B chromosomes in Brazilian rodents

B chromosomes are now known in eight Brazilian rodent species: Akodon montensis, Holochilus brasiliensis, Nectomys rattus, N. squamipes, Oligoryzomys flavescens, Oryzomys angouya, Proechimys sp. 2 and Trinomys iheringi. Typically these chromosomes are heterogeneous relative to size, morphology, banding patterns, presence/absence of NORs, and presence/absence of interstitial telomeric signals after FISH. In most cases, Bs are heterochromatic and late replicating. Active NORs were detected in two species: Akodon montensis and Oryzomys angouya. As a rule, Bs behave as uni or bivalents in meiosis, there is no pairing between Bs and autosomes or sex chromosomes and also their synaptonemal complexes are isopycnotic with those in A chromosomes.