Incomplete genetic compensation and countergradient variation of blood-oxygen transport in deer mice

We combined lab and field data to quantify maladaptive (fitness-reducing)
hematological responses to hypoxia and cold in deer mice (Peromyscus
maniculatus) across their ~4000 m elevational range. In laboratory-raised
mice native to low elevations, individuals increased their hemoglobin
concentration and hematocrit in response to simulated high-elevation, a
response that is maladaptive if unmitigated. In contrast, deer mice from
high elevation increased hematocrit and hemoglobin to a lesser degree,
consistent with genetic compensation. Unlike the predictions under
complete genetic compensation, we observed a positive slope between
hematological traits and elevation in the field, although this slope was
lower than that observed in lowlanders in the lab. Our results suggest
that deer mice have attenuated maladaptive hematological responses to
high-elevation via genetic compensation that is incomplete, which has led
to weak countergradient variation. We suggest this phenomenon is the
result of a balance between positive selection for increased oxygen
carrying capacity and antagonistic selection against elevated blood
viscosity.