Chloroplast variation is incongruent with classification of the Australian bloodwood eucalypts (genus Corymbia, family Myrtaceae).

Tanja M Schuster,Stephen Wilcox,Todd G B Mclay,Sabrina D Setaro,Rachael M Fowler,Erin L Batty,Peter K Ades,Josquin F G Tibbits,Michael J Bayly

doi:10.1371/journal.pone.0195034

Abstract

Previous molecular phylogenetic analyses have resolved the Australian bloodwood eucalypt genus Corymbia (~100 species) as either monophyletic or paraphyletic with respect to Angophora (9–10 species). Here we assess relationships of Corymbia and Angophora using a large dataset of chloroplast DNA sequences (121,016 base pairs; from 90 accessions representing 55 Corymbia and 8 Angophora species, plus 33 accessions of related genera), skimmed from high throughput sequencing of genomic DNA, and compare results with new analyses of nuclear ITS sequences (119 accessions) from previous studies. Maximum likelihood and maximum parsimony analyses of cpDNA resolve well supported trees with most nodes having >95% bootstrap support. These trees strongly reject monophyly of Corymbia, its two subgenera (Corymbia and Blakella), most taxonomic sections (Abbreviatae, Maculatae, Naviculares, Septentrionales), and several species. ITS trees weakly indicate paraphyly of Corymbia (bootstrap support <50% for maximum likelihood, and 71% for parsimony), but are highly incongruent with the cpDNA analyses, in that they support monophyly of both subgenera and some taxonomic sections of Corymbia. The striking incongruence between cpDNA trees and both morphological taxonomy and ITS trees is attributed largely to chloroplast introgression between taxa, because of geographic sharing of chloroplast clades across taxonomic groups. Such introgression has been widely inferred in studies of the related genus Eucalyptus. This is the first report of its likely prevalence in Corymbia and Angophora, but this is consistent with previous morphological inferences of hybridisation between species. Our findings (based on continent-wide sampling) highlight a need for more focussed studies to assess the extent of hybridisation and introgression in the evolutionary history of these genera, and that critical testing of the classification of Corymbia and Angophora requires additional sequence data from nuclear genomes.

Highlights

The bloodwood eucalypts are sclerophyllous trees (c. 100 species) [1], currently classified in the genus Corymbia K.D.Hill & L.A.S.Johnson, which was taxonomically segregated from the genus Eucalyptus L’Her. in 1995 [2]
The relationships within Eucalyptus generally confirm those of previous High-Throughput Sequencing (HTS) studies [41] and our results for relationships among Eucalyptus will not be discussed further as our focus here is on Corymbia and Angophora
A key result of the current study is that cpDNA relationships in Corymbia (Fig 3) are largely incongruent with the current circumscriptions of subgenera and sections, and with relationships inferred based on nrDNA (Fig 6)

Summary

Introduction

The bloodwood eucalypts are sclerophyllous trees (c. 100 species) [1], currently classified in the genus Corymbia K.D.Hill & L.A.S.Johnson, which was taxonomically segregated from the genus Eucalyptus L’Her. in 1995 [2]. These groups occur primarily in northern or eastern Australia (Fig 1), where they are well-represented in monsoonal, tropical savannahs, and there are two small groups of red bloodwoods restricted to the south-west and south-east of Australia (sections Calophyllae and Corymbia, respectively) in areas with Mediterranean, temperate climates. Corymbia is part of the "eucalypt group" [16] (tribe Eucalypteae) [17] that includes the sclerophyll genera Eucalyptus (>665 species) [1] and Angophora (9–10 species) [1,18], and the rainforest genera Arillastrum (1 species) [19], Stockwellia (1 species) [20], Allosyncarpia (1 species) [21], and Eucalyptopsis (2 species) [22]

Methods

Results

Conclusion