Selective recruitment of bacteria during embryogenesis of an earthworm.

Earthworms of the family Lumbricidae harbor specific and stable populations of Acidovorax-like bacteria within their excretory organs, the nephridia. The symbionts of Eisenia foetida are deposited into the egg capsules during mating and the nephridia of the juveniles are colonized before they hatch. The timing and mechanisms governing bacterial recruitment and colonization are unknown for the earthworm-Acidovorax association. This study examined the process of colonization of the symbiotic organ during development of the embryos within the egg capsules. Bacteria associated with the developing embryos were visualized using in situ hybridization to bacterial cells and laser scanning confocal microscopy. Bacterial cells were associated with earthworm embryos during the earliest stages of development-the ova through to hatching. Three-dimensional examination of stages of development revealed an embryonic duct that recruits the Acidovorax-like symbiont cells. As each segment matures, Acidovorax-like symbiotic bacteria are recruited into this duct, excluding most other bacterial types, and remain there for a period of days prior to migration into the nephridium. After colonization of the nephridial ampulla, the canal remains bacteria-free. In addition to the known Acidovorax-like bacteria, multiple types of bacteria interact with the embryos externally and internally during the full course of development, and ultimately fill the gut lumen near the end of development prior to hatching. Colonization of the correct tissues by specific bacteria during differentiation and maturation of the organs must involve selective host defenses and signaling between the two partners to prevent over growth of nascent tissues.