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Investigating taxonomic barriers to gene flow between plantation and native eucalypts

CCA-diversity

The Currency Creek Arboretum is home to over 700 eucalypts from across the taxonomic and geographic range of the genus.  Images: Matt Larcombe.



It has long been recognised that closely related eucalypt species often have weak reproductive barriers that allow inter-specific hybridisation to occur. Through the process of back crossing from hybrid individuals to the pure parental species, hybridisation can result in the movement (or introgression) of genetic material from one species into the gene pool of another, or - in other words - in "gene flow" occurring between species (Potts et al. 2003).

With this propensity for hybridisation, it is possible that plantation eucalypt species could hybridise with native eucalypt species, resulting in hybrid combinations and gene flow to species that would not occur in nature, particularly where the plantation species is planted outside its natural range.

Australia’s hardwood plantation estate reached one million hectares in 2010. The main plantation species is Eucalyptus globulus, which is planted well outside its natural range of Tasmania and southern Victoria. The potential for exotic gene flow from E. globulus plantations was identified through work undertaken by project 4.2.6 pioneer Dr Robert Barbour (formerly of UTAS) (Barbour et al. 2008).

Although hybridisation is widespread across the genus there is a range of barriers to hybridisation among eucalypts. For example, there have been no reported hybrid combinations between species from the two largest subgenera (Symphyomyrtus and Eucalyptus) of Eucalyptus. There are also likely to be barriers between different taxonomic sections within the subgenera and also possibly between taxa within sections.  Most commercially important plantation species of Eucalyptus (e.g., E. globulus, E. nitens, E. grandis, E. urophylla, E. camaldulensis) belong to subgenus Symphyomyrtus.  Eucalyptus globulus and E. nitens are the most widely planted plantation species in temperate regions of Australia.

Hence, the question arises: which eucalypt species are likely to be at risk from exotic gene flow through hybridisation with E. globulus or E. nitens? UTAS PhD student Matt Larcombe is attempting to answer this question by undertaking a large crossing program at the Currency Creek Arboretum (CCA) in South Australia.  CCA is an amazing private arboretum developed by Dean Nicolle, who has been collecting and growing eucalypts since before he was 10 years old. There are over 900 vouchered specimens growing at the arboretum, offering a very convenient convergence of geographically and taxonomically divergent species (see images, left).

The aim of the crossing program is to asses post-pollination barriers to gene flow. The study will involve adding E. globulus and E. nitens pollen to the flowers of as many species in subgenus Symphyomyrtus as possible over the next year.  Seed from the female parents will be collected one year later and grown in the glasshouse to be screened for the presence of putative hybrids between the plantation and mother species. First generation (F1) hybrids in eucalypts are typically intermediate in morphology between the two parent species and this allows rapid visual screening of large numbers of seedlings. Putative hybrids will be verified using molecular markers.

Natural pollination will be mimicked by utilising the supplementary pollination technique (Barbour et al. 2005), where competition from intra-specific pollen will be present (as opposed to controlled pollination where the crossed flower is isolated so that only the experimentally added pollen has the opportunity to pollinate the flower). The project will focus on species that are known to occur in the main plantation zones in Tasmania, Gippsland (Victoria), The Green Triangle (Victoria and South Australia), and southwest Western Australia (Barbour et al. 2005) and species from across all 15 sections of subgenus Symphyomyrtus, to detect taxonomic trends in the likelihood of hybridisation.

It is hoped that data from this study (along with data from a range of other projects in Matt’s PhD that are investigating other barriers to interspecific gene flow among eucalypts - click here to download a 5.5 MB pdf of Matt's introductory PhD seminar) will feed into decision support tools for plantation managers so that they can assess the likelihood of plantation species hybridising with neighbouring native species at the plantation design stage and implement mitigation strategies where necessary. A similar tool has been developed based on work undertaken by Robert Barbour in collaboration with the Forest Practice Authority (see FPA technical note).

Crossing got underway in late June at CCA.  Over 1000 flowers from 16 species representing 3 sections of subgenus Symphyomyrtus have been crossed so far, but with over 400 Symphyomyrtus species growing at CCA, there is still a lot to do!

References

Barbour RC, Otahal Y, Vaillancourt RE, Potts BM (2008) Assessing the risk of pollen-mediated gene flow from exotic Eucalyptus globulus plantations into native eucalypt populations of Australia. Biological Conservation 141: 896-907. [read]

Barbour RC, Potts BM, Vaillancourt RE (2005) Gene flow between introduced and native Eucalyptus species: crossability of native Tasmanian species with exotic E. nitens. Australian Journal of Botany 53: 465-477. [read]

Potts BM, Barbour RC, Hingston AB, Vaillancourt RE (2003) Genetic pollution of native eucalypt gene pools - identifying the risks. Australian Journal of Botany 51: 1-25. [read]

For more information, contact:

Matthew Larcombe
School of Plant Science
University of Tasmania
Biobuzz issue twelve, August 2010