The origins of kin discrimination—telling kith from kin

Last week Sarah Jane explained the evolutionary advantage of helping out family members, an idea known as kin selection. While the benefits are clear, we don't know how kin selection initially evolves—is it directly selected for or can it arise as a beneficial side effect of other adaptations?

You can't look out for your family if you don't know who they are. We usually don't have trouble picking out relatives, but that's because of our social habits. You probably can't tell whether someone you've never met is a relative—based on just their scent, for example. Some animals can. Ants and bees are coated with complex organic molecules which they use to identify members of their colony, who are their relatives; there's also evidence that plants can identify their kin.

A team of researchers turned to the social bacterium Myxococcus xanthus to explore how kin discrimination can evolve. When faced with a lack of nutrients, M. xanthus cells cooperate to build a fruiting body. When different colonies meet, they may work together to make a mixed fruiting body or they may be incompatible, leading to an empty border zone between them. The team collected three M. xanthus isolates from the wild and tested whether they formed fruiting bodies with each other. Colonies readily formed fruiting bodies when they met another colony from the same isolate, but only rarely with colonies from another isolate. In other words, the colonies seem to be able to identify related colonies and prefer to make fruiting bodies with them.

To figure out how this discrimination came about, the team turned to evolutionary experiments in the lab. They grew 104 M. xanthus populations for many generations; the populations were grown on different nutrient media, setting each off on its own evolutionary trajectory. After some time, they tested pairs of colonies to see how well they formed fruiting bodies together. First, they paired off colonies from the final generation with their ancestor from the first generation. Fruiting bodies formed in roughly half of the pairs; in the other half, the ancestor and descendent had diverged too much to recognize each other as kin. Next, they tested pairs from successive generations to figure out when the break happened. In about half of these lineages, ancestors and descendants diverged gradually; how well a pair cooperated depended on how many generations separated the two colonies. In the other half, the break was sudden. These patterns show that M. xanthus can evolve two different kin recognition mechanisms: an all-or-nothing type based on a single genetic difference or a graded distinction based on many genetic differences.

The final test was to pair kin-discriminating colonies from different lineages and see how they responded to each other. If these pairs failed to make fruiting bodies, it would mean that their kin discrimination system worked across evolutionary lineages, not just within them. Again, fruiting bodies formed in roughly half the pairs but not in the other half.

The main point of this research is that kin discrimination appeared within and between M. xanthus lineages even though it was never selected for. This doesn't mean that kin discrimination doesn't evolve as an adaptive trait; it simply shows that it can emerge as a consequence of other evolutionary changes. Once they're in place, kin discrimination and kin selection can act as important evolutionary forces, but that doesn't necessarily mean they were originally selected as adaptive traits; useful traits may start off as side effects of other changes. It's an important lesson not only for thinking about kin selection, but also about evolution more generally.

Ref
Rendueles, O., Zee, P.C., Dinkelacker, I. et al. Rapid and widespread de novo evolution of kin discrimination. PNAS 112:29(9076-9081) 2015. doi:10.1073/pnas.1502251112

Image credits
The M. xanthus image is by Wikimedia user Trance Gemini and is distributed under a CC-BY-SA license.