title: B chromosomes and sex in animals.
abstract: Supernumerary (B) chromosomes are dispensable elements found in many eukaryote genomes in addition to standard (A) chromosomes. In many respects, B chromosomes resemble sex chromosomes, so that a common ancestry for them has frequently been suggested. For instance, B chromosomes in grasshoppers, and other insects, show a pycnotic cycle of condensation-decondensation during meiosis remarkably similar to that of the X chromosome. In some cases, B chromosome size is even very similar to that of the X chromosome. These resemblances have led to suggest the X as the B ancestor in many cases. In addition, sex chromosome origin from B chromosomes has also been suggested. In this article, we review the existing evidence for both evolutionary pathways, as well as sex differences for B frequency at adult and embryo progeny levels, B chromosome effects or B chromosome transmission. In addition, we review cases found in the literature showing sex-ratio distortion associated with B chromosome presence, the most extreme case being the paternal sex ratio (PSR) chromosomes in some Hymenoptera. We finally analyse the possibility of B chromosome regularisation within the host genome and, as a consequence of it, whether B chromosomes can become regular members of the host genome.
authors: Departamento de Genética, Universidad de Granada, Spain. jpmcamac @ ugr.es
authors: J P M
authors: JP
authors: Camacho
authors: M
authors: M
authors: Schmid
authors: J
authors: J
authors: Cabrero
mesh_terms: D000818:Animals; D002875:Chromosomes / Q000235:genetics*; D005260:Female; D008297:Male; D012727:Sex Characteristics; D012744:Sex Ratio