Evolutionary genomics of pelvic spine reduction in Gasterosteus aculeatus

Marine threespine sticklebacks have colonized and adapted to freshwater lakes. Freshwater sticklebacks often have reduced body armor, like lateral plates and sometimes pelvic spines. Pelvis reduction in North American sticklebacks is convincingly linked to the PelA and PelB enhancers of the Pitx1 gene. In a subarctic Norwegian watercourse, an upper lake consists of sticklebacks with normal, asymmetrical and no pelvic spines. A downstream lake and a nearby marine site only have normally spined sticklebacks. This study examines mechanisms for the pelvic spine reduction of sticklebacks in this watercourse. Sanger and next generation sequencing were applied to examine the Pel enhancers and mitochondrial and nuclear genomic differences between the three populations. The Pel enhancers of sticklebacks in the upper lake seems not associated with pelvic spine reduction. Two distinct mitochondrial genome groups were identified in the upper lake, and only one of them was found in the downstream lake. Both the mito and the nuclear genomes varied more among marine than freshwater fish. The nuclear genomes were more different between the two freshwater populations than between the freshwater and the marine fish.