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

Genetic variation within the core E. odorata complex as a cline

Our findings of extensive introgression among members of the E. odorata complex and co-occurring box species is not unexpected given that previous studies on E. section Adnataria have shown extensive hybridisation leading to morphological taxa not forming genetic clades ( Flores-Rentería et al. 2017). However, in regard to the taxonomy of the E. odorata complex, the nature of the core of the clade as a discontinuous cline of morphological and genetic variation running from the Flinders Ranges, south and then east through south-eastern SA, east through the Wimmera and then the Goldfields of Victoria and then north to West Wyalong in NSW also plays a significant role in disagreements among authorities over where taxonomic boundaries should be drawn. This clinal genetic variation has been taxonomically divided by different authors at different points on the basis of different factors they consider most important for classification and, in this paper, we have, a priori, broken it into the seven largely geographically distinct species ( E. odorata , E. cajuputea , E. wimmerensis , E. polybractea , E. filiformis , E. walshii and E. yarriambiack ).

We see evidence for this cline in the most distal populations (Flinders Range and West Wyalong) being the most distinct, and in relationships between neighbouring populations fluctuate between analyses. This includes, for instance, E. filiformis clustering with E. wimmerensis in one network ( Fig. 2 b ) but being more closely related to E. polybractea from the Victorian goldfields in other analyses ( Fig. 2 a , 5), and the swapping of relationships between the three main groups (samples from west of the Murray River Basin, E. wimmerensis and allied samples, and eastern E. polybractea and allied samples) in the core group between analyses, even if the alternate relationships are not supported ( Fig. 4 ). This may be indicative of recent rapid diversification of a widespread ancestral population that has undergone vicariance at multiple locations at approximately congruent times. Clines along which diversification has occurred, often resulting in morphologically distinct species with a hybrid zone between them, have previously been observed in other eucalypt groups, including E. populnea and E. brownii Maiden & Cambage ( Holman et al. 2003), E. melanophloia F.Muell. and E. whitei Maiden & Blakely ( Holman et al. 2011), and the green ashes ( E. sect. Eucalyptus ; Rutherford et al. 2018). The strong support for IBD ( Fig. 6 , r: 0.597) in our data is congruent with the existence of a genetic cline.

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.

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).

Fig. 6. Isolation by distance plot of core E. odorata complex samples (E. odorata, E. cajuputea, E. wimmerensis, E. walshii, E. yarriambiack, E. filiformis, E. polybractea and E. viridis from south-eastern Queensland). Geographic distances are kilometres between collection coordinates and genetic distances are uncorrelated-P distances.