Symbiosis: the evolution of host symbiont dependence

Toby Kiers and colleagues have unraveled the factors that drive hosts, from insects to squid, to become more or less dependent on their bacterial symbionts over evolutionary time. Their results reveal how hosts become strongly dependent on relationships with symbionts that provide food, rather than protection. They also found that symbionts passed down from parents are more important to the host survival than those picked up from the environment.

07/06/2017 | 11:26 AM

Food or defense
Bacterial symbionts can provide many functions for their hosts, producing chemicals to ward off the enemies, or providing the most simple basic blocks of nutrition. If these symbionts are lost or removed, the host may die. But are some functions more important to host survival than others? When Kiers asked this question, she found that symbiotic relationships where an exchange of food, such as a nutrient, is involved, host show more dependence than the ones where the symbiont acts as defender of its host.

From parents to offspring
Kiers and colleagues also found that it matters how the hosts find their symbionts. Symbiotic relationships develop in two different ways: the host organism picks up its symbiotic bacteria species from the environment, without any help from parents. This is called horizontal transmission. The other way is for parents to pass symbionts directly on to their offspring. This is the so-called vertical transmission. Kiers showed how hosts, across the tree of life, become more dependent on inherited symbiotic relationships via their parents than those acquired from the environment.

The results, published in Nature Communications, help us predict which hosts will suffer the most from losing their symbiotic partners. In some cases, for example in marine flat worms found in tropical subtidal sediments, host dependence on symbionts is so extreme that the hosts have lost their mouths and digestive track for eating, and rather just absorb nutrition from their bacterial partners.

Database and modeling
For their research the team compiled a huge database with 110 diverse symbioses, from insects to squids, in order to test hypotheses of how host dependence evolves. They developed models to test various hypotheses, using the database as input for these models. Kiers: “At the start of the project, we were initially overwhelmed at the diversity of symbiotic partnerships. However, as more datapoints came in, we started seeing broad patterns emerge. It’s beautiful to connect the dots.” 

Photo: A light-producing bacteria that helps the squid defend against predators.
Credits: C. Frazee, and contributed by M. McFall-Ngai