Vertebrate mothers pass antibodies to their offspring via milk (Hanson, 1998) or the placenta in mammals, and through yolk in birds and fish (Grindstaff et al., 2003). In these cases, the mechanism of immunity is clear. Functionally homologous phenomena have been observed in insects, but the mechanisms have yet to be uncovered.
In a study on bumblebees, it was shown that there are higher induced levels of antibacterial activity in offspring from mothers who had received a bacterially based immune challenge, prior to colony founding and subsequent egg laying (Sadd et al., 2005). Cross-fostering experiments confirmed this result and showed that the cue for the trans-generational effect is passed through the egg and persists into the adulthood of the offspring
(Sadd and Schmid-Hempel, 2007). The phenomenon of trans-generational immunity has also been demonstrated in mealworm beetles (Moret, 2006). While none of these studies have dealt with the issue of specificity, a study in the crustacean Daphnia demonstrated maternal transfer of specific resistance. The resistance transferred from mother to offspring was highly specific, differentiating on the level of bacterial strains (Little et al., 2003). One proposed molecular basis for creating immune receptor diversity, Dscam (see Chapter 5), seems conserved between Daphnia and insects (Brites et al., 2008). Given this, we may well expect trans-generational immune priming of a similar level of specificity in insects.
In trans-generational immunity, both the mother and her environment influence the phenotype of the offspring. This will be a particular advantage when the environment experienced by the mother, in this case the prevailing parasite and pathogen community, is closely related to that encountered by her offspring. Such defence-associated trans-generational effects will enable offspring to reap the benefits of defence when required, avoid the costs of investment when it is not needed, and avoid the potential lags that are often involved in induced defences (Agrawal et al., 1999). While unlikely to be directly relevant for the issue of specificity, offspring immunity and resistance in invertebrates may also be influenced by maternal environmental cues, outside of those related to immunological experience. For example, mother Daphnia subjected to an environment with poor food during reproduction produce offspring that are more resistant to a bacterial pathogen than offspring of mothers reproducing in a high-food environment (Mitchell and Read, 2005).
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