Usually, two principal origins of B chromosomes have been considered

The karyotypes of about 70 mammalian species have been characterised by the presence of B chromosomes, of which 50 species belong to the get Rodentia. In Brazilian rodents, heterogeneous supernumeraries are recognized in the genome of 9 species. Amongst them, and symbolizing about 10% of all identified B-made up of species of Rodentia, are Holochilus brasiliensis, Nectomys squamipes, Nectomys rattus, Oligoryzomys flavescens and Sooretamys angouya, belonging to the monophyletic Oryzomyini , a team comprising 33 Neotropical genera, and about 140 species, which originated from a quick adaptive radiation.Oryzomyini have an outstanding selection of diploid and fundamental numbers, with polymorphism of autosomes and sex chromosomes in some species. Their B chromosomes display wonderful diversity in terms of number, dimension, morphology of Bs, DNA replication, banding styles and existence/absence of interstitial telomeric signals .


Usually, two principal origins of B chromosomes have been considered. The most extensively approved see is that they are derived from an intragenomic fragment of the autosomal complement, as in the crops Crepis capillaris and Secale cereale, in Drosophila subsilvestris and Abracris flavolineata, in Rattus rattus and in the Siberian roe deer Capreolus pygargus. Despite the fact that considerably less well supported, B chromosomes could be derived from the sexual intercourse chromosomes, as proposed in the case of the W chromosome in the New Zealand frog Leiopelma hochstetteri, and the Y chromosome in the collared lemming, Dicrostonyx groenlandicus. An option look at is that interspecific hybridization supplies foreign DNA from a intently relevant species, as is presumed for the Amazon molly Poecilia formosa and as shown in two species of the wasp Nasonia.

The presence of Bs in some species is linked with amplification of particular genome areas and their initial accumulation in the population may arise as a result of meiotic generate .In mammals, supernumerary chromosomes have been investigated by FISH with telomeric or rDNA sequences, by chromosome microdissection, by BAC mapping and by DNA sequencing. These modern approaches to investigating B chromosomes from animals and crops have enhanced our understanding of their origin, composition, sequence accumulation, gene content material and derived transcripts for assessment see. Although this sort of different amounts of DNA-sequence comparisons have been used to investigate the origin of Bs, in Oryzomyini they have been studied only for their similarity to autosomes and sexual intercourse chromosomes in conditions of morphology, size or meiotic behaviour.