Maternal provisioning of an obligate symbiont

Vertical transmission of microbial symbionts is interpreted as all offspring within a clutch being provided a similar number of symbionts irrespective of reproductive output (fecundity). This interpretation, however, stems primarily from oviparous insects and, thus, has yet to consider other major reproductive strategies. We used the viviparous sponge Halichondria panicea and its obligate symbiont "Candidatus Halichondribacter symbioticus" to test the hypothesis that offspring receive quantitatively similar numbers of its obligate symbiont. This quantitative strategy of vertical transmission was not observed. Instead, we find that H. panicea has a maternal pool of Ca. H. symbioticus that is partitioned proportionally to reproductive output and allometrically by offspring size. Moreover, Ca. H. symbioticus could not be experimentally reduced in larvae by antibiotics, while the total bacterial community could be depleted. The ability to undergo metamorphosis was unaffected by this perturbation. Together, this demonstrates that the obligate symbiont Ca. H. symbioticus is maternally provisioned and, thus, provides an additional strategy for how microbes can be vertically transmitted.