Cracking the kinship code: Measuring animal dispersal across generations with DNA
Dispersal is a key component of the ecology and evolution of animal populations. It allows animals to colonize new habitats, escape deteriorating conditions, and locate mates. When animals disperse and breed successfully in new habitats that are already occupied by the same species, there will be an exchange of genes. This exchange is essential in maintaining the health of animal populations, which relies on minimising inbreeding and maximising their ability to evolve and adapt to new conditions.
Dispersal has normally been measured in animal populations through a “mark and recapture” approach, where animals are caught, marked in some way such as through tags or bands (applicable to larger animals like birds and mammals) or fluorescent dust (applicable to insects and small animals), and then released before being captured a second time. This approach provides a measure of dispersal between the two capture intervals. However the approach has limitations. When animals are captured and marked, they may behave unnaturally – they may become agitated and therefore disperse further than normal. Tags and other marking methods can also interfere with the animals; for instance, tracking collars or conspicuous markings can make animals more prone to predation by spoiling their camouflage. Animals can also become stressed from handling during multiple captures and handling.
LEFT: A mosquito marked with fluorescent dust (Mengjia Liu). RIGHT: a tigress with a GPS tracking collar (Wikimedia commons).
Once an animal is tagged, movement patterns can only be established across time intervals determined by the capture effort. It is difficult to measure animal movement across the entire lifespan of an animal unless they can be tagged at birth. Moreover, the movement of genes across the landscape cannot be easily measured because this depends on animals successfully reproducing after they have dispersed. Ideally the flow of genes needs to be measured across generations.
To overcome some of these issues, an alternative approach to measuring dispersal and its impact across generations is to use high density molecular markers. These markers can effectively fingerprint individuals to allow them to be identified within a population. Importantly, the approach also allows individuals to be tested for relatedness to other individuals in a population. Thus it is possible to link parents to their offspring and siblings to other siblings. With thousands of genetic markers available for marking individuals, it is even possible to link individuals to their first cousins. This approach therefore provides a way of measuring successful dispersal across one, two and even three generations.
The dispersal components that separate pairs of mosquitoes of each kinship category
In our latest research published in Molecular Ecology Resources, we have developed and applied this approach to investigate movement patterns in mosquitos that transmit Dengue in Kuala Lumpur. We show that in high rise buildings of 18 stories the siblings are almost always found in the same building within an apartment complex. Mosquitos tend to move around a single building, but not across to another building in the same complex. Dispersal events across buildings starts to happen across two or three generations. Cousins can therefore often be found in adjacent buildings, but rarely siblings.
These findings help to guide local efforts to control mosquitos. Where there is an outbreak of mosquito-transmitted disease in a building, it may be sufficient to target mosquito control efforts to that building alone. However if efforts are directed at long term suppression of mosquito populations, the entire apartment complex will need to be targeted, otherwise the building will be recolonised across 3 generations by mosquitoes re-colonising from another building.
Lines indicate pairs of full-siblings (a), half-siblings (b), and cousins (c).
Importantly, the approaches we have developed can be applied to measure dispersal within and across generations for any species, including not only pests that need to be controlled but also species of conservation concern where the main interest might lie in the protection of habitat that is used by populations. By establishing patterns of dispersal across multiple generations of a threatened species across an area, it becomes possible to identify land that needs to be prioritised for protection. And because DNA can be sourced from tissue samples such as hair samples and scats, the threatened animals don’t necessarily have to be captured in this approach. It is therefore ideal for species whose conservation status prevents them from being handled.