Eucalyptus odorata, Behr, Behr

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 : 430-431

Taxonomy within the E. odorata complex

With the extensive level of interspecific geneflow, our dataset has provided evidence for, and the lack of resolved relationships among populations in the E. odorata complex; we do not feel that it is appropriate to make major taxonomic changes on the basis of our study without further work to investigate patterns of diversity in finer detail. However, we see several approaches that could be taken to clarify the taxonomy of the group as outlined in Table 6 . The simplest is to uphold the current morphology-based species classification, with adjustments to species distributions where necessary to match resolved phylogenetic relationships. This approach, essentially applying the morphological species concept, would require accepting the recognition of hybrid entities as species, which is being realised as a major driver of plant evolution ( Mayr 2000). The one clear change that is well supported in our data is that populations currently regarded as E. polybractea in the Flinders Ranges are not related to the eastern populations of that species and the circumscription of E. cajuputea should be expanded to include these populations.

The second approach is to greatly reduce the number of species in the complex to only those that are supported as monophyletic, strictly applying the phylogenetic species concept ( Baum 1992). This approach would likely reduce the complex back to two or three species, depending on what future study reveals regarding the distinctness of the northern populations of E. viridis . Eucalyptus aenea and E. castrensis would be synonymised with E. viridis and the cline of the rest of the complex broken up as follows: E. odorata to include populations west of the Murray Basin, E. wimmerensis to cover all populations in the western Wimmera of Victoria and adjacent areas of SA, and E. polybractea to accommodate populations from the eastern Wimmera, Victorian goldfields and around West Wyalong. Additionally, the name E. viridis var. latiuscula may need to be resurrected and its taxonomic rank re-assessed to accommodate the northern E. viridis populations that may represent a further extension of this cline. We would advocate that this approach is less than optimal, because each of these taxa would cover a large range of morphological variation that is somewhat correlated with geography and there are outstanding questions regarding the monophylly of these taxa. For these reasons, at this point, we would advocate a looser application of the phylogenetic concept and using the framework of integrative taxonomy to consider both the morphological and molecular evidence, synonymising species with molecular evidence against them being distinct entities, while maintaining morphological taxa with inconclusive molecular support for their status as distinct populations.

Following this reasoning, E. hawkeri and E. silvestris may need to be synonymised because both represent intergrades between E. wimmerensis and E. microcarpa . Although also showing introgression from E. microcarpa , E. walshii is perhaps better synonymised with E. wimmerensis , given its much closer genetic ties to this species. Recognition of E. filiformis is problematic, because it is clonal and, given its close placement to E. polybractea in our phylogeny and networks ( Fig. 2 , 5), possibly an outlying population of E. polybractea that has unique morphology owing to the small population size causing bottlenecking and genetic drift. A similar problem exists for E. yarriambiack because our data suggest that it is not experiencing introgression from the co-occurring E. largiflorens , but does not represent a distinct lineage from E. polybractea , rather being another potential small, isolated population undergoing genetic drift or a genetic bottleneck that should be synonymised with this species. However, in the case of E. yarriambiack , the population is not clonal, holds greater genetic diversity than does the clonal E. filiformis and is far enough outside the range of E. polybractea that there is likely to be no ongoing gene flow with populations of that species, all suggesting that the species status for E. yarriambiack is not unreasonable. For E. viridis , E. aenea and E. castrensis , we recommend that further phylogenetic studies are undertaken before taxonomy is re-assessed, because, although we have shown these three taxa are each other’s closest relatives, we have not sampled widely enough to determine whether E. aenea and E. castrensis are distinct lineages from E. viridis or isolated populations experiencing introgression from the co-occurring and more locally abundant grey-box species E. albens . In addition, further work is needed to investigate the relationships of the Queensland populations currently regarded as E. viridis to that species and to E. polybractea , or whether the name E. viridis var. latiuscula needs to be resurrected and given the rank of species.

Figures

Fig. 2. Neighbour-net networks of E. odorata complex taxa and co-occurring members of E. section Adnataria created in Splitstree V.4 using uncorrelated-P distances of SNPs generated by A. ddRADseq and B. DArTseq. Tips are coloured by species, with shapes used to distinguish different major groups: E. series Hetereophloiae (inverse tri- angles), E. series Melliodorae (hexagons), E. series Buxeales (triangles), the grey-box taxa (diamonds), mallee members of E. series Subbuxeales not in the E. odorata complex (squares), and the E. odorata complex (circles) coloured consistent with Fig. 1.