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The Wood From The Trees - Issue Five - Geographic variation in blackbutt

By Mervyn Shepherd, Tim Sexton, Dane Thomas, Michael Henson and Robert Henry

The conclusion that there are two geographic races in blackbutt (Eucalyptus pilularis) has important implications for genetic improvement and gene conservation in this species. This finding arose from a recently completed, range-wide study of geographic variation in blackbutt, a component of the CRC for Forestry project 'Association genetics for solid wood properties in Eucalyptus pilularis'. Researchers at the Centre for Plant Conservation at Southern Cross University collaborated with Forests NSW to study patterns of genetic variation by looking at microsatellites (short, variable repeating sections of the DNA).

Blackbutt is one of the most important timber trees in New South Wales and southern Queensland, supplying wood from both native forests and plantations.

Eucalyptus pilularis

Eucalyptus pilularis (blackbutt) from Eden NSW (photo: R Allen)

Geographic variation is expected in a tree species like blackbutt with a wide latitudinal range, extending from Fraser Island in Queensland to the far south of NSW, due to adaptation and isolation by distance. Coastal and inland ecotypes of blackbutt had been identified previously in genecological studies and provenance trials (ecotypes are genetically based varieties associated with contrasting environments—coastal blackbutt populations differ from inland ones in their flowering time and leaf morphology). However, no clear pattern of regional geographic differences has previously been recognised. In the CRC project, genetic diversity and structure parameters were estimated from 12 microsatellite loci in 426 individual trees from 48 locations across the latitudinal range and both inland and coastal ecotypes of blackbutt.

The patterns of variation in the microsatellite markers showed that although genetic differentiation was weak overall, some geographically based variation was evident. Three zones were identified, a southern zone, encompassing provenances located on the coast to the south of Sydney; a northern zone including provenances from Coffs Harbour northwards to the species’ northern limit at Fraser Island and a third transition zone between these two. The southern zone, which included six provenances from Yerriyong south of Sydney down to Eden, was distinguished by a high and uniform level of ancestry from a single population with a southern geographic focus. The northern zone was more diverse and heterogeneous in its population ancestry whereas the transition zone was intermediate in its levels of population admixture.

The correspondence of genetic zones in blackbutt with biogeographic zones suggested that genome-wide reproductive isolation may be associated with pollinator endemism—that is to say, pollinators (insects, and possibly birds) do not move pollen between the northern and southern zones.

The study also showed that gene diversity in blackbutt (expected heterozygosity, He = 0.78) was typical of other widespread eucalypts and was relatively homogeneous, suggesting the maintenance of large population sizes and strong linkage between populations within zones, due to gene flow. Ecotypes of blackbutt were not genetically differentiated, which suggested that adaption to local site conditions occurs despite extensive gene flow between coastal and inland provenances. The differentiation at neutral genetic markers found in this study, combined with limited evidence for adaptive differentiation in south coast material from provenance tests, suggests northern and southern geographical races should be recognised in blackbutt in addition to coastal and inland ecotypes.

Tim Sexton

CRC PhD Student Tim Sexton collecting foliage from a blackbutt tree at DoubleDuke State Forest, NSW

The recognition of geographic races has implications for association genetics studies in the species. If there is substructure, alleles at candidate genes may be statistically associated with alleles in genes underlying quantitative traits but physically unlinked in the genome. This can lead to false positives in association studies. The recognition of two geographic races in blackbutt suggests substructure may need to be controlled for in association studies that span the full geographic range of the species. However substructure is not likely to be a problem for the CRC’s association study on blackbutt now underway, because this study is restricted to material in the northern geographic race.

The recognition of geographic races also has wider implications for gene pool management, and may indicate a need to develop seed transfer guidelines for plantations or reforestation of blackbutt. 


Merv Shepherd
02 6620 3412