Latitudinal Cline in Chromosome Numbers of Ice Cod A. glacialis (Gadidae) from Northeast Greenland

Abstract

The ice cod Arctogadus glacialis (Peters, 1872) is one of the few fish species endemic to the Arctic. With a circumpolar distribution, the species is confined to the fjords and shelves of the Arctic seas. Biological information on A. glacialis is scarce, with genomic information restricted to microsatellites. Within the frame of the TUNU-Programme: Arctic Ocean Fishes—Diversity, Adaptation and Conservation, we studied A. glacialis at the chromosomal level to explore fish diversity and evolutionary aspects. The analysis of over 50 individuals from the Northeast Greenland fjords between latitudes 71°09′ N and 76°42′ N revealed a remarkable intraspecific diversity epitomized by chromosome numbers spanning from 28 to 33, the occurrence of putative B chromosomes, and diversified patterns of distribution of heterochromatin and rDNAs. The number of B chromosomes followed a latitudinal gradient from 0–2 in the north to 2–5 in the south. Considering the benthic and rather stationary life history of this species, the observed chromosomal differences might have arisen independently, possibly driven and/or fostered by the dynamics of repetitive sequences, and are being fixed in relatively isolated fjord populations. The resulting latitudinal cline we observe today might have repercussions on the fate of local populations facing the ongoing climate-driven environmental changes.

Chromosomal fusions and large-scale inversions are key features for adaptation in Arctic codfish species

“…The reduction in chromosome number seen both in the Antarctic notothenioids and Arctic codfishes is intriguing and raises questions about the adaptive significance of chromosomal fusions in species inhabiting high-latitude oceans. For Arctic cod observed population-level karyotype variation seems to follow a latitudinal cline, supporting the idea of chromosomal reduction being advantageous in cold-water conditions[30]. Moreover, high numbers of chromosomal inversions have been reported for many gadids, and are likely to play important roles in the separation of co-occurring cryptic ecotypes[34,35], as well as for local adaptation to different environmental conditions[36–39].…”

“…Moreover, inter-and intraspecies chromosome number variation has been reported in previous studies within the gadid family (see Figure 1 and Table S1). Among the cold-water adapted species residing in the Arctic and sub-Arctic regions [29,30] fewer chromosomes have been identified compared to the presumed ancestral teleost karyotype (n=24-26) [31][32][33]. The reduction in chromosome number seen both in the Antarctic notothenioids and Arctic codfishes is intriguing and raises questions about the adaptive significance of chromosomal fusions in species inhabiting high-latitude oceans.…”

Ice Cod Arctogadus glacialis (Peters, 1874) in Northeast Greenland—A First Sketch of Spatial Occurrence and Abundance

“…During years 1995-2008, a total of 20 specimens were caught at Jan Mayen Island [14]. Whether these fish are strays from Northeast Greenland or represent a population of its own remain to be verified by popu lation genetics [36].…”

Fish Cytogenetics: Present and Future

“…In this species, the presence of an XY chromosome system and mapping of repeated sequences repeats allowed the identification of the main mechanisms of chromosome rearrangement that have driven karyotypic evolution in the genus [ 19 ]. Other papers reported polymorphism related to the presence of B chromosomes [ 20 , 21 ]. This is the case of the common nase Chondrostoma nasus , which belongs to Leuciscidae, a freshwater family whose species are usually characterized by a stable diploid number.…”

“…Similarly, in the marine ice cod Arctogadus glacialis endemic to the Arctic Sea, the cytogenetic analysis revealed a remarkable intraspecific chromosome polymorphism: six karyotype variants associated with the presence of B chromosomes along a latitudinal cline, and different patterns of heterochromatin and rDNA distribution. These karyotype variations can be associated with the presence of isolated fjord populations at different latitudes/environmental condition, raising concerns on the fate of such populations in the light of the ongoing climate-driven environmental changes [ 21 ]. Finally, chromosome mapping of major and minor ribosomal gene clusters was applied in a comparative cytogenetic analysis of Muraeinidae, an ancient and poorly studied family.…”