Maternal provisioning of an obligate symbiont in a sponge

Abstract

AbstractThe transmission of microbes from mother to offspring is an ancient, advantageous, and widespread feature of metazoan life history. Despite this, little is known about the quantitative strategies taken to maintain symbioses across generations. The quantity of maternal microbes that is provided to each offspring through vertical transmission could theoretically be stochastic (no trend), consistent (an optimal range is allocated), or provisioned (a trade‐off with fecundity). Examples currently come from animals that release free‐living eggs (oviparous) and suggest that offspring are provided a consistent quantity of symbionts. The quantity of maternal microbes that is vertically transmitted in other major reproductive strategies has yet to be assessed. We used the brooding (viviparous) spongeHalichondria paniceato test whether offspring receive quantitatively similar numbers of maternal microbes. We observed thatH. paniceahas a maternal pool of the obligate symbiontCandidatusHalichondribacter symbioticus and that this maternal pool is provisioned proportionally to reproductive output and allometrically by offspring size. This pattern was not observed for the total bacterial community. Experimental perturbation by antibiotics could not reduce the abundance ofCa.H. symbioticus in larvae, while the total bacterial community could be reduced without affecting the ability of larvae to undergo metamorphosis. A trade‐off between offspring size and number is, by definition, maternal provisioning and parallel differences inCa.H. symbioticus abundance would suggest that this obligate symbiont is also provisioned.