title: The B chromosomes of the African cichlid fish Haplochromis obliquidens harbour 18S rRNA gene copies.
abstract: BACKGROUND
  Diverse plant and animal species have B chromosomes, also known as accessory, extra or supernumerary chromosomes. Despite being widely distributed among different taxa, the genomic nature and genetic behavior of B chromosomes are still poorly understood.
  
  
  RESULTS
  In this study we describe the occurrence of B chromosomes in the African cichlid fish Haplochromis obliquidens. One or two large B chromosome(s) occurring in 39.6% of the analyzed individuals (both male and female) were identified. To better characterize the karyotype and assess the nature of the B chromosomes, fluorescence in situ hybridization (FISH) was performed using probes for telomeric DNA repeats, 18S and 5S rRNA genes, SATA centromeric satellites, and bacterial artificial chromosomes (BACs) enriched in repeated DNA sequences. The B chromosomes are enriched in repeated DNAs, especially non-active 18S rRNA gene-like sequences.
  
  
  CONCLUSIONS
  Our results suggest that the B chromosome could have originated from rDNA bearing subtelo/acrocentric A chromosomes through formation of an isochromosome, or by accumulation of repeated DNAs and rRNA gene-like sequences in a small proto-B chromosome derived from the A complement.
authors.affiliation: UNESP - Universidade Estadual Paulista, Instituto de Biociências, Departamento de Morfologia, Botucatu, SP, Brazil. andreiapol@ibb.unesp.br
authors.forename: Andréia B
authors.initials: AB
authors.lastname: Poletto
authors.forename: Irani A
authors.initials: IA
authors.lastname: Ferreira
authors.forename: Cesar
authors.initials: C
authors.lastname: Martins
mesh_terms: D000818:Animals; D002874:Chromosome Mapping; D023681:Cichlids / Q000235:genetics*; D004275:DNA, Ribosomal / Q000235:genetics; D005260:Female; D017404:In Situ Hybridization, Fluorescence; D007621:Karyotyping; D008297:Male; D008540:Meiosis; D008677:Metaphase; D012337:RNA, Ribosomal, 18S / Q000235:genetics*; D017422:Sequence Analysis, DNA
chemical_list: D004275:DNA, Ribosomal; D012337:RNA, Ribosomal, 18S