Microbes as manipulators of developmental life-history

Marine invertebrates mainly reproduce by energy-poor eggs that develop into feeding larvae or energy-rich eggs that develop into non-feeding larvae1-4. Transitions between these reproductive strategies have been studied in detail5-7, yet the evolutionary factor(s) responsible for these switches remains elusive. Here, we use theoretical models to show that microbes with the capacity to manipulate host reproduction are one possible factor. We report that microbial manipulators create a sperm-limited environment that selects for larger eggs by shifting the hosts sex ratio towards female dominance and, as a result, serve as the evolutionary driver of transitions in the developmental life-history for marine invertebrates. Loss of a microbial manipulator can then recover the ancestral developmental life-history. We also document more than a dozen genera of marine invertebrates from throughout the worlds oceans that fit the framework of a microbe-induced switch between these predominate reproductive strategies. We anticipate that microbial manipulators have a yet-to-be appreciated influence on the life-history strategies of marine invertebrates. We find it paramount to understand if transitions in developmental life-history also occur without microbial manipulators as well as if the underlying mechanisms of these manipulations are convergent with terrestrial systems.