Global patterns of nuclear and mitochondrial genetic diversity in marine fishes

Genetic diversity is a fundamental component of biodiversity. Examination
of global patterns of genetic diversity can help highlight mechanisms
underlying species diversity, though a recurring challenge has been that
patterns may vary by molecular marker. Here, we compiled 6862 observations
of genetic diversity from 492 species of marine fish and tested among
hypotheses for diversity gradients: the founder effect hypothesis, the
kinetic energy hypothesis, and the productivity-diversity hypothesis. We
fit generalized linear mixed effect models (GLMMs) and explored the extent
to which various macroecological drivers (latitude, longitude, temperature
(SST), and chlorophyll-a concentration) explained variation in genetic
diversity. We found that mitochondrial genetic diversity followed
geographic gradients similar to those of species diversity, being highest
near the Equator, particularly in the Coral Triangle, while nuclear
genetic diversity did not follow clear geographic patterns. Despite these
differences, all genetic diversity metrics were correlated with
chlorophyll-a concentration, while mitochondrial diversity was also
positively associated with SST. Our results provide support for the
kinetic energy hypothesis, which predicts that elevated mutation rates at
higher temperatures increase mitochondrial but not necessarily nuclear
diversity, and the productivity-diversity hypothesis, which posits that
resource-rich regions support larger populations with greater genetic
diversity. Overall, these findings reveal how environmental variables can
influence mutation rates and genetic drift in the ocean, caution against
using mitochondrial macro-genetic patterns as proxies for whole-genome
diversity, and aid in defining global gradients of genetic diversity.