Population structure and genetics

Why is understanding population structure and genetic health important for river dolphin conservation?

Although genetic studies may sound like they belong solely in the realm of laboratories and academic journals, they have very important conservation applications. While we know that almost all river dolphin populations are small and threatened, understanding more about a population’s potential genetic uniqueness, or separation from other neighbouring populations can lend additional urgency to efforts to protect that population. If genetic studies prove that a particular population can no longer be ‘replenished’ by interbreeding with neighbouring populations, or if a small population is found to have so little genetic diversity that it is at risk of in-breeding related health risks, it may be necessary to consider more aggressive conservation measures. On a more positive note, the granting of full species status to a population or subspecies can create a sense of pride for stakeholders, who feel a sense of ownership and investment in protecting ‘their’ species, and spur more effective conservation action.

How do we study dolphin population structure and genetic health?

The first step in genetic studies is the collection of tissue samples that can be sent to a lab for analysis. Only a tiny sliver of skin or other tissue (tooth, bone, blood or muscle) is required, and samples can be stored in ethanol, or a concentrated saline solution, at room temperature for several days after collection before freezing for long term storage. As such, it is relatively easy to collect samples from stranded or bycaught carcasses, or from live animals during rescue or satellite tagging operations.  Samples can also be collected from bone or dried tissue in museums or other historical collections. 

Once samples have been collected they can be analysed and used to address the following types of questions:

• Questions of taxonomy: Does a particular population, inhabiting a particular river basin represent a sub-population, a subspecies, or a species in its own right? There are clearly prescribed criteria for designating species and subspecies, which are continually being reviewed in light of new genetic techniques that are developed^{1, 2}, and it can often take a long time to compile the necessary lines of evidence to make a strong case for species status. Genetic analysis, accompanied by detailed morphometric studies, has been used to support the recognition of the Ganges and Indus river dolphins as two separate species³⁵. Deliberations are still underway to determine whether the Bolivian river dolphin, which is currently recognised as a sub-species of the Amazon river dolphin (Inia geoffrensis boliviensis), should be recognised as a full species. There is also a very strong case based on genetics, morphological differences and physical barriers to mixing to recognise the Araguaian river dolphin (putative species Inia araguaiaensis) as a separate species ⁶¹⁶.
• Questions of population status or fragmentation: Genetics can also be used to demonstrate where and how populations are diverging into separate units that can no longer interbreed due to fragmentation of their habitat. This is the case in the Yangtze river, where genetic studies showed that barriers are preventing gene flow between the three remaining fragmented populations^{17, 18}.
• Questions of genetic diversity and health: Species whose populations are already small may be reduced to such low levels that they are more susceptible to other problems associated with low population size, such as increased levels of inbreeding and the loss of genetic diversity through genetic drift. As a result, their risk of extinction may be substantially increased. This is  a pervasive problem in river dolphins around the world where population sizes are decreasing. A study of Irrawaddy dolphins in the Mekong found that 85% of 60 sampled dolphins all shared the same maternally inherited haplotype, an indication of severe inbreeding depression, with the current population size only being 5% of its original/ancestral size¹⁹. The Indus river dolphin also suffers from very low genetic diversity³ as do the three remaining Yangtze finless porpoise populations^{17, 18}.
• Other information that can be obtained from genetics: Advances in modern laboratory techniques now make it easier to generate longer genomic sequences that have increased the ability to understand things such as gene flow, past demographic events, historical population size, and the detection of genes under past or current selection. It is possible to use genetic fingerprinting to identify individuals thereby allowing for genetic capture-recapture methods to estimate population abundance, or identify movements of known finger-printed individuals, and genetics can also be used to identify the species of products for sale in markets or that are being traded illegally. Genetics is used for management by identifying ‘evolutionarily significant units’ that need to be managed separately from other components of a population.