Data from: Adaptive divergence between lake and stream populations of an East African cichlid fish

Divergent natural selection acting in different habitats may build up
barriers to gene flow and initiate speciation. This speciation continuum
can range from weak or no divergence to strong genetic differentiation
between populations. Here, we focus on the early phases of adaptive
divergence in the East African cichlid fish Astatotilapia burtoni, which
occurs both in Lake Tanganyika and inflowing rivers. We first assessed the
population structure and morphological differences in A. burtoni from
southern Lake Tanganyika. We then focused on four lake-stream systems and
quantified body shape, ecologically relevant traits (gill raker and lower
pharyngeal jaw) as well as stomach contents. Our study revealed the
presence of several divergent lake-stream populations that rest at
different stages of the speciation continuum, but show the same
morphological and ecological trajectories along the lake-stream gradient.
Lake fish have higher bodies, a more superior mouth position, longer gill
rakers, and more slender pharyngeal jaws, and they show a plant/algae and
zooplankton biased diet, whereas stream fish feed more on snails, insects
and plant seeds. A test for reproductive isolation between closely related
lake and stream populations did not detect population-assortative mating.
Analyses of F1 offspring reared under common garden conditions indicate
that the detected differences in body shape and gill raker length do not
constitute pure plastic responses to different environmental conditions,
but also have a genetic basis. Taken together, the A. burtoni lake-stream
system constitutes a new model to study the factors that enhance and
constrain progress toward speciation in cichlid fishes.