Crowea

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 : 375

Relationships of Crowea , Eriostemon and Philotheca section Corynonema

The placement of Crowea , Eriostemon and Philotheca section Corynonema in a highly supported clade (Clade 4A), together with strong support for relationships in that clade, improved on previous understanding of the ptDNA relationships of these taxa. Duretto et al. (2023) also resolved these three taxa in a well-supported clade in two of their analyses, but found somewhat conflicting relationships between the taxa according to different datasets; they found weak support for a sister relationship between Eriostemon and Crowea in their combined nrDNA–ptDNA phylogeny, weak support for a sister relationship between Philotheca section Corynonema and Crowea in their ptDNA phylogeny, and the taxa were not resolved in a clade in their nrDNA phylogeny. We have not identified obvious morphological synapomorphies that unite all three groups, but several morphological studies have proposed a sister relationship between Crowea and Eriostemon that is incongruent with our placement of Eriostemon (and Crowea angustifolia Sm. ; see next paragraph for discussion of that species) as sister to Philotheca section Corynonema ( Armstrong 1991; Bayly 2001). Crowea and Eriostemon differ from the rest of the Eriostemon group in having petals with at least three main veins (rather than one main vein) originating from their base ( Bayly 2001). Armstrong (1991) also considered Crowea and Eriostemon united by staminal filaments that are arranged pyramidally over the ovary at anthesis, although this was questioned by Bayly (2001), who argued that the feature is not as striking in Eriostemon , and that staminal arrangement in Eriostemon is not dissimilar to some species of Philotheca (= Eriostemon section Nigrostipulae Paul G.Wilson ). In addition, chromosome numbers for Crowea (n = 19) and Eriostemon (n = 17) are unique in the Eriostemon group, where n is most commonly 14 ( Asterolasia , Geleznowia , Drummondita , Muiriantha , Philotheca sectin Philotheca , P. section Erionema , P. section Cyanochlamys , some Diplolaena ) or 16 ( Correa , Leionema , Nematolepis , Phebalium ) ( Smith-White 1954; Armstrong 1991; Stace and Armstrong 1992; Bayly 2001). Chromosome numbers for species in Philotheca section Corynonema are unknown, and future cytological investigation of this group would provide valuable insight into how well the cytology of this section aligns with Crowea and Eriostemon .

Our recovery of Crowea as polyphyletic in ptDNA analyses is perhaps not surprising. Duretto et al. (2023) found support for the monophyly of Crowea to vary between nrDNA and plastid datasets, with ptDNA suggesting that the genus may be non-monophyletic because of the only western Australian species, C. angustifolia (using different accessions to the current study), forming a polytomy with the eastern Australian Crowea and Eriostemon . Our results have improved the clarity of ptDNA relationships in this group and confirmed that ptDNA suggests Crowea is non-monophyletic with the placement of C. angustifolia as sister to Eriostemon with high support. However, our phylogenetic analyses of nrDNA sequences (see the Supplementary ‘Methods and results’ section) mirror the findings of Duretto et al. (2023) and suggest the genus is monophyletic. Such cytonuclear discrepancy may arise from one, or a combination of, incomplete lineage sorting (ILS), horizontal gene transfer and organellar genome capture ( Rieseberg and Soltis 1991; Tsitrone et al. 2003; Toews and Brelsford 2012). Given that both Eriostemon species occur only in eastern Australia, it is unlikely that their sister placement to C. angustifolia in ptDNA trees is the result of the last two processes, as they require gene flow between the genera (or their common ancestors) to occur; on the basis of the current distribution of taxa, it is simpler to infer that there has been no historical reconnection between C. angustifolia and Eriostemon , rather than infer that there has been exclusive connectivity between the two (across the whole of Australia) in the absence of gene flow with the other species of Crowea . Although we cannot rule out the latter scenario, we feel a more likely explanation involves ILS of the plastome of C. angustifolia , because this matches the simpler scenario outlined above and is consistent with ILS having a greater influence on plastid markers due to larger effective population sizes, and thus longer coalescence times than for nrDNA markers that are subject to concerted evolution ( Buckler and Holtsford 1996; Clowes et al. 2022). Further work is required to deduce the cause of this discordance.