Genetic bottlenecks in Pristis sawfishes in northern Australian waters

Abstract

Northern Australia has been identified as the last stronghold for the dwarf sawfish Pristis clavata, green sawfish P. zijsron, and largetooth sawfish P. pristis, making these populations key in global conservation efforts for each species. This research assesses the levels of genetic diversity in these 3 sawfishes in Australian waters, testing for the presence of population bottlenecks using data at microsatellite loci. Levels of observed heterozygosity in each species from the west coast of Australia and the north-eastern Gulf of Carpentaria were generally high. M ratio tests suggest that assemblages of P. zijsron and P. pristis on the west coast and P. clavata and P. zijsron in the Gulf of Carpentaria may have experienced population bottlenecks. The bottlenecks are especially pronounced in P. zijsron populations and in P. clavata from the Gulf of Carpentaria. Demographic analyses, based on mtDNA data, indicate relatively recent (evolutionarily) range expansions in Pristis sawfishes in northern Australian waters, which could account for the population bottlenecks. A more recent range expansion in each of P. clavata and P. zijsron, as evidenced by more recent population divergence and more recent/higher rates of historic maternal gene flow, could account for the more pronounced bottlenecks in these species when compared to P. pristis. Given that Pristis sawfishes in Australian waters have experienced population bottlenecks, whether they be historic, contemporary or both, the preservation of remaining genetic diversity should be a high conservation priority.

Mixed-marker approach suggests maternal philopatry and sex-biased behaviours of narrow sawfish Anoxypristis cuspidata

“…as per Fauvelot et al 2003). Similar genetic drift and bottlenecks were reported for Pristis sawfish populations from northern Australia and considered to reflect founder events that occurred during range expansions into new habitats during their evolutionary history in northern Australia (Phillips et al 2017). Genetic drift/bottleneck events as a driver of population structure is supported by the finding of negative Fu's F s values, which indicate the presence of an excess of private mutations that would not be expected if populations had reached equilibrium (Fu 1997).…”

“…Elasmobranch mtDNA evolutionary rates are low (Martin et al 1992), and sawfish mitochondrial diversity is at the lower end of the range for elasmo- branchs (see D'Anastasi, 2010, Phillips et al 2017.…”

“…Male-biased dispersal may provide a number of benefits for populations by reducing inbreeding (Pu sey 1987), dispersing local resource competition (Greenwood 1980) and reducing mate competition (Dobson 1982). Phillips et al (2016Phillips et al ( , 2017 hypothesised that differing patterns in habitat use may explain the observed differences in dispersal bias among Pristis sawfish. However, our findings suggest involvement of other drivers.…”

Is this the last stand of the Critically Endangered green sawfish Pristis zijsron in the Arabian Gulf?

“…Genetic studies investigating the green sawfish in Australia emphasize that this species forms regional assemblages with little parental movement between populations (Phillips 2012). While the boundaries of these areas remain unknown, the implication is that local extinction will not be replenished by outside individuals and that it is critical to identify and manage these individual assemblages as independent populations (Phillips 2012). This is also supported by telemetry data, which suggests that sawfish movements tend to be less than 300 km from the shoreline (Peverell & Pillans 2004.…”

Biology and ecology of sawfishes: global status of research and future outlook

“…Sometimes, when populations of organisms decline severely, genetic diversity is compromised and populations can become inbred, which is problematic for recovery. Studies in the USA and Australia have shown generally high genetic diversity despite precipitous declines (Chapman et al 2011, Phillips et al 2017 this Theme Section), which should provide optimism for recovery, although in Australia, some assemblages of Pristis and Anoxypristis sawfishes may have experienced population bottlenecks or founder effects at some time (Phillips et al 2017, Green et al 2018.…”