Digging Deep: What New Research Shows About the Genetic Diversity of Indian Gibbons
A recent study from CSIR-Centre for Cellular and Molecular Biology, Hyderabad, sheds light on the population structure of western hoolock gibbons (Hoolock hoolock) in the northeastern region of India. The species occupies a region marked by large rivers that flow throughout the year and provide an important geographic barrier to populations on either side of the mix. Human-caused disturbances of the landscape have made the problem even more acute, giving rise to more isolated subpopulations that are part of a larger “metapopulation”.
In order to map the genetic diversity of western hoolock gibbons, the researchers collected fecal samples from across their geographic range. In order to obtain these samples, individuals of western hoolock gibbon were closely monitored for a few days to ensure a robust sampling strategy.
Fecal remains are useful because they consist not only of epithelial cells from the intestine, but also of undigested food remains. In an email conversation with indianexpress.com, Dr Govindhaswamy Umapathy, the corresponding author of the study, explained that “when we extract DNA from faecal samples, we find all kinds of DNA, including including food, but we’re just amplifying it further by using primates.” specific markers for further analysis where DNA from other species will be left behind.
Fecal DNA has been widely used in the past to understand the occupancy, population size, and diet of a wide range of species. Additionally, “barcoding of undigested and digested food materials can be done using barcode markers to identify food species,” adds Dr. Umapathy. Finally, faecal samples were supplemented by blood samples from gibbons kept in captivity in zoos in Assam and Manipur.
The study taps into mitochondrial DNA (mtDNA) from the remains, which, unlike nuclear DNA (nrDNA), is monoploid. In other words, mtDNA does not exist as a pair of chromosomes and instead exists as a small circular chromosome. Since it does not undergo recombination like rDNA does, it is inherited “as is” from mother to offspring, making it highly susceptible to demographic bottlenecks like epidemics. These characteristics make mitochondrial DNA a very useful molecular marker in ecological studies.
Ultimately, genetic diversity within and between populations must be quantified. Since mtDNA is inherited uniparentally as a unique set of traits (aka haplotype), a good metric is “haplotype diversity”. In other words, haplotype diversity describes the number and diversity of different mitochondrial haplotypes. Another metric used by Trivedi et al. (2022) was nucleotide diversity (), which quantifies how different genetic sequences are on average (a strand of DNA is nothing but nucleotides stitched together).
Finally, the study calculated fixation statistics (FST) which measure allele frequencies between two or more populations. An allele is basically a type of a particular gene, for example for a gene that determines the color of flowers, one allele will code for yellow, another allele for pink and so on. Simply put, the higher the FST, the higher the genetic difference between two populations and vice versa.
For the western hoolock gibbon metapopulation, the authors found high haplotype diversity and low nucleotide diversity. These values are nevertheless higher than those of most primates, which “suggests a complex evolutionary history…with multiple instances of bottlenecks and diversification…rivers and other geographic barriers in this region may have influenced the high diversities of haplotypes and weak nucleotides in the west”. gibbons hoolock.’
The study identifies three subpopulations with 27 haplotypes. The “southern” subpopulation apparently has a genetic continuum with their counterparts in Bangladesh. The “northern” subpopulation includes those in parts of Meghalaya, Assam (Tinsukia and Hollongapar) and Arunachal Pradesh (Roing). Samples collected from Wakro, Arunachal Pradesh form a third distinct subpopulation.
FST calculations show a fairly high value among all populations (i.e. North-South, South-Wakro, North-Wakro), implying a high level of genetic differentiation and limited gene flow among all. The authors speculate that this is likely due to the river system dividing the region and preventing populations of western hoolock gibbons from mixing with each other.
The study notes, however, that these populations are stable and resilient to habitat fragmentation and that their work will allow for more targeted intervention in conservation efforts.
The author is a researcher at the Indian Institute of Science (IISc) in Bengaluru and a freelance science communicator. He tweets at @critvik.