Using a genetic approach to optimise dugong (Dugong dugon) conservation management
Tikel, Daniela (1997) Using a genetic approach to optimise dugong (Dugong dugon) conservation management. PhD thesis, James Cook University.
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Abstract
Distributed throughout the coastal tropical and sub tropical waters of the Indo-west Pacific, the dugong (Dugong dugon) is considered rare over much of its range and listed by the IUCN (1996) as 'vulnerable to extinction'. The largest numbers of dugongs are believed to occur in Australian waters. The main threats to dugong numbers are anthropogenic activities, such as accidental netting, habitat deterioration and Indigenous hunting. The primary objective of this study is to recognise intraspecific genetic subdivision for dugong management. Complementing ecological studies, these findings have immediate and practical relevance to the conservation management of the dugong.
Genetic material was sampled from various types of dugong tissue including skin, muscle, bone, and their faeces. Advances upon established sampling approaches for marine mammals include a biopsy system tailored to dugongs and the extraction, amplification and sequencing of dugong DNA from their faeces. Samples from approximately 250 dugongs were collected by carcass salvage, from dugongs hunted by Indigenous peoples, and by remote sampling of free ranging dugongs. Not all the samples collected were analysed, mainly because of low sample quality.
From dugong samples collected from Australia (n=92), West Indian Ocean (n=4) and Asia (n=7), three genetic markers were investigated: the cytochrome b gene and control region of mitochondrial DNA (mtDNA), and microsatellites. Cytochrome b proved to contain insufficient variation for an interpopulation comparison, whereas a block of DNA sequence (194 bases), positioned 5' in the control region of mtDNA, was identified as suitable for interpopulation comparison because of its high variation. A substantial foundation for the development of microsatellite markers for future research was established by this study. In addition to recognising a six base repeat located 3' in the control region of mtDNA, five GT-AC imperfect microsatellites were sequenced from a dugong genome library. The application of microsatellites was not developed further than their location and characterisation.
The hypervariable region 1 of mtDNA was sequenced for a total of 103 dugongs, as well as in an outgroup, the Florida manatee (Trichechus manatus latirostris). Among the dugong sequences, 39 variable sites and 37 haplotypes (specific DNA sequences) were found. Phylogenetic trees constructed from the mtDNA haplotypes showed three clusters: West Australian, East Australian and Asian. These haplotype clusters suggest that Asian and East Australian dugongs are more closely related to each other than either group is to the West Australian dugongs. Surprisingly, haplotypes of West Indian Ocean dugongs are extremely similar to the haplotypes of dugongs from East Australia despite their large geographical separation. This suggests that the West Indian Ocean and East Australian dugongs shared a more recent ancestor compared to the Asian or West Australian dugongs. The geographical range of the Asian mtDNA haplotypes does not overlap with the Australian haplotypes. The two Australian mtDNA haplotype clusters overlap geographically in the Great Barrier Reef region.
The intraspecific genetic partitioning of dugong populations from Australia, Asia, and the West Indian Ocean, can be partly explained in terms of historical geography. The distinction of the two Australian mtDNA haplotype clusters may be attributed to the Torres Strait (land bridge) acting as a periodic barrier to dugong movements during the Pleistocene low sea level phases. It is of particular interest to management that West Australian haplotypes have such a limited spread to the south and east Australian dugong range. Similarly, East Australian haplotypes do not extend north and west along the Australian coast beyond Torres Strait. Considering the dugong's potential for dispersal, the spread of the two Australian haplotype clusters is remarkably limited.
Dugongs have a high intraspecific genetic diversity (average nucleotide diversity = 3.425, and haplotypic diversity = 0.766), and a rate of evolution (suggested to be 2% per Myr per lineage for the HVR-l) that is comparable to most land mammals. The suggested low level of female mediated gene flow between neighbouring populations indicates that successful recolonisation of a depleted area may be extremely slow. For conservation management of the Australian dugong, populations should be treated as distinct units with some degree of overlap. To maintain continuity among these populations, and to preserve the existing high intraspecific genetic diversity, management initiatives should be directed at limiting further fragmentation of the dugongs' range.
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