How do nematodes and annelids differ from phyla

Symbioses with partner swaps

In worm-bacteria symbioses, some microbes remain loyal to their host, others to their location

If your favorite landlord changes town - will you move with them or are you looking for a new local hangout? In the case of bacteria living in symbiosis with sea worms, this depends entirely on whether they prefer to sit in front of the bar or in the bar with their host. Scientifically speaking: Bacteria living on the surface of the body are loyal to the host, whereas the bacteria living inside their hosts are more faithful to their location, as researchers at the Max Planck Institute for Marine Microbiology in Bremen have now shown.

Nematodes and annelids. These are roundworms and annelids. You might think that they're all worms. "In fact, these two strains are about as different as humans and turtles," says Judith Zimmermann from the Max Planck Institute for Marine Microbiology in Bremen. "Nevertheless, they harbor closely related bacterial symbionts."

The lodgers of the two worm animal strains all belong to a group of very closely related bacteria, the Candidatus Are called thiosymbion. These bacteria provide their hosts with food. On those belonging to the roundworms Stilbonematinae they live on the surface of the body. “They wrap the worm like a sleeping bag that only leaves the head and the tip of the tail uncovered,” says Zimmermann. In the case of the gutless little bristles (oligochaetes) belonging to the annelid worms, however, the bacteria live inside the body under the skin and supply their hosts with food so well that they have even lost their mouth and intestines.

“We found out that bacteria in this group seem to have switched several times in the course of evolution between roundworms and ringworms and thus also their way of life between ecto- and endosymbiosis, says Zimmermann. “Such flexibility is really amazing, since symbiotic bacteria are normally only adapted to one way of life and only one host group,” explains Cecilia Wentrup, who was also involved in the study. "It was only through the large amount of new data that we were able to reconstruct the closely interwoven evolution between the symbionts of these sea worms."

Superficial and loyal or intimate and erratic

Despite the great flexibility, the symbionts are very loyal to their hosts in some respects. Here too, Zimmermann and her colleagues had a surprise. Because contrary to expectations, the superficial roommates seem to be more loyal to their hosts in the long term than the internalized lodgers. "Long-term here means over millions of years," explains Zimmermann. “The relationship between the host and the symbiote is very stable in the nematode and their bacteria. They seem to have developed together in the course of evolution without changing partners. ”In the case of nematodes, hosts that are closely related also have closely related symbionts. The Bremen researchers found this loyalty to the host in roundworms and their symbionts all over the world - from Sylt to the Caribbean, from the Mediterranean to Australia.

The annelid worms present a different picture. “Annelworms and their symbionts are less organized,” explains Wentrup. Here it is not just the type of host that decides which bacteria will be found. Rather, the place of residence also seems to play an important role. The researchers often found distantly related host species in a certain region that harbor very closely related bacteria. Closely related annelids, on the other hand, often had different tenants if the samples came from different geographic regions. “Closely related annelids that live in Australia and the Caribbean do not necessarily have closely related symbionts,” says Wentrup. "This suggests that the annelids have repeatedly exchanged their symbiotic lodgers for other local bacteria."

The present results show how changeable and full of surprises the marine communities are. "Now we want to find out which factors are responsible for the different stability and lifestyle of the symbionts," says Nicole Dubilier, Director at the Max Planck Institute for Marine Microbiology. What does it depend on whether the bacteria are from Candidatus Thiosymbion stick to the surface of the worm's skin or under the skin that is anchored? How do the hosts and their bacteria recognize each other? And is it more likely that an ectosymbiont will become an endosymbiont or vice versa? The researchers at the Max Planck Institute want to find out in further studies.