Eucalyptus section, Adnataria

Fahey, Patrick S., Udovicic, Frank, Nicolle, Dean, McLay, Todd G. B. & Bayly, Michael J., 2022, A phylogenetic investigation of the taxonomically problematic Eucalyptus odorata complex (E. section Adnataria series Subbuxeales): evidence for extensive interspecific gene flow and reticulate evolution, Australian Systematic Botany 35 (5), pp. 403-435 : 427-428

Phylogeny of E. section Adnataria

Although this study was focussed on the E. odorata complex, we have also produced one of the most resolved phylogenies for section Adnataria ( Fig. 5 ). Previous phylogenetic studies of section Adnataria have either relied on plastid DNA ( Flores-Rentería et al. 2017; Alwadani et al. 2019), which does not resolve phylogenetic relationships in the eucalypts because of a large cyto-nuclear discrepancy ( Steane et al. 1998; Jackson et al. 1999), internal transcribed spacer sequences, which do not provide enough resolution to reconstruct relationships within the section ( Steane et al. 2002, 2007; Thornhill et al. 2019), or have not resolved any relationships between series and species with support ( Woodhams et al. 2013). However, because our sampling was comprehensive only for taxa that commonly co-occur with members of the E. odorata complex, we sampled only four of the nine series in E. section Adnataria that are recognised in the classification of Nicolle (2019). Three of the sampled series ( Heterophloiae , Melliodorae and Subbuxeales ) were monophyletic, with the placement of E. populnea as sister to E. series Heterophloiae , rendering E. series Buxeales polyphyletic ( Fig. 5 ). A possible explanation for the placement of this sample is introgression from E. conica H.Deane & Maiden , a member of E. series Heterophloiae , which occurs at the collection locality and that we did not sample in this study. As we sampled only 3 of the 15 species placed in E. series Buxeales by Nicolle (2019), including a single E. populnea individual, and as the group shares several morphological traits ( Brooker 2000), we recommend further phylogenetic investigation of the series to test its monophyly.

Brooker (2000) placed E. series Melliodorae in E. subsection Terminales along with E. series Heterophloiae and several series not sampled in our study, although he placed E. series Buxeales and Subbuxeales in E. subsection Apicales . This does not match our phylogeny, which shows E. series Melliodorae as more closely related to the last two series than to E. series Heterophloiae ( Fig. 5 ). No previous phylogenetic studies have resolved these subsections to be reciprocally monophyletic ( Woodhams et al. 2013; Flores-Rentería et al. 2017), suggesting a re-assessment of the subsectional classification within E. section Adnataria may be needed. Brooker’s E. subsection Terminales was defined as including species with inflexed stamens and an outer ring of staminodes ( Brooker 2000), which, if our phylogeny is a true representation of evolutionary relationships, suggests that these traits may either be the ancestral state of E. section Adnataria or independently derived in both series, especially given the placement of E. porosa , which lacks staminodes, in E. series Melliodorae . The close relationship between E. series Buxeales and Subbuxeales is consistent with established classifications, with some authors not recognising E. series Subbuxeales as distinct from E. series Buxeales ( Brooker 2000; Brooker et al. 2015).

We are able to confirm that E. porosa is best placed in E. series Melliodorae ( Fig. 5 ), despite morphological similarities to members of the E. odorata complex ( E. series Subbuxeales ), including its typically mallee growth habit, lack of an operculum scar and axillary inflorescence arrangement ( Brooker et al. 2015). This aligns with the classification of Nicolle (2019), but not that of Brooker (2000) who considered this species a member of E. series Buxeales , which included the E. odorata complex in their classification. Eucalyptus series Melliodorae is united by several key morphological traits that E. porosa also shares, namely, the outer operculum being held to anthesis, axillary inflorescences, an intramarginal vein remote from the leaf edge, and a tardily deciduous, broad staminal ring. Only the first two of these characteristics are shared with members of E. series Subbuxeales ( Brooker et al. 2015) , whereas E. froggattii is the only member of E. series Subbuxeales with a remote intramarginal vein ( Brooker and Nicolle 2013). A final trait shared by the other members of E. series Melliodorae we sampled but lacked by E. porosa and two other members of the series we have not sampled ( E. bosistoana F.Muell. and E. argophloia Blakely ) is a sterile outer ring of stamens, which fits with the placement of E. porosa in this study as sister to the rest of the series ( Fig. 5 ). Additional support for the placement of E. porosa in E. series Melliodorae comes from hybrids of E. porosa × E. leucoxylon being common ( Nicolle 2014), but no E. porosa × E. odorata complex hybrids having been recorded.

The seedling of ‘ E. aff. polybractea (Mount Jeffcott) ’ was also part of the E. series Melliodorae clade in our full dataset phylogeny ( Fig. 4 ). The maternal parent of this seedling is found at Mount Jeffcott within 100 m of the only known stand of E. filiformis , from which it differs primarily in its glaucous juvenile foliage and smooth bark ( K. Rule, pers. comm., 22 November 2018). Of the other E. section Adnataria taxa that occur at the site ( E. microcarpa , E. largiflorens and E. leucoxylon ), E. leucoxylon is by far the most abundant species perhaps explaining why, despite the extensive geneflow between E. microcarpa and members of the E. odorata complex in the Wimmera, our results, both the phylogeny ( Fig. 4 ) and ABBA-BABA tests ( Table 4 ), showed that this seedling is a E. filiformis × E leucoxylon hybrid. This finding fits with the observations of its morphology ( K. Rule, pers. comm., 3 February 2021), although, because we did not sample the maternal parent tree, we cannot say if it is also a E. leucoxylon hybrid, potentially explaining its unique morphology.

The two clades within E. series Subbuxeales ( Fig. 5 ) are supported by morphology, as the typically single-trunked grey-box species and mallee box species form reciprocally monophyletic clades. This is in line with previous studies that have shown that the mallee growth form may be tied to genetic lineages in other eucalypt groups ( Hines and Byrne 2001). Although grey boxes are very morphologically similar and establishing geographic boundaries among the taxa are difficult because they intergrade with one another ( Bean 2009; Flores-Rentería et al. 2017), E. albopurpurea and E. froggattii have not previously been hypothesised to be closely related in the literature. Eucalyptus albopurpurea has often been regarded as more morphologically similar to members of the E. odorata complex, in particular E. odorata itself, with which it intergrades with on Kangaroo Island ( Nicolle 2000, 2014), than to E. froggattii , which is set apart morphologically by its square buds and fruits ( Brooker et al. 2015). However, these two species do have morphological similarities not shared by the members of the E. odorata complex, including apparently terminal compound inflorescences, generally broader leaves, and slightly larger buds and fruit ( Brooker et al. 2015). More work is needed to clarify the relationship between these two species and the E. odorata complex.

Figures

Fig. 5. Maximum-likelihood phylogeny generated using RAXML, excluding samples with strong evidence for hybridisation and introgression in the combined ddRADseq and DArTseq dataset. Support values on branches are those from the ML and MP analysis, with branches with greater than 80% bootstrapping support in both analyses thickened. Series are labelled as per Nicolle (2019).

Fig. 4. Maximum-likelihood phylogeny generated using RAXML including all samples in the combined ddRADseq and DArTseq dataset. Support values on branches are those from the ML and MP analysis, with branches with greater than 80% bootstrapping support in both analyses thickened. Series are labelled as per Nicolle (2019).