The neo-X neo-Y sex pair in Acrididae,its structure and association

In most of the fifty described cases of neo-X neo-Y sex determining systems in Acrididae the pairing regions during meiosis are limited to distal regions. A comparative study on the structure and pairing mechanisms of Dichroplus silveiraguidoi (2n=8); Dichroplus bergi (2n=22) and Dichroplus vittatus (2n=20) has been undertaken. — The sex bivalents of these three grasshoppers are different: the neo-X centromere is associated with the neo-Y telomere in D. silveiraguidoi; in D. bergi the neo-X is related through the short arm telomere to the centromere of neo-Y and both members of the sex pair are associated by the telomeres in D. vittatus. Centromeric and telomeric C-band positive blocks are present in both members of the pair in the three species. D. silveiraguidoi also presents an interstitial block in the neo-X. These blocks are brightly fluorescent with quinacrine mustard and Hoechst 33258 at low concentration (0.05 g/ml). The region of neo-X corresponding to the primitive X takes an intermediate staining during the early meiotic prophase with C-banding and Hoechst 33258. — The structure of the sex bivalent and the particular staining of the X region are discussed in relation to the available information on the presence of different types of DNA in this segment. The possibility that the neo-X interstitial block of D. silveiraguidoi plays a role in preventing the spreading of heterochromatinization along the chromosome is also discussed. The classical interpretation of the neo-X neo-Y association during meiotic prophase as the result of a terminalized chiasma is considered in the light of optic and electronmicroscopic data. Other possible mechanisms of relationship between both chromosomes are also presented by these three orthopteran species.