Ge McKinnon

Incongruence between chloroplast and species phylogenies in Eucalyptus subgenus Monocalyptus (Myrtaceae).

Seventy-eight polymorphic cpDNA (chloroplast DNA) characters were found in 13 closely related taxa from Eucalyptus series Amygdalinae (subgenus Monocalyptus) and seven potential outgroup taxa. The strict consensus of six cladograms generated from cpDNA data confirmed monophyly of Monocalyptus. However, cpDNA phylogeny within Monocalyptus was incongruent with taxonomic classification, being more related to geography, even when accessions were from divergent series. Monocalyptus cpDNA formed two major clades. On the island of Tasmania cpDNA was restricted to a single clade, exhibited very little variation, and was phylogenetically related to cpDNA found in central and western Victoria. In contrast, cpDNA of mainland monocalypt taxa was more variable, even within the Amygdalinae. Four out of six Tasmanian Amygdalinae species were polymorphic. The difference between cpDNA of replicates was often greater than differences between species from different series. The low level of cpDNA variation and extensive morphological intergradation between the Tasmanian endemics suggest recent speciation. However, the transfer of cpDNA through hybridization between lineages is the most likely explanation for the observed sharing of cpDNA across series. This study highlights that the geographical pattern to cpDNA variation in Eucalyptus may be an important source of information on past plant distributions in Australia.

Chloroplast DNA phylogeography of Eucalyptus globulus

The pattern of variability in chloroplast DNA (cpDNA) of Eucalyptus globulus Labill. (Myrtaceae) was studied using 270 samples from southern Australia. Forty variable sequence characters were found, defining 105 haplotypes. Haplotypes were assigned to three major cpDNA clades based on their phylogeny. The pattern of cpDNA variation did not conform to subspecies boundaries; however, there was a strong geographic structure to the distribution of clades and haplotypes. One clade (JC) was geographically central and widespread and was found in 163 samples from Tasmania and continental Australia. Less-common clades occurred in more localised regions— southern (Js, 77 samples) and eastern (Jet, 12 samples). The distribution of the Jet and Js clades coincides with hypothesised glacial refugia in Tasmania, suggesting limited seed-mediated dispersal since deglaciation.

An AFLP marker approach to lower-level systematics in Eucalyptus (Myrtaceae)

Genus Eucalyptus, with over 700 species, presents a number of systematic difficulties including taxa that hybridize or intergrade across environmental gradients. To date, no DNA marker has been found capable of resolving phylogeny below the sectional level in the major subgenera. Molecular markers are needed to support taxonomic revision, assess the extent of genetic divergence at lower taxonomic levels, and inform conservation efforts. We examined the utility of 930 amplified fragment length polymorphisms (AFLPs) for analyzing relationships among Tasmanian taxa of subgenus Symphyomyrtus section Maidenaria. Phenetic and cladistic analyses resolved species into clusters demonstrating significant genetic partitioning, largely concordant with series defined in the most recent taxonomic revision of Eucalyptus. Some departures from current taxonomy were noted, indicating possible cases of morphological convergence and character reversion. Although the resolution obtained using AFLP was greatly superior to that of single sequence markers, the data demonstrated high homoplasy and incomplete resolution of closely related species. The results of this study and others are consistent with recent speciation and reticulate evolution in Maidenaria. We conclude that a combination of phylogenetic and population genetic approaches using multiple molecular markers offers the best prospects for understanding taxonomic relationships below the sectional level in Eucalyptus.

The rare silver gum, Eucalyptus cordata, is leaving its trace in the organellar gene pool of Eucalyptus globulus

The process of genetic assimilation of rare species by hybridizing congeners has been documented in a number of plant genera. This raises the possibility that some of the genetic diversity found in phylogeographical studies of widespread species has been acquired through hybridization with species that are now rare or extinct. In this fine‐scale phylogeographical analysis, we show that a rare eucalypt species is leaving its trace in the chloroplast genome of a more abundant congener. The heart‐leafed silver gum, Eucalyptus cordata, is a rare endemic of south‐eastern Tasmania. Its populations are scattered amidst populations of more abundant related species, including the Tasmanian blue gum, Eucalyptus globulus. Using 339 samples from across the full range of both species, we compared chloroplast (cp) DNA haplotype phylogeography in E. globulus and E. cordata. The genealogy and distribution of chloroplast haplotypes suggest that E. globulus has acquired cpDNA from E. cordata in at least four different mixed populations. Shared haplotypes are highest in E. globulus sampled within 2 km of known E. cordata populations and drop to zero at a distance of 25 km from the nearest known E. cordata population. Localized haplotype sharing occurs in the absence of obvious hybrid zones or locally shared nuclear ribosomal DNA sequences. Given that the future loss of E. cordata from some mixed populations is likely, these findings indicate that phylogeographical analyses of organellar DNA should consider the possibility of introgression, even from species that have been eliminated from the sites of interest.

Population and phylogenetic analysis of the cinnamoyl coA reductase gene in Eucalyptus globulus (Myrtaceae)

Low-copy number nuclear genes are currently emerging as new markers for phylogenetic and phylogeographic analysis. This study used the single-copy gene for cinnamoyl coA reductase (CCR) to gain insights into the evolutionary history of the forest tree Eucalyptus globulus Labill. (subgenus Symphyomyrtus, section Maidenaria). A population analysis based on CCR restriction fragments from E. globulus was combined with a phylogenetic analysis of 1.5 kb of CCR sequence from the major haplotypes. Two highly divergent CCR lineages were found in E. globulus. One lineage was prominent throughout the species’ range and was identified in 16 other Maidenaria species by restriction analysis. The second lineage, which was prominent in the northern part of the species’ range, was found only in species of the E. globulus complex and surprisingly showed homology to CCR from Eucalyptus saligna Smith (subgenus Symphyomyrtus, section Latoangulatae). This finding may reflect either incomplete lineage sorting in CCR, or reticulate evolution. No statistically significant phylogeographic structure (geographic clustering of closely related haplotypes) was detected. However, patterns of CCR haplotypic diversity were congruent with patterns of chloroplast DNA diversity in several respects, and divided the range of E. globulus into four regions, supporting (1) former gene flow between King Island and western Tasmania, (2) gene flow between northern Tasmania, the Furneaux Group and Victoria, and (3) a genetic disjunction between north-eastern and south-eastern Tasmania, consistent with separate histories for these two regions.