Genome Sizes of Teleostean Fishes: Increases in Some Deep-Sea Species

The primitive teleostean superorder Protacanthopterygii and the derived Paracanthopterygii contain many deep-sea fishes, which inhabit the cold, dark, and impoverished oceanic depths, and which have shallow-marine relatives typical of more diverse and productive environments. Genome sizes, as estimated by DNA amounts per diploid nucleus, vary widely among members of both superorders. We investigated genome sizes and karyotypes of deep- and shallow-living representatives of relatively primitive and advanced protacanthopterygian orders and of an advanced paracanthopterygian order to see if DNA amount is somehow correlated with depth of habitat. DNA amounts almost quadruple between shallow-marine and bathypelagic (deep oceanic) representatives of the primitive paracanthopterygians (Salmoniformes: Argentinoidei). This may have been caused by "saltatory replications," mostly in large two-armed chromosomes. Gradual tandem duplication of genes seems incapable of producing the huge DNA increases. Polyploidy, which would tend to disrupt the well-developed chromosomal sex mechanism of argentinoids, is not indicated by a concordant increase in chromosomal arm numbers. Extensive gene duplication in the large linkage groups of deep-living argentinoids may fix specializations for life in the fishes' vast and constant environment. The "X" chromosome of the strongly dimorphic sex pair is disproportionately enormous and must sequester replicated sets of hemizygous genes that are inherited en bloc without recombination. In general, the great redundancy of replicated sets may preclude their selective elimination. Most encoding of vital body functions, then, may occur within the large and impervious two-armed chromosomes. More generalized karyotypes may have inhibited analogous DNA increases within the more advanced orders. The bathypelagic myctophoid fish (Myctophiformes) lacks dimorphic sex elements and large two-armed chromosomes. Large and stable linkage groups may promote a natural tendency for gene duplication and genome specialization. This is permitted in the cloistered oceanic depths, but may usually be counteracted by strong selection for the efficiency correlates of small DNA amounts, such as small cell size and rapid development, in diverse and variable environments. The amount of increase may simply reflect the evolutionary age of the deep-sea form. The moderately large DNA amounts in mesopelagic (middepth) myctophoid lanternfishes may serve their highly speciali