Barramundi (Lates calcarifer) is a centropomid teleost with a wide distribution across the
Indo Pacific. In Australia, barramundi are native to the tropical zone from Exmouth Gulf
in Western Australia, across the northern part of the continent, to the Mary River in
Queensland. Barramundi are protandrous hermaphrodites, and are euryhaline, with a
catadromous life history. Barramundi are a valuable Australian resource, with important
commercial and recreational fisheries and aquaculture production to the value of $11
million dollars per year. Recent declines in the availability of the fish in some rivers has led
to an interest in the possibility of restocking rivers with barramundi from other areas.
Determining the genetic structure of barramundi populations in Australia is important for
understanding biogeographic history, and appropriate management practices for both
aquaculture and recreational and commercial fishing. Previous studies have concentrated
on the east coast of Australia, and have largely ignored the western populations. In this
study, I obtained DNA data from barramundi populations across the Australian range of
the species, as well as populations from Papua New Guinea and Indonesia. The aims of
this study were to use the genetic data to determine: 1. if populations in Western Australia
show genetic differences between geographic regions 2. if these populations show an
ancestral split from populations in the east of Australia and 3. the ancestral origins of
Australian barramundi.
Previous studies of DNA data from barramundi have discovered an east/west split
occurring at the Torres Strait that was assumed to be caused by the closing of the strait
during lowered sea levels. However, these studies suffered from a bias in sampling area,
concentrating either on the eastern half of the range of barramundi, or on the western tip
of the range. Data from these studies were combined and reanalyzed. Two major clades
were discovered, with considerable biogeographic structuring, but their geographic
locations did not coincide with the reported vicariance event at the Torres Strait. Instead,
historical divisions among freshwater drainage systems appeared to have driven the
evolutionary history of barramundi in Australia.
In order to investigate these historical divisions further, a 290 bp section of the
mitochondrial DNA control region was sequenced in 284 barramundi from seven
populations across the Australian geographic range of the species and from one population
in Papua New Guinea and one population in Indonesia. Analyses of molecular variance
within and among populations showed significant geographic structuring, based on
biogeographical provinces and drainage divisions. Nested clade analyses indicated that
these geographical associations were the result of restricted gene flow, range expansion,
and past fragmentation events. I hypothesise that the Ord River area in the west of the
continent was the ancestral source population for the rest of the species range across
Australia, with Indonesia being the most likely origin of this source. Populations of
barramundi from the Pilbara region are genetically distinct and geographically isolated, with
strong evidence of an ancestral divide along geographical barriers to dispersal. There is a
strong association between Papua New Guinea and Australia, although further investigations using the cytochrome b region of mitochondrial DNA indicated a more
ancestral divide between the two than is currently evident, which could reflect an ancient
geographical divide between the two, or could be evidence of a secondary migration route
to Australia.
For a more detailed study of evolutionary processes acting on populations of barramundi in
Western Australia, allelic diversity was examined at five microsatellite loci. All loci were
polymorphic and genotypic frequencies conformed to Hardy-Weinberg expectations, with
no significant linkage between loci evident in any population. Measures of within
population diversity were significantly related to latitude, suggesting southerly migration
from a northern source population. The Ord River was the most genetically diverse
population, and the most likely ancestral migration source to the area, with diversity
decreasing down the west coast. Although there were significant differences among
populations, the nuclear microsatellite data do not indicate the same degree of genetic
structuring as is evident in the mitochondrial data. This may be a consequence of rapid
evolutionary change at microsatellite loci, with past separations or population differences
masked by recombination and back mutation of the microsatellite alleles. However, the
nature of nuclear and mitochondrial inheritance may also indicate life history differences
between the sexes, where significant genetic contribution to gene flow by males and limited
female gene flow may lead to preservation of maternally inherited population substructure.
The principal findings from this study are:
There is no genetic evidence for an east/west division of barramundi populations
in Australia, as suggested by previous research.
Despite barramundis catadromous life history, and ability to disperse through
marine waters, the present genetic structure indicates a division principally among
river drainages. From a population genetic viewpoint, the species can be regarded
as freshwater, rather than marine.
The most likely origin of barramundi in Australia is the Ord River region, with
Indonesia as the route of migration.
Differences in the population structure demonstrated by nuclear and mitochondrial
data indicate possible life history differences between the sexes.
Barramundi populations in different biogeographical provinces may have been
substantially isolated over a long period of time, and may therefore represent
independently evolving populations. This has important implications for fishery
management and translocation issues for restocking rivers.
| Identifer | oai:union.ndltd.org:ADTP/221639 |
| Date | January 2005 |
| Creators | cmarshall@tobob.com, Carina Rynn Ecremen Marshall |
| Publisher | Murdoch University |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://www.murdoch.edu.au/goto/CopyrightNotice, Copyright Carina Rynn Ecremen Marshall |
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