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.

Chromosomal Mapping of Repeat DNA in <b><i>Bergiaria westermanni</i></b> (Pimelodidae, Siluriformes): Localization of 45S rDNA in B Chromosomes

“…Thus, the analysis of B chromosomes of B. westermanni allowed inferences to be drawn about the occurrence of variant chromosome copies as a probable escape route of "neutralization forces" of these chromosomes in the host genome. Some studies suggest that B chromosomes influence sex determination [Burt and Trivers, 2006;Camacho et al, 2011;Yoshida et al, 2011;Valente et al, 2014]. In our study, both males and females had a high frequency of B chromosomes, with this variable showing no relationship to sex.…”

Genomic content and new insights on the origin of the B chromosome of the cichlid fish Astatotilapia latifasciata

“…Although the Bs of A. latifasciata occur in males and females and the CGH experiments did not reveal a sexspecific relationship, we cannot exclude their participation in the sex determination of the cichlid species. However, as previously discussed (Camacho et al 2011), the similarities between Bs and sex chromosomes may occur by analogy and not homology. Such an assumption will be better addressed though the integration of genomic and cytogenetic analyses for the B chromosome of A. latifasciata as well as through advancing studies of sex determination among cichlids.…”

“…Relationships between sex and B chromosomes have been broadly reported among animals and it is evident that these chromosomes share traits such as chromatin distribution, repetitive DNA accumulation and loss of gene activity, which are distinct in relation to the A chromosomes (Camacho et al 2011). One interesting example of the relationship between the sex and B chromosomes comes from the African cichlid fish of Lake Victoria Lithocromis rubripinnis with the presence of female-specific B chromosome that leads to a female-biased sex ratio in the species, although no sex determining genes were identified on the Bs (Yoshida et al 2011).…”

Sex chromosome composition revealed in Characidium fishes (Characiformes: Crenuchidae) by molecular cytogenetic methods

“…These chromosomes share certain characteristics, such as the differential accumulation of heterochromatin and repetitive DNAs, as well as a lower proportion of active genes, that clearly distinguish them from the autosomes (e.g., Camacho et al 2011). A particularly interesting case is that of the African cichlid Lithocromis rubripinnis (Seehausen et al 1998) with B chromosomes apparently derived from the sex chromosome pair and greatly correlated to females (its presence being sex-specific), even though up to that time no sexrelated genes have been identified in the supernumerary chromosomes of this species (Yoshida et al 2011).…”