationships in Philotheca was provided by Duretto et al. (2023). Our analyses and that of Duretto et al. (2023) both suggest that Philotheca section Corynonema and Philotheca section Cyanochlamys are phylogenetically distant from the type of the genus ( P. salsolifolia ). Undoubtedly, they should be placed in separate genera from Philotheca and each other. However, in the case of P. section Cyanochlamys , contrasting support for monophyly (ptDNA; Fig. 3) v. paraphyly (nrDNA; see the Supplementary ‘Methods and results’ section) in our analyses raises questions about the appropriate taxonomic status of that group relative to the related, monotypic genus Muiriantha .

Within Clade 1, we have identified three ptDNA lineages of Philotheca species that render Philotheca paraphyletic with respect to Drummondita and Geleznowia , namely the following: (1) Philotheca section Erionema , supported as

monophyletic and sister to the rest of the clade; (2) Philotheca s.str., not resolved as monophyletic but strongly placed in a clade with Drummondita ; and (3) Philotheca ‘Nigrostipulae’, supported as monophyletic and sister to Geleznowia . Taxonomic interpretation of these relationships could include ‘lumping’ (e.g. placing all of Clade 1 in a single genus, Philotheca ) or ‘splitting’ to various extents. Given that many of the species in this clade have been sampled only for a small number of plastid markers ( Fig. 4), and that nrDNA sampling here (see the Supplementary ‘Methods and results’ section) and by Duretto et al. (2023) is very limited and does not resolve distinct nrDNA groups corresponding to Philotheca s.str. and Philotheca ‘ Nigrostipulae ’, some aspects of relationships in the group remain uncertain.

Given limitations of the results presented here, and the new questions they have raised, we consider that further investigation of the relationships of Philotheca is warranted before taxonomic changes are proposed. In particular, sequencing approaches that generate large amounts of data from the nuclear genome, such as using the MyBaits Expert Plant Angiosperms353 bait set (Arbor Bioscience, Ann Arbor, MI, USA; Johnson et al. 2019) for targetsequence capture ( H. K. Orel, unpubl. data), could prove useful for elucidating unresolved relationships, because of the faster evolutionary rates of nuclear genes. Such data would also enable comparison with the ptDNA results presented here so as to evaluate the extent of cytonuclear discordance in this group.