Karyotypes of basal lineages in notothenioid fishes: the genus Bovichtus
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Using comparative cytogenetic techniques, we characterized the chromosomes of fishes from the family Bovichtidae, the basal lineage of the largely Antarctic suborder Notothenioidei. We focused on three Sub-Antarctic species of the genus Bovichtus that differ greatly in their circumpolar distributions: B. diacanthus (Tristan da Cunha Island Group), B. variegatus (New Zealand) and B. angustifrons (Tasmania). Chromosomes were analyzed both by conventional karyotyping and by cytogenetic mapping of ribosomal genes using fluorescence in situ hybridization. The three species displayed a strongly conserved karyotype consisting entirely of telocentric chromosomes (diploid number = 48; Fundamental Number = 48), in agreement with our previously published hypothesis that the bovichtid karyotype is the basal state for notothenioid fishes. The chromosomal distribution of ribosomal genes differed from those of most notothenioid species studied to date, with the 45S and 5S genes separated on two different chromosome pairs. Separation of two classes of ribosomal genes is the most widespread condition in teleosts, including the bovichtids. Most notothenioid lineages on the other hand exhibit a derived consolidation of these genes.