Fig. 4

Comprehensive schematic summarizing vertical transmission of microorganisms in sponges, as reported previously [18, 19, 46, 75, 83]. Microbes (red dots) occur freely in the sponge mesohyl (A) or within bacteriocytes (B). While it is not known how microbes in bacteriocytes are transferred (?), microbes from the mesohyl are transmitted from mother to offspring through a variety of pathways (C–Q), irrespective of whether oocytes and embryos are brooded (black arrows) or develop externally (blue arrows). Oocytes can acquire microbes by direct engulfing them (C) or through nurse cells. Nurse cells engulf microbes from the mesohyl (D), which are subsequently transferred to the oocyte through cytoplasmic bridges (E) or by nurse cells being phagocytosed by the oocyte (F). Oocytes charged with microbes give rise to embryos that have intracellular microbes in the blastomeres, irrespective of whether the embryo was brooded (H) or was released into the environment (I) for external fertilization and development (J). Alternatively, oocytes may not acquire microbes (K) but an embryo (morula) can acquire them through two pathways. First, follicle cells and/or nurse cells charged with microbe that enter the embryo by migrating through the cleavage furrows (L–Q). Second, microbes infiltrated in the space between the follicle and the embryo migrating themselves through the spaces between blastomeres (M–P). These two extracellular transfers of microbes to embryos occur both in brooded embryos (L–M) and externally fertilizing oocytes (N) leading to externally developing embryos (P–Q). Therefore, the resulting larvae may have microbes in their epithelial cells (R), their internal extracellular medium (S), or their internal cavity (T), if any. By combining mechanisms for intracellular and extracellular transference, some species have larvae containing intracellular and extracellular vertically transmitted microbes (U)