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Organisms are locally adapted when members of a population have a fitness
advantage in one location relative to conspecifics in other geographies.
For example, across latitudinal gradients, some organisms may trade off
between traits that maximize fitness components in one, but not both, of
somatic maintenance or reproductive output. Latitudinal gradients in life
history strategies are traditionally attributed to environmental selection
on an animal's genotype, without any consideration of the possible
impact of associated microorganisms ("microbiota") on life
history traits. Here, we show in Drosophila melanogaster, a key model for
studying local adaptation and life history strategy, that excluding the
microbiota from definitions of local adaptation is a major shortfall.
First, we reveal that an isogenic fly line reared with different bacteria
varies the investment in early reproduction versus somatic maintenance.
Next, we show that in wild fruit flies, the abundance of these same
bacteria was correlated with the latitude and life history strategy of the
flies, suggesting geographic specificity of the microbiota composition.
Variation in microbiota composition of locally adapted D. melanogaster
could be attributed to both the wild environment and host genetic
selection. Finally, by eliminating or manipulating the microbiota of fly
lines collected across a latitudinal gradient, we reveal that host
genotype contributes to latitude-specific life history traits independent
of the microbiota and that variation in the microbiota can suppress or
reverse the differences between locally adapted fly lines. Together, these
findings establish the microbiota composition of a model animal as an
essential consideration in local adaptation.
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