Developing earthworm embryos weed out unwanted microbes, while attracting their favoured bacterial symbionts to colonize their excretory organs.
Eisenia foetida bequeath their offspring a gift of Acidovorax-like bacteria, deposited directly into egg capsules where embryos will develop. Seana Davidson and David Stahl of the University of Washington in Seattle watched as the bacteria colonized the developing earthworm embryo. They found that the Acidovorax-like bacteria were selectively recruited to a small canal in each segment of the earthworm, where they lingered until excretory organs called nephridia developed sufficiently for the bacteria to colonize them. This selectivity for their symbionts suggests that the embryos fend off colonization by other microbes.