Landscape genomics of a dryland river keystone species, Eucalyptus coolabah: the influence of hydrochory on landscape genetic structure

Ignition Grant Round 7 (July 2017)

Dryland river systems within the Lake Eyre Basin (LEB) represent some of the few remaining naturally functioning, unregulated desert river systems globally. Given their natural, unregulated, hydrological regimes, these rivers provide critical reference systems against which to compare highly regulated systems such as those of the Murray Darling Basin. The Lake Eyre Basin therefore provides unique and valuable insights into the nature and impact of hydrological dysfunction and possible restorative actions. The associated biodiversity values of these dryland river systems are driven by the naturally functioning hydrology of the system.

The iconic Coolibah or Eucalyptus coolabah is one of the widest-ranging eucalypts in Australia. Eucalyptus coolabah subsp. arida is the dominant subspecies occurring along the dryland river systems within the arid LEB. 

Coolibah is a long-lived perennial and plays a highly significant role in directly influencing ecosystem structure, composition and function of the riparian communities of LEB dryland river systems. In this capacity Coolibah is performing as a keystone species, a biodiversity indicator and a potentially valuable surrogate for monitoring the biodiversity health of the river system. Coolibah is fundamental to the ongoing biodiversity health of the river system.

Coolibah life history, particularly expressed by the synchronous timing of reproductive phenology with a higher incidence of summer flooding events, is dependent on the river system’s naturally functioning hydrological regime. Coolibah fruit matures rapidly and the resulting seed is promptly shed for wide dispersal during major flooding events, both longitudinally down the system and laterally across the adjacent floodplain, a process known as hydrochory. As a result, major recruitment events of Coolibah are highly dependent upon major, uncommon, extensive flood events over a sequence of years.

Throughout the study area (LEB), clusters of an assortment of Coolibah cohorts are visible in the landscape, marking the outer edge of historical floods, which have reached to varying extent across the adjacent floodplains. Recent results have revealed that the most apparently conspicuous of these cohorts, as revealed by 14C dating is contemporaneous with the major flooding events of the 1970’s. Cohorts such as these manifest physically as ‘flood fingerprints’ structuring the landscape; records of floods past. 

There is a dearth of material in the literature relating to the patterns and driving processes associated with the recruitment and distribution of Coolibah at a landscape level let alone the influences structuring genetic variability. An obvious potential outcome of the process of hydrochory would be the impact upon Coolibah gene flow and associated genetic diversity both temporally and spatially. It is possible that, in contrast to typical landscape genetic patterns where relatedness increases with proximity, Coolibah relatedness may be more associated with individuals’ age than proximity. 

During a recent autecological investigation of Coolibah, Eucalyptus coolabah (Gillen 2017) a collaboration was developed between the ANU and the CSIRO Australian Tree Seed Centre (ATSC). This project represents the opportunity both to develop and expand upon this collaboration.

While undertaking field work, Coolibah seed from across the range of the Lake Eyre Basin (LEB) was collected for the ATSC seed bank. Prior to storage at the ATSC a sub-sample of deposited seed from all seed sources was subjected to a mandatory viability analysis. The ATSC generously supplied seed collecting materials as well as equipment for the collection of tree cores for valuable radiocarbon dating as below. During this autecological investigation, Coolibah leaf material was collected opportunistically and systematically with the intent of future genomic analyses should funds be available.

An initial insight into the landscape genomics of Coolibah, in addition to progressing autecological knowledge of the species, would also provide the ATSC with valuable information regarding genetic variability in relation to seed provenance. Future collaborative work envisaged is the identification of genetic markers under selection; an insight into adaptation to a range of environmental factors such as salinity and flooding regimes.

On a larger scale, we are optimistic of using the data generated in this study to seed a larger multidisciplinary investigation in to landscape / continent wide spatial patterning, linking landscape and genetic data for Coolibah.

It is envisaged that the findings from this Ignition proposal would be integrated into this multidisciplinary investigation which would incorporate an expansion of radiocarbon dating work to develop a greater understanding of Coolibah population dynamics and demography.

It is intended that this Ignition Grant will undertake preliminary genomic / SNP analysis of 400 Coolibah samples using Genotyping-By-Sequencing (GBS) to test the above proposal.

In addition, we aim to provide insight into:

  • the structure of genetic variability of Coolibah across multiple river catchments within the Lake Eyre Basin;
  • the nature of genetic variability down the Diamantina/Warburton system;
  • the potential influence of environmental gradients upon genetic composition and variability within the system;
  • the local site level the genetic composition and relatedness between differently aged cohorts;
  • the level of genetic relatedness between Coolibah populations representing two river systems of relatively close geographic proximity (the Cooper and Diamantina systems);
  • the level of genetic relatedness between differently aged and flood related cohorts of Coolibah; and
  • the role of hydrochory on population genetic structure.