Phebalium, Vent.

Duretto, Marco F., Heslewood, Margaret M. & Bayly, Michael J., 2023, Generic and infrageneric limits of Phebalium and its allies (Rutaceae: Zanthoxyloideae), Australian Systematic Botany 36 (2), pp. 107-142 : 134-135

Phebalium and Microcybe

Phebalium (30 of 35 species, 4 of the 6 subspecies of P. glandulosum Hook. , and 5 of the 8 subspecies of P. squamulosum Vent. sampled) and Microcybe (3 of 4 species and all 4 subspecies sampled) form a well-supported clade in all analyses (Clade 4: Fig. 1 , 3 , S 1 , S 2 ) supporting the conclusions of Wilson (1970, 1998 b, 2013 b, 2013 c) and Mole et al. (2004). The main contentious issue within the clade is the position of M. ambigua . The species is sister to the remainder of Microcybe in both the combined analyses (0.93 PP, 76% JK) and the analyses based on nuclear data (1.00 PP, 88% JK), although with only moderate support. In the analyses utilising plastid data, M. ambigua is sister to Phebalium with moderate support (0.93 PP, 86% JK). The remainder of Microcybe and all of Phebalium are both robustly supported clades in all analyses, as is the entire Microcybe + Phebalium clade. Missing from this study are M. albiflora Turcz. , P. appressum Paul G.Wilson , P. brevifolium Paul G.Wilson , P. calcicola S.Dema & I.Telford , P. cicatricatum A.J.Ford & Duretto , P. graniticola I.Telford & J.J.Bruhl , and P. microphyllum Turcz. Microcybe albiflora is morphologically similar to both M. multiflora and M. pauciflora . The south-western Australian species, i.e. P. appressum , P. brevifolium and P. microphyllum , are morphologically similar to other south-western species such as P. filifolium Turcz. and P. tuberculosum (F.Muell.) Benth. (see also Wilson 1970, 1998 b, 2013 b), whereas the eastern Australian species, i.e. P. calcicola , P. cicatricatum and P. graniticola , are part of the P. squamulosum group of species (see Telford et al. 2019; Ford and Duretto 2020; Dema et al. 2021).

Microcybe ambigua has had a complicated taxonomic history. It was originally described as M. pauciflora var. uniflora D.A.Herb. ( Herbert 1922) and then placed in Phebalium (as P. ambiguum ) by Gardner (1943) without much explanation, only to be moved back to Microcybe by Wilson (2013 c). Microcybe ambigua shares with the remain-der of Microcybe the reduced number or carpels (1–4, v. 5 in Phebalium ). It differs from virtually all other species in both genera in having solitary, sessile flowers. All other species of Microcybe have sessile flowers but have inflorescences containing 5–20 flowers. Phebalium clavatum C.A.Gardner is the only species in Phebalium to have solitary and sessile flowers and it also has unusual leaf morphology ( Wilson 2013 c). That species groups here within a weakly supported clade (1.00 PP, 72% JK), nested well within the south-western Australian clade of Phebalium , that also contains P. drummondii Benth. , P. elegans Paul G.Wilson , P. filifolium , P. lepidotum (Turcz.) Paul G.Wilson , and P. obovatum (Paul G.Wilson) Paul G.Wilson (see also Mole et al. 2004, where it forms a clade with P. elegans and P. filifolium ).

Wilson (2013 c) indicated that Microcybe ambigua was anomalous and discussed two alternative options to his treatment when he transferred it to Microcybe from Phebalium . These were to place it in a monotypic genus or alternatively expand Phebalium to include it and other species of Microcybe . Both these options are consistent with the results presented here. The isolated position of the species in the Microcybe + Phebalium clade certainly requires acknowledgement but the adoption of a monotypic genus that would be defined on few, non-unique, morphological characters seems to be unwarranted. A third option could be to retain M. ambigua in Microcybe but to erect a monotypic section for it, but that is problematic because its close relationship with Microcybe is only moderately supported in some analyses and is not supported by plastid data. We consider that expanding Phebalium to include Microcybe is warranted, as the genera share several apomorphies, including terminal umbels, similar seeds, and having stellate-lepidote trichomes and the relationship is robustly supported here. Reducing Microcybe sens . strict. to the rank of section is appropriate given both clades are robustly supported. The uncertain placement of M. ambigua is recognised by making it the sole member of a newly described and monotypic section (see Taxonomy below). The recognition of sections on the basis of carpel number mirrors our treatment of Asterolasia , although there is not the clear geographical split that is seen in that genus (see above).

The contrasting positions of Microcybe ambigua in this study may suggest a hybrid origin for the species. We saw no signature in the sequences suggestive of this, such as high levels of polymorphism at nuclear loci, which often characterise hybrids. We recorded only two polymorphic sites in ETS and none in ITS. Sequences from M. ambigua show a high number of uninformative substitutions at various loci, indicative of a long period of genetic isolation from both Microcybe sens . strict. and Phebalium . Microcybe ambigua is also lacking most of the distinctive indels of the plastid data that characterised the other two species of Microcybe used in this study.

Phebalium sens . strict. forms two well-supported geographic clades, namely, one from eastern Australia and another one from south-western Australia. A pattern of geographically separated clades is also seen here in Asterolasia , Crowea and Nematolepis and elsewhere in Rutaceae such as in Boronia sections Boronia ( Duretto et al. 2023) and Valvatae (Benth.) Engl. ( Duretto 1999). In Phebalium , the eastern clade is more strongly supported (Clade 9 E: 1.00 PP, 99% JK) than is the south-western clade (Clade 9 W: 0.98 PP, 72% JK), which has no support in the analyses utilising plastid data alone. Because morphological characters could not be found to support these clades and because this genus is part of a more comprehensive study by other workers, no formal taxonomic changes within Phebalium sens . strict. are proposed here.

This study has also highlighted the risks associated with sampling only single exemplars or single loci. In Phebalium , there may have been ongoing genetic exchange leading to retention of ancestral polymorphisms in a range of taxa. Almost no taxa represented by multiple accessions were resolved as monophyletic. Further work with a greater density of within-species sampling is needed to determine whether the groupings seen are distinct lineages or whether the phylogenetic signal has been confounded by incomplete lineage sorting. For future studies, it is imperative to include multiple accessions of each taxon to determine whether taxa can be resolved before potentially flawed interpretations are made about relationships.

Figures

Fig. 1. Simplified version of the Bayesian 50% majority-rule consensus tree of combined molecular data, with thick lines showing supported clades (≥0.95 PP). Genera have been collapsed on the basis of sampling density, to provide a snapshot of the overall relationships.

Fig. 3. Bayesian 50% majority-rule consensus tree of combined molecular data, with thick lines showing supported clades (≥0.95 PP). Jackknife supports from MP analysis are indicated with icons above branches: 100% JK (solid triangles); 90–99% JK (solid squares); 75–89% JK (open triangles); 50–74% JK (open squares). This tree details relationships within the more derived clades. New or revised sectional assignments are indicated in bold. Clade numbering has been defined in the text.

Fig. 2. (Caption on next page)