Demographic collapse and low genetic diversity of the Irrawaddy dolphin population inhabiting the Mekong River

Abstract

In threatened wildlife populations, it is important to determine whether observed low genetic diversity may be due to recent anthropogenic pressure or the consequence of historic events. Historical size of the Irrawaddy dolphin (Orcaella brevirostris) population inhabiting the Mekong River is unknown and there is significant concern for long-term survival of the remaining population as a result of low abundance, slow reproduction rate, high neonatal mortality, and continuing anthropogenic threats. We investigated population structure and reconstructed the demographic history based on 60 Irrawaddy dolphins samples collected between 2001 and 2009. The phylogenetic analysis indicated reciprocal monophyly of Mekong River Orcaella haplotypes with respect to haplotypes from other populations, suggesting long-standing isolation of the Mekong dolphin population from other Orcaella populations. We found that at least 85% of all individuals in the two main study areas: Kratie and Stung Treng, bore the same mitochondrial haplotype. Out of the 21 microsatellite loci tested, only ten were polymorphic and exhibited very low levels of genetic diversity. Both individual and frequency-based approaches suggest very low and non-significant genetic differentiation of the Mekong dolphin population. Evidence for recent bottlenecks was equivocal. Some results suggested a recent exponential decline in the Mekong dolphin population, with the current size being only 5.2% of the ancestral population. In order for the Mekong dolphin population to have any potential for long-term survival, it is imperative that management priorities focus on preventing any further population fragmentation or genetic loss, reducing or eliminating anthropogenic threats, and promoting connectivity between all subpopulations.

Mitochondrial DNA diversity and population structure in the Irrawaddy dolphin (Orcaella brevirostris) from the Gulf of Thailand and the Mekong River

“…The 28 samples analyzed in this study would then represent 32% of the total population, suggesting reduced genetic diversity. Reduced genetic diversity both at the mitochondrial and nuclear levels was also reported in the study by Kr€ utzen et al (2018). However, one would expect that a population with such a little abundance would have reduced levels of genetic diversity.…”

“…Krützen et al . () present a phylogenetic analysis with a shorter fragment of the mitochondrial control region (384 bps) and they suggest reciprocal monophyly between the haplotypes found in the Mekong River and those found in other populations, all of them coastal. However, the Bayesian posterior probability support value for this monophyletic clade is low ( PP = 0.66).…”

“…) and, recently, a study on genetic diversity and demographic trends of the Mekong River population was published, suggesting reduced genetic diversity and steep population declines, with the current population representing only 5.2% of the ancestral population size (Krützen et al . ).…”

“…Reduced genetic diversity both at the mitochondrial and nuclear levels was also reported in the study by Krützen et al . (). However, one would expect that a population with such a little abundance would have reduced levels of genetic diversity.…”

“…To date, no systematic genetic sampling and analyses have been undertaken for O. brevirostris along its range. There is only one published descriptive study presenting mitochondrial diversity of the population found in the Chilika Lagoon in India (Jayasankar et al 2011) and, recently, a study on genetic diversity and demographic trends of the Mekong River population was published, suggesting reduced genetic diversity and steep population declines, with the current population representing only 5.2% of the ancestral population size (Kr€ utzen et al 2018).…”