Classical and molecular cytogenetics of the pufferfish tetraodon nigroviridis

Because of its highly compact genome, the pufferfish has become an importan t animal model in genome research. Although the small chromosome size rende rs chromosome analysis difficult, we have established both classical and mo lecular cytogenetics in the freshwater pufferfish Tetraodon nigroviridis (T NI). The karyotype of T. nigroviridis consists of 2n = 42 biarmed chromosom es, in contrast to the known 2n = 44 chromosomes of the Japanese pufferfish Fugu rubripes (FRU). RBA banding can identify homologous chromosomes in bo th species. TNI 1 corresponds to two smaller FRU chromosomes, explaining th e difference in chromosome number. TNI 2 is homologous to FRU 1. Fluorescen ce in-situ hybridization (FISH) allows one to map single-copy sequences, i. e. the Huntingtin gene, on chromosomes of the species of origin and also on chromosomes of the heterologous pufferfish species. Hybridization of total genomic DNA shows large blocks of (species-specific) repetitive sequences in the pericentromeric region of all TNI and FRU chromosomes. Hybridization with cloned human rDNA and classical silver staining reveal two large and actively transcribed rRNA gene clusters. Similar to the situation in mammal s, the highly compact pufferfish genome is endowed with considerable amount s of localized repeat DNAs.