SIMPLE animals such as insects, worms and crustaceans may have more complicated immune systems than we thought. Water flea babies have been found to inherit immunity to bacteria from their mothers, suggesting that biologists may have to abandon the idea that invertebrates do not have advanced immune systems that 鈥渞emember鈥� pathogens.
Tom Little and his team at the University of Edinburgh infected female water fleas (Daphnia magna) with a strain of the bacterium Pasteuria ramosa. They then challenged the fleas鈥� offspring with either the same strain or a different one. Fewer babies became infected when exposed to the same pathogen that had infected their mother.
鈥淎lthough it was a small difference, [these] offspring not only produced more babies, but also earlier,鈥� says Little. He calculates that Daphnia with this inherited immunity would produce double the population within three generations (Current Biology, vol 13, p 489).
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Invertebrates lack the cells to produce the antibodies that vertebrates use to remember and kill pathogens that have infected them before. Instead they produce anti-microbial peptides. Scientists have assumed they lack antibodies because they do not live long enough to encounter the same pathogen twice.
But Little says that an immune system that remembers makes evolutionary sense: the short lifespans of invertebrates mean an individual is very likely to encounter the same parasites as its parents. And many invertebrates, including Daphnia, produce clones of themselves, so inheriting immunity would be a great benefit. How immunity is passed from mother to offspring remains a mystery, but Little thinks mothers may add the anti-microbial peptides to their eggs.
Michael Siva-Jothy, at the University of Sheffield, says the finding shows that just because invertebrates do not have antibodies, we should not assume they lack an advanced immune system. 鈥淧erhaps the underlying mechanism is less important than the consequences. The beauty of it is that it shows that a pretty unimpressive organism can do some phenomenal stuff.鈥�