Relationships of Philotheca section Cyanochlamys and Muiriantha

The placement of Philotheca section Cyanochlamys sister to Muiriantha in Clade 2 is consistent with the findings of Duretto et al. (2023). These taxa are quite distinct from each other morphologically, and were speculatively thought to be allied to Philotheca (for P. section Cyanochlamys) and the Phebalium group (for Muiriantha) on morphological grounds (Wilson 1970). The only species of Muiriantha, M. hassellii (F.Muell.) C.A.Gardner, and both species of P. section Cyanochlamys, P. spicata (A.Rich.) Paul G.Wilson and P. nodiflora (Lindl.) Paul G.Wilson, are endemic to south-western Australia, so this relationship is geographically sensible. Muiriantha hassellii has pendulous flowers with imbricate yellowish-green petals that form an elongated tube (Fig. 7 a), whereas the flowers of Philotheca section Cyanochlamys are borne erectly, with petals spreading and ranging from pink to blue in colour (Fig. 7 b, c).

On the basis of Duretto et al. (2023) and our nrDNA analyses (see the Supplementary ‘Methods and results’ section), it is apparent that the close relationship of Philotheca section Cyanochlamys and Muiriantha is consistent across nrDNA and ptDNA. However, in contrast to our ptDNA results, our analyses of nrDNA resolved Muiriantha nested inside P. section Cyanochlamys, with very high support (see the Supplementary ‘Methods and results’ section). Duretto et al. (2023) found the same discrepancy in the position of Muiriantha between ptDNA and nrDNA datasets, but the nesting of Muiriantha in P. section Cyanochlamys in their nrDNA phylogeny was not well supported.

Despite obvious morphological differences between Philotheca section Cyanochlamys and Muiriantha, they are broadly similar in having a subshrub habit and leaves with a somewhat papery texture. Further morphological examination by the authors has also shown similarities in carpel surfaces and branchlet indumenta (Fig. 7).

In Muiriantha and P. section Cyanochlamys, the abaxial surface of the carpels is conspicuously pitted with small glands (Bayly 2001; Fig. 7 d–f). This feature is otherwise known only in Philotheca pinoides (section Corynonema) (Bayly 2001) and is, therefore, considered apomorphic for Muiriantha and section Cyanochlamys in the context of major clades of the Eriostemon group.

In Philotheca section Cyanochlamys, two types of branchlet hairs are found (Bayly 2001). The first type is long (~ 0.6 mm), silky, distally ascending and somewhat appressed, and is fasciculate (having multiple hairs arising from a single base) but not spreading; these hairs are always present in P. nodiflora, and are sometimes absent in P. spicata . The second type is fasciculate–stellate, with multiple hairs arising from a single base that spread out in a stellate manner, and occurs in both species of P. section Cyanochlamys (Bayly 2001) . Muiriantha also possesses two types of hairs; the first is similar to the long, silky, appressed hairs of P. section Cyanochlamys, but differs in being singular (i.e. one hair, not multiple from a single base), the second is fasciculate hairs that are the same as those of P. section Cyanochlamys . Fasciculate hairs of this type (Fig. 7 g –i) are an apparent synapomorphy for Clade 2, as they do not occur in any other members of the Eriostemon group.

Finally, from comparing descriptions of Philotheca sections Cyanochlamys and Muiriantha (Wilson 1970, 2013 a, 2013 b), it is apparent that seed features between these taxa are superficially similar. Preliminary investigations of seeds in these taxa by the authors has confirmed that they share a subreniform shape, linear hilum and sub-basal raphe, but it remains unclear whether these characteristics are synapomorphic or plesiomorphic. More detailed investigations of seed morphology may clarify whether or not these similar features should be treated as synapomorphic, and provide further characters that could be used to assign these taxa to a morphologically diagnosable taxonomic group (e.g. subtribe).