A new study on the genetic structure of the Indian tiger has found that the current tiger population now only retain a small proportion of the historical mitochondrial DNA haplotypes. The new study has just been published in Proceedings of the Royal Society B (currently freely available) by a international team comprising researchers from the University of Bangalore (Samrat Mondol and Uma Ramakrishnan) and Cardiff University (Michael Bruford).
The study describes how the tiger’s range has declined and there is thought to be only about 1500 individuals in India, with these individuals responsible for retaining the genetic diversity of what used to be a population of over 40,000 individuals only about 100 years ago. The tigers have declined due to years of hunting pressure, habitat changes and the destruction of connecting wildlife corridors, that has resulted in the population becoming isolated and genetically subdivided. The fragmentation of the region now means that those remaining individuals can no longer be considered as a single population, but constitute many smaller isolated populations of between 20-100 individuals. Even though the tiger is now protected, and there has been a modest population growth in recent years, the population as a whole is still considered endangered.
This current study examined the genetics of modern and museum samples from across the species contemporary and historical range. Historical samples came from the natural history museums in London and Edinburgh. Some of those samples came from areas such as Afghanistan where the tiger is no longer present.
The study examined two types of DNA markers (mitochondrial and nuclear) that allowed Mondol et al. to gain an insight into the historical DNA variance of the tiger and also see how it compared to the contemporary population today.
The results showed that the historical samples contained 25 mitochondrial DNA haplotypes, only four of which are still present in the modern population. The modern population contained an additional 31 haplotypes, which were not seen in the historical samples. Analysis of the nuclear DNA showed that the modern samples contained a higher number of alleles than the historical samples. In the nuclear DNA, the modern samples showed that there were three genetic populations: 1. Peninsular India (Western Ghats and Deccan Plateau) and NE India samples formed one genetic population, 2: the semi-arid region (towards the NW of India) formed a second, and 3: the Terai (in the north of India, sharing a border with Nepal) landscape. When the data was joined with the historical samples, there were actually fewer genetic populations inferred; Peninsular India and NE India samples formed one cluster, and N Indian samples (the semi arid and the Terai landscape) dominated the second population. In fact, it appeared that the genetic diversity had actually doubled for the modern samples, in both nuclear and mitochondrial DNA!
In the face of increased genetic diversity, my initial thoughts were that surely this was a good thing?! The genetic diversity of an endangered species was now higher than it was when compared to historical samples. What could the problem be? Mondol et al. highlight a number of issues. The mitochondrial DNA is inherited through the female line and when you look at the ecology of tigers, you see that tiger dispersal tends to be male mediated, with the females tending to stay closer to home. This means that local populations contain the same mitochondrial DNA haplotypes resulting in region specific haplotypes. As females tend to stay close to the natal site, the loss of habitat in those areas further exasperates the loss of the female inherited genetics, wiping out local genetic variation. The historical samples used in this study were from areas that were heavily hunted and those populations are now extinct. The modern samples tended to come from areas where tigers were hunted less intensively (if at all), and as a result historical samples are not available from those sites.
The nuclear DNA showed additional problems. As the population of tigers has dramatically decreased over the last number of years (despite small recent increases), I expected a loss of genetic diversity. Instead, when Mondol et al. looked at the historical versus modern samples, they found that there was in fact much higher diversity in the modern samples. They suspect that due to habitat fragmentation and the isolation of the remaining populations, that these animals are now becoming more isolated, more differentiated and as a result they are more genetically different to the next population. Mondol et al. say this a ‘red flag’ situation, as genetic subdivision can occur in a population prior to extinction of the isolated population.
However, Mondol et al. acknowledge that sampling bias may have influenced the dramatic results we see here. The museum samples used in this study tended to be from regions where the tiger is no longer present. This means that the modern and historical samples don’t exactly match up, and if historical samples were available from some of the modern sampling locations (and vice versa), we would expect to find some or more overlap between the contemporary and historical genetic diversity of the species.
Mondol et al. conclude that the information gained from this study should now be put into conservation practice. They recommend that the species is managed so that further genetic subdivision and population isolation is avoided. The authors acknowledge that current practice is focusing on habitat connectivity, but they believe that connectivity is needed at a much larger scale and this will involve cross border initiatives.
Mondol S, Bruford MW, & Ramakrishnan U (2013). Demographic loss, genetic structure and the conservation implications for Indian tigers. Proceedings. Biological sciences / The Royal Society, 280 (1762) PMID: 23677341