Bayliss, S. L. J., Scott, Z. R., Coffroth, M. A., and C. P. terHorst. 2019.

Bayliss, S. L. J., Scott, Z. R., Coffroth, M. A., and C. P. terHorst. 2019. Genetic variation in
Breviolum antillogorgium, a coral reef symbiont, in response to temperature and nutrients.
Ecology & Evolution 9: 2803-2813. PDF

Symbionts within the family Symbiodiniaceae are important on coral reefs because
they provide significant amounts of carbon to many different reef species. The breakdown
of this mutualism that occurs as a result of increasingly warmer ocean temperatures
is a major threat to coral reef ecosystems globally. Recombination during sexual
reproduction and high rates of somatic mutation can lead to increased genetic variation
within symbiont species, which may provide the fuel for natural selection and
adaptation. However, few studies have asked whether such variation in functional
traits exists within these symbionts. We used several genotypes of two closely related
species, Breviolum antillogorgium and B. minutum, to examine variation of traits
related to symbiosis in response to increases in temperature or nitrogen availability
in laboratory cultures. We found significant genetic variation within and among symbiont
species in chlorophyll content, photosynthetic efficiency, and growth rate. Two
genotypes showed decreases in traits in response to increased temperatures predicted
by climate change, but one genotype responded positively. Similarly, some
genotypes within a species responded positively to high‐nitrogen environments,
such as those expected within hosts or eutrophication associated with global change,
while other genotypes in the same species responded negatively, suggesting context‐
dependency in the strength of mutualism. Such variation in traits implies that
there is potential for natural selection on symbionts in response to temperature and
nutrients, which could confer an adaptive advantage to the holobiont.
climate change, evolutionary rescue, genetic variation, mutualism, natural selection

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