Abstract:
The Magadi tilapia (Alcolapia grahami) is a
cichlid fish that inhabits one of the Earth’s most extreme
aquatic environments, with high pH (*10), salinity
(*60 % of seawater), high temperatures (*40 C), and
fluctuating oxygen regimes. The Magadi tilapia evolved
several unique behavioral, physiological, and anatomical
adaptations, some of which are constituents and thus
retained in freshwater conditions. We conducted a transcriptomic
analysis on A. grahami to study the evolutionary
basis of tolerance to multiple stressors. To identify the
adaptive regulatory changes associated with stress
responses, we massively sequenced gill transcriptomes
(RNAseq) from wild and freshwater-acclimated specimens
of A. grahami. As a control, corresponding transcriptome
data from Oreochromis leucostictus, a closely related
freshwater species, were generated. We found expression
differences in a large number of genes with known functions
related to osmoregulation, energy metabolism, ion
transport, and chemical detoxification. Over-representation
of metabolism-related gene ontology terms in wild individuals
compared to laboratory-acclimated specimens
suggested that freshwater conditions greatly decrease the
metabolic requirements of this species. Twenty-five genes
with diverse physiological functions related to responses to
water stress showed signs of divergent natural selection
between the Magadi tilapia and its freshwater relative,
which shared a most recent common ancestor only about
four million years ago. The complete set of genes
responsible for urea excretion was identified in the gill
transcriptome of A. grahami, making it the only fish species
to have a functional ornithine-urea cycle pathway in
the gills—a major innovation for increasing nitrogenous
waste efficiency.
