Philotheca section Erionema

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 : 376-379

Relationships of Philotheca section Erionema , P. section Philotheca, Drummondita and Geleznowia

Our well-supported recovery of Clade 1, containing Philotheca section Erionema , P. section Philotheca , Drummondita and Geleznowia , is consistent with results of Duretto et al. (2023), and was also recovered with low support by Appelhans et al. (2021). Within this clade, the monophyly and divergence of Philotheca section Erionema , placed as sister to the other taxa, is supported by several morphological synapomorphies ( Bayly 2001; Batty et al. 2022), including anther and seed characters. In particular, the seeds of P. section Erionema are unique in the Eriostemon group and readily recognisable in being ellipsoid and laterally flattened with a linear hilum on the adaxial face and having a basal raphe and chalazal region ( Fig. 8 ; Wilson 1998 a; Bayly 2001). In contrast, seeds of Philotheca section Philotheca , Drummondita and Geleznowia ( Fig. 8 ), which share a strong morphological resemblance in comparison to other members of the Eriostemon group, are more or less reniform, substantially thicker than their length, have a smaller hilum that is round to deltoid and located, together with the raphe, on the adaxial face of the seed.

Our supermatrix approach expands on the results of previous molecular studies with more comprehensive sampling from Philotheca section Philotheca . Our results show Philotheca section Philotheca as polyphyletic, with members of this section falling into two separate clades, one including Drummondita and one sister to Geleznowia ( Fig. 4 ).

The species of section Philotheca that group with Drummondita constitute (with the exception of P. coateana ) an assemblage that, prior to the revision of Wilson (1998 a), was considered the entirety of Philotheca . This narrow concept of Philotheca (referred to hereafter as Philotheca s.str.; Fig. 4 , black triangle), first circumscribed by Bentham (1863), included only species with 10 fertile, monadelphous stamens and a spreading corolla. Drummondita also has monadelphous stamens, and this feature presumably led Mueller (1869) to include that genus in Philotheca . However, Drummondita was reinstated at generic rank by Wilson (1971) on the basis of key differences including stamen fertility (only five fertile in Drummondita ), anther attachment (dorsifixed in Drummondita , versatile in Philotheca ), petal texture (glumaceous in Drummondita , soft in Philotheca ) and carpel apex (unbeaked in Drummondita and beaked in Philotheca ); as a result, Philotheca (sensu Wilson 1971) returned to its original, narrow circumscription. Wilson (1998 a) expanded Philotheca to include Philotheca s.str., plus all species previously included by Wilson (1970) in Eriostemon section Nigrostipulae .

Among members of Philotheca s.str., fusion of the staminal filaments is present in Philotheca salsolifolia , P. ciliata , P. basistyla , P. tubiflora , P. acrolopha and P. sericea ( Bayly 2001) , but the degree of fusion varies among these species. In particular, the filaments of P. acrolopha and P. sericea are fused only very minutely at the base ( Bayly 2001). In our analyses, the only species to be placed in this clade that does not have fused filaments is Philotheca coateana , which was not considered part of Philotheca s.str. by Bayly (2001); the free nature of filaments in this species was verified in the current study by using scanning electron microscopy (see Supplementary Fig. S5 ).

The remaining members of Philotheca section Philotheca that are not part of Philotheca s.str. are equivalent to Eriostemon section Nigrostipulae sensu Wilson (1970) (referred to herein as Philotheca ‘Nigrostipulae’; Fig. 4 , black square). In his revision of Eriostemon, Wilson (1998 a) recognised the similarity of Geleznowia to his newly circumscribed Philotheca section Philotheca and contemplated that the arrangement he presented may render Philotheca paraphyletic with regards to Geleznowia . In particular, Wilson (1998 a, 2013 a) noted that Geleznowia and some members of Philotheca section Philotheca (including Philotheca s.str. and Philotheca ‘Nigrostipulae’) possess filiform sclereids, similar seeds and the same chromosome number. Although Geleznowia shares these morphological features with Philotheca section Philotheca in the broad sense ( sensu Wilson 1998 a ), our recovery of a sister relationship specifically between Geleznowia and Philotheca ‘Nigrostipulae’ is not supported by any morphological synapomorphies that we can identify, probably because of the highly derived morphology of Geleznowia and a high degree of morphological homoplasy in Clade 1.

Figures

Fig. 8. Seed morphology in Clade 1, showing the adaxially central raphe in Philotheca section Philotheca, Drummondita and Geleznowia, and the basal raphe in Philotheca section Erionema. (a–c) Seeds typical of P. section Philotheca [P. linearis; G.J. White s.n., NE 52727]. (d–f) Seeds typical of Drummondita [D. longifolia; H. Demarz 10361, PERTH 959707]. (g–i) Seeds typical of Geleznowia [G. verru- cosa; L. Broadhurst 14, PERTH 5547822]. (j–l) Seeds typical of P. section Erionema [P. verrucosa; MJB 249, HO523410]. (a, d, g, j) Lateral views. (b, e, h, k) Adaxial views. (c, f, i, l) Longitudinal sec- tions through the raphe. (a, g) Drawn with the placental portion of endocarp still attached to the seed; for all other draw- ings the placental endocarp was removed. Drawings are modified from Bayly (2001) and are not to scale.

Fig. 4. (Caption on next page)

Fig. 5. Results of the likelihood-mapping analysis of loci. (a) Plot of phylogenetic informativeness of individual loci, where a higher percentage of fully resolved quartets for a locus indicates greater support for tree-like evolution. Dashed lines on the plot indicate the three cut-off values that were tested for potentially improving phylogenetic resolution of the polytomy (i.e. 50, 60 and 70%). Bars along the x-axis denote the cluster that each locus was placed in during the analysis of tree space; some loci were excluded from the tree-space analysis, owing to incomplete representation of samples. (b) Phylogenetic network (NeighborNet) showing splits of the backbone polytomy; constructed from a concatenated alignment of all loci with>60% of quartets fully resolved. For Crowea, EA, eastern Australia; WA, western Australia.