Transmission of the Symbionts

When gutless oligochaetes become fully mature they develop so-called genital pads, a pair of sack-like pockets on the ventral side of the worms close to the oviporus, the opening through which eggs are deposited. These genital pads are packed with symbiotic bacteria and are only separated from the environment by a very thin cuticle layer of the worm (Giere and Langheld 1987). While all genital organs and the maturing eggs inside of the worm appear to be free of bacteria, the freshly laid egg is surrounded by bacteria that are enclosed in an extracellular space between the inner egg membrane and the outer egg integument (Krieger 2000). It is assumed that these bacteria are transmitted from the parent to the egg via the genital pads, which are believed to rupture during egg deposition (Giere and Langheld 1987). Several hours after egg deposition, the bacteria accumulate at one pole of the egg and penetrate the egg membrane (Krieger 2000).

In contrast to all other oligochaetes that deposit their eggs in cocoons, the eggs of gutless oligochaetes are deposited singly and immediately adhere to the surrounding sediment because of a sticky mucous covering on the outside of the egg integument (Giere and Langheld 1987). In addition to the vertical transmission of symbionts via the genital pads, the

Fig. 2. Phylogenetic placement of the symbiotic bacteria in gutless oligochaetes, based on parsimony analyses of 16S rRNA sequences. The host species are from Bermuda (I. leukodermatus in dark blue), the Bahamas (I. makropetalos in light blue), the island of Elba in the Mediterranean (O. algarvensis in light green and O. ilvae in dark green), the Australian Great Barrier Reef (O. loisae in purple), and the continental margin of Peru (O. crassitunicatus in yellow). As many as six bacterial phylotypes can co-occur in a single host (e.g., O. crassitunicatus). Bar 10% estimated sequence divergence

Fig. 2. Phylogenetic placement of the symbiotic bacteria in gutless oligochaetes, based on parsimony analyses of 16S rRNA sequences. The host species are from Bermuda (I. leukodermatus in dark blue), the Bahamas (I. makropetalos in light blue), the island of Elba in the Mediterranean (O. algarvensis in light green and O. ilvae in dark green), the Australian Great Barrier Reef (O. loisae in purple), and the continental margin of Peru (O. crassitunicatus in yellow). As many as six bacterial phylotypes can co-occur in a single host (e.g., O. crassitunicatus). Bar 10% estimated sequence divergence deposition of the eggs into the surrounding sediment would offer free-living bacteria from the environment an opportunity to invade the egg. Thus, it is conceivable that some of the symbionts are inherited vertically from the parents, and some horizontally from the environment. Even if all symbiont phylotypes are inherited vertically in recent oligochaetes, the first steps in the development of these multiple symbioses may have originally evolved through horizontal infection of the egg, followed by a successful establishment as a symbiont and transfer to a vertical mode of infection.

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