Variable B chromosomes frequencies between males and females of two species of pufferfishes (Tetraodontiformes)

Cytogenetic analysis in two sympatric species of pufferfish from Paraná Coast, Brazil, Sphoeroides spengleri (Tetraodontidae) and Chilomycterus spinosus (Diodontidae) revealed the presence of 2n = 46 and 2n = 50 chromosomes, respectively. In S. spengleri, the double FISH technique with 5S and 18S rDNA probes showed no syntenical organization, with both multigene families occupying a terminal position on the chromosomes. In S. spengleri the poor pattern of heterochromatin distribution is discussed about its relation with the compactness of their genome. On the other hand C. spinosus presented conspicuous blocks occupying the short arms of several chromosomes, possibly consequence of an accumulative process. Heterochromatic supernumerary chromosomes were detected in both species, however in S. spengleri they were presented as microchromosomes varying of 0–3 and being more frequent in females than in males, possibly as result of a preferential segregation. Conversely in C. spinosus an extra chromosome was restricted to the males, which to behave as univalent during meiosis. In view of these peculiarities we hypothesized a possible relationship between B chromosome and sex determination, although others aspects on accumulation, maintenance, and frequency of these extra elements were also considered, focusing on the parasitic model. In spite of the karyotypic diversity and others genetic peculiarities in Tetraodontiformes, this corresponds the third report of the occurrence of chromosomes B in the group.     

An unusual ZZ/ZW sex chromosome system in Characidium fishes (Crenuchidae, Characiformes) with the presence of rDNA sites

Characidium fishes with a sex chromosome system form a monophyletic group. This work presents data of Characidium lanei from the South Atlantic basin (Brazil), including an unknown type of ZW sex chromosome system for the groups including the presence of rDNA sites on sex chromosomes.