Evolution of ion transporter Na/K-ATPase expression in the osmoregulatory maxillary glands of an invasive copepod.

While many freshwater invaders originate from saline habitats, the physiological mechanisms involved are poorly understood. We investigated the evolution of ion transporter Na/K-ATPase (NKA) protein expression between ancestral saline and freshwater invading populations of the copepod (Atlantic clade of the complex). We compared NKA expression between populations under common-garden conditions at three salinities in the maxillary glands. We found the evolution of reduced NKA expression in the freshwater population under freshwater conditions and reduced plasticity (canalization) across salinities, relative to the saline population. Our results support the hypothesis that maxillary glands are involved in ion reabsorption from excretory fluids at low-salinity conditions in the saline population. However, mechanisms of freshwater adaptation, such as increased ion uptake from the environment, might reduce the need for ion reabsorption in the freshwater population. These patterns of ion transporter expression contribute insights into the evolution of ionic regulation during habitat change.