Genomic testing of landlocked Kildin cod (Gadus morhua

14 Kildin cod is a small landlocked population of Atlantic cod reproductively isolated from 15 marine counterparts for around 1500-2000 years. The Kildin cod lives in a shallow 16 meromictic lake in the five-meter intermediate layer of water with sharp gradients of oxygen 17 and salinity. The cod had an effective population size of around one hundred individuals and 18 evolved unique physiological, morphological and behavioral features. The marine Atlantic 19 cod has two ecologically distinct forms: the stationary (coastal) and migratory (deep-water) 20 ecotypes that differ in migratory behavior and habitat preferences (the depth, oxygen content, 21 salinity and temperature). To understand the origin and genetic properties of Kildin cod, we 22 scrutinized genomic regions associated with the cod ecotypes differentiation (LG1, LG2, and 23 LG7) and found out that Kildin cod’s regions LG2 and LG7 were fixed with the migratory 24 variants, whereas polymorphic LG1 had a higher frequency of the stationary variant, that 25 could be explained by the possible strong genetic drift. The lake cod investigated had four 26 times lesser genome diversity than marine population. Our finding suggests that Kildin cod 27 originated from the migratory ecotype of the marine cod. 28 PeerJ Preprints | https://doi.org/10.7287/peerj.preprints.2497v1 | CC BY 4.0 Open Access | rec: 3 Oct 2016, publ: 3 Oct 2016