Marine stickleback have repeatedly invaded freshwater environments. On the Pacific coast of Canada, these freshwater lakes are colder during the winter than the marine environment – so how do marine invaders survive?
One possible way to cope with the cold is by increasing mitochondrial content of muscle tissue. Mitochondria are the “powerhouse of the cell.” Cold temperatures reduce the production of energy generated by each unit of mitochondria, which can affect long-term survival in the cold. Fish are not unprepared for this; the effects of the cold can be compensated by increasing the volume density of mitochondria in muscle fiber. In other words, if the cold is suppressing energy production per unit of mitochondria, making more mitochondrial units can increase overall energy production.
|In this experiment, a marine population of stickleback from Bamfield Inlet and a freshwater population from nearby Frederick Lake were collected by BMSC researchers and raised under different temperatures. Heart and pectoral muscle were extracted and examined for mitochondrial content using electron microscopy. Importantly, marine stickleback were raised under freshwater conditions, to investigate how initial colonists of freshwater lakes would have responded to the cold. We found that heart muscle had high mitochondrial content that did not change with temperature in either marine or freshwater stickleback, suggesting that mitochondrial levels were already high enough for survival. Pectoral muscle, however, had more mitochondrial content when acclimated to the cold. Interestingly, this pattern was most obvious in the marine stickleback, suggesting that they were “pre-adapted” to cold freshwater conditions. In support of this hypothesis, marine stickleback raised in fresh water could tolerate temperatures that were as cold as those tolerated by their freshwater counterparts.
The image shows mitochondria in pectoral (top) and cardiac (bottom) muscle, in cold- (left) and warm- (right) acclimated stickleback.
Morris M, Wuitchik SJS, Rosebush J, Rogers SM (2021). Mitochondrial volume density and evidence for its role in adaptive divergence in response to thermal tolerance in threespine stickleback. Journal of Comparative Physiology B. Online early.
Link to article: 1007/s00360-021-01366-w