Figure 9.

A compiled scheme of karyotype evolution in Pinnipedia, including the species studied here and species for which whole-genome sequencing data are available. The fusion of ancestral elements is designated by “/”. +H: heterochromatin addition, ENC: evolutionarily new centromere, inv (CR?): inversion or centromere repositioning. ^G G-banding only, ^P painting data, ^S whole-genome sequencing data, and ^C-S chromosome level genome assembly (C-Scaffolds) [19,68]. The cytogenetic data are consistent with the tree topology based on published Carnivora phylogenies affirming pinniped monophyly [69]. The basal position of the walrus is supported by two extra fusion events [70]. The lineage of the northern fur seal first split from Otariidae ~8 mya [71,72]. The separation of the bearded seal ~12 mya from the rest of Phocinae [70] is confirmed here. Substantial enrichment with repeated sequences with variation of nucleotide composition has occurred during long-term monotypic evolution in O. rosmarus, C. ursinus, and E. barbatus in contrast to fewer heterochromatin changes in evolutionarily recent pinniped species. Cytogenetically, Phoca and Pusa and likely other genera with 2n = 32 on this branch are linked by a single fusion (ACK 5/15). Overall, pinniped karyotype evolution has had a slow rate of genome rearrangements with less than one rearrangement per 10 million years [5].