The dicentric nature of the metacentric B chromosome of Metaleptea brevicornis adspersa (Acridinae, Acrididae)

The meiotic behaviour of a large B isochromosome of Metaleptea brevicornis adspersa (Acridinae, Acrididae) was studied in both sexes using conventiona l preparations and in males by silver staining of surface-spread synaptonem al complexes and kinetochores. In males, both arms of the B chromosome syna pse at zygotene-early pachytene suggesting its isochromosomal nature. Both arms separate precociously at late pachytene and remain joined only through the formation of a single distal chiasma in 92.5% and 91.7% of male and fe male meiocytes, respectively. In both sexes, the B chromosome tends to migr ate precociously to one of the spindle poles, auto-orientating at the equat or in 19.2% and 16.6% of metaphase I cells of males and females, respective ly. In males, this produces a high frequency of anaphase I/telophase I lagg ing with the consequent inhibition of cytokinesis and the formation of seco nd division restitution nuclei. For a better understanding of the synaptic and segregation behaviour of the B chromosome in males, pachytene synaptone mal complex (SC) formation was studied. In 0B males, pachytene nuclei invar iably showed 11 fully paired SCs, each one with a densely stained pair of t erminal kinetochores, and a single axis with a single kinetochore correspon ding to the X univalent. In 1B males, both arms of the B chromosome appeare d fully paired in 93.8% of the nuclei, showing complete asynapsis in the re st. This frequency is almost exactly the same as distal chiasma formation i n diplotene. The centric region of the SC of the B chromosome had two kinet ochores connected by a single axial filament of variable length. The dicent ric nature of the B was confirmed by silver staining of kinetochores at met aphase I and anaphase II stages. This is the first time that a dicentric B isochromosome has been described. Based on these results, two alternative m odels for the origin of isochromosomes are postulated.