Microbes as manipulators of egg size and developmental evolution

Marine invertebrates mainly reproduce by energy-poor eggs that develop
into feeding larvae or energy-rich eggs that develop into non-feeding
larvae. Evolutionary transitions between these developmental modes have
been studied in detail, yet the evolutionary factor(s) responsible for
these switches remains elusive. Here, we use theoretical models to support
the premise that microbes with the capacity to manipulate host
reproduction may be one possible factor. Our model predicts that microbial
manipulators could create a sperm-limited environment that selects for
larger eggs by shifting the host’s sex ratio toward female dominance and,
as a result, drive an evolutionary transition in the developmental mode
for marine invertebrates. The loss of a microbial manipulator could then
recover the ancestral egg size and developmental mode. We also suggest
more than a dozen genera of marine invertebrates from throughout the
world’s oceans that fit the framework of a microbe-induced evolutionary
transition between these predominant developmental modes. We anticipate
that microbial manipulators have a yet-to-be-appreciated influence on the
developmental evolution of marine invertebrates. We find it paramount to
understand whether evolutionary transitions in developmental mode occur
with and without microbial manipulators as well as whether the underlying
mechanisms of these manipulations are convergent with terrestrial systems.