Guido van den Thillart, Inger Wilms, Maaike Nieveen, Roy E. Weber, and Frans Witte
In a previous study, broods of the Lake Victoria cichlid Haplochromis ishmaeli raised under hypoxic or normoxic conditions showed striking differences in isohemoglobin (isoHb) pattern that were not observed in two other cichlids that do not belong to the Lake Victoria species flock. We therefore hypothesized that the adaptive mechanism seen in H. ishmaeli in response to hypoxia constitutes a trait that the Lake Victoria species flock inherited from ancestors that lived in hypoxic environments. We tested this hypothesis by designing split-brood experiments with three other representative species from the same species flock: the insectivorous Haplochromis thereuterion, the mollusk-shelling Platytaeniodus degeni and the zooplanktivorous Haplochromis piceatus, while keeping H. ishmaeli as a reference. Split broods were raised, under either normoxia or hypoxia. All hypoxia-raised (HR) individuals of each of the four species exhibited a distinctly different isoHb pattern compared with their normoxia-raised (NR) siblings. The hemoglobin of HR H. thereuterion showed higher O2 affinity compared with NR siblings particularly in the presence of ATP and GTP, indicating that blood of HR juveniles has significantly improved O2-binding affinity under hypoxic conditions. We also tested the capacity to acclimate at greater age in two species by reversing the O2 condition after 7 (H. thereuterion) and 4 (H. ishmaeli) months. After reacclimation for 1 and 2 months, respectively, we found incomplete reversal with intermediate isoHb patterns. As three of the four species do not encounter hypoxic conditions in their environment, this unique trait seems to be a relic inherited from predecessors that lived in hypoxic environments.