In mice and humans, the gut is lined with a layer of mucous -- and it is assumed to be the same in other mammals. But this style of gut lining is a new development, evolutionarily speaking; most invertebrates protect their gut walls with a membrane made of chitin, which acts as a barrier to microbes and helps prevent infection. But until now, scientists were unsure about which came first, or if they are even related. But a team led by Dr. Keisuke Nakashima of OIST's Marine Genomics Unit were keen to find the answers.
Chitin is an abundant biological material that is produced by a wide variety of organisms. From crab shells to the cell walls of fungi, chitin is second only to cellulose when it comes to its commonplace existence. A versatile building material for nature, chitin is even found in the gut linings of invertebrates, where it acts as a defensive barrier against potentially harmful microbes.
Vertebrates - Share - Gut - Layer - Bacteria
But strangely, most vertebrates don't share this chitin-lined gut. Instead they protect themselves with a layer of mucous that bacteria are able to colonize but not penetrate. Dr. Nakashima's team noticed a group of animals that had both chitin and mucous coexisting in its gut: Tunicates.
Also known as sea squirts, tunicates are simple animals that live by filtering food particles from seawater. While they don't have a spine, they are closely related to the vertebrates, with a similar genomic background that make them ideal for studying their evolution. But it's the uniqueness of their gut lining that attracted the research team: "We noticed that tunicates seemed to have an intermediate kind of gut lining that could show that mammalian and invertebrate gut linings shared an evolutionary link," said Dr. Nakashima.
Linings - Chordates
It wasn't only tunicates that had these transitional linings -- other simple chordates...
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