evolution: symbiosis review (Introduction)

by David Turell , Tuesday, October 04, 2022, 16:09 (779 days ago) @ David Turell

A huge review article covering current research:

https://www.the-scientist.com/features/symbiotic-organs-extreme-intimacy-with-the-micro...

"All multicellular creatures interact with bacteria, but some have taken the relationship to another level with highly specialized structures that house, feed, and exploit the tiny organisms.

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"While many of these symbiotic organs have traditionally been studied as peculiarities of particular species, some researchers are now pushing to consider them collectively, as extreme examples of what happens when multicellular organisms develop intricate relationships with the microbes around them. In all of these cases, “you create this emergent organ that would only exist in the context of the interaction,” says Joel Sachs, an evolutionary biologist at the University of California, Riverside (UCR) who studies bacteria-housing root nodules that endow many plant species with the ability to fix nitrogen. “Once that occurs, it reshapes the evolution of both the host and the symbiont. And that’s the commonality where I think it makes sense to join these crazy, diverse systems and start to compare them side by side to see these similar dynamics.”

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"Despite the obvious differences across scales and phyla, there are important similarities in how these organs establish their symbioses, Sachs and UCR postdoc David Fronk argue in a recent paper. For a start, symbiotic organs are well equipped to control where a symbiont can and can’t settle. Nutrient-filled crypts, for example, appear in symbiotic organs across the animal kingdom, suggesting that there are benefits to confining bacteria in this way. Restricting interactions to these specific areas stops a symbiont from taking over other host tissues while letting the host focus its energy expenditure on feeding and housing the microbes in that space, Sachs says.

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"Symbiotic organs also employ common mechanisms for ensuring that they only welcome desired guests. In beewolves and attine ants, for example, symbionts are transmitted directly among individuals in a population, eliminating some of the risk of environmental contamination. (While the beewolves have their brood cell secretions, the ants propagate microbes largely through physical contact between adult ants.) This sort of inheritance can have important consequences for bacterial evolution, notes Kaltenpoth. His group showed recently that beewolf symbionts are undergoing a reduction in genome size and complexity, consistent with their protected existence and reliance on hosts for transmission.

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"However hard a host tries to attract the symbiont it wants, there’s always a risk that the microbes won’t hold up their side of the bargain. Bacteria reproduce much faster than the host they live in, and any strain that manages to hold onto its house without doing the costly work the host wants is likely to gain an advantage over its hardworking peers. Consequently, many multicellular organisms with symbiotic organs have evolved mechanisms to monitor and punish microbial cheaters.

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"Despite such advances in understanding the biology of symbiotic organs, much about the intricacies of host-symbiont communication have yet to be worked out. Some symbionts, such as the bacteria living in tubeworms, are still impossible to culture in the lab, notes Cavanaugh, who also studies symbioses in bivalve mollusks and anemones. Other microbes are being sequenced and scanned for clues as to how they find their hosts, signal to those hosts that they’re performing their work, or interact with the host immune system to maintain their unusual relationship. Such studies could shine a light on microbial interactions across multicellular organisms, not just those that have developed separate organs for the purpose, Nyholm says.

"For example, “by understanding how the innate immune system is used to tell the difference between symbiotic and pathogenic or not-symbiotic bacteria, we can really discover some evolutionarily conserved mechanisms by which all animals detect bacteria,” he explains. “This is an open question still in symbiosis, whether you’re talking about the human microbiome, or a mouse, or a squid, or a zebrafish, or a plant: How do the partners find each other, and what’s the language they use to talk to each other?”

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"It can be just as useful to study collapse in symbiosis as it is to study how it arises, notes Sachs, adding that while many plant species produce nodules, others seem to have lost the trait. Studying symbiont loss can help researchers understand not only the costs and benefits of symbiotic relationships, but also the long-term effects of the relationship on a species’ physiology and genetics. It’s a reminder, too, that even when you evolve an entire organ to host your microbes of choice, “symbiosis is this knife-edge,” Sachs says. “It’s beneficial for the host under a certain set of scenarios. But you alter the ecology, and suddenly it becomes neutral or even harmful.'”

Comment: I've only reproduced the commentary. Read to see the amazing symbiotic arrangements.