title: The dicentric nature of the metacentric B chromosome of Metaleptea brevicornis adspersa (Acridinae, acrididae).
abstract: The meiotic behaviour of a large B isochromosome of Metaleptea brevicornis adspersa (Acridinae, Acrididae) was studied in both sexes using conventional preparations and in males by silver staining of surface-spread synaptonemal complexes and kinetochores. In males, both arms of the B chromosome synapse 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 female meiocytes, respectively. In both sexes, the B chromosome tends to migrate precociously to one of the spindle poles, auto-orientating at the equator in 19.2% and 16.6% of metaphase I cells of males and females, respectively. In males, this produces a high frequency of anaphase I/telophase I lagging with the consequent inhibition of cytokinesis and the formation of second division restitution nuclei. For a better understanding of the synaptic and segregation behaviour of the B chromosome in males, pachytene synaptonemal complex (SC) formation was studied. In 0B males, pachytene nuclei invariably showed 11 fully paired SCs, each one with a densely stained pair of terminal kinetochores, and a single axis with a single kinetochore corresponding to the X univalent. In 1B males, both arms of the B chromosome appeared fully paired in 93. 8% of the nuclei, showing complete asynapsis in the rest. This frequency is almost exactly the same as distal chiasma formation in diplotene. The centric region of the SC of the B chromosome had two kinetochores connected by a single axial filament of variable length. The dicentric nature of the B was confirmed by silver staining of kinetochores at metaphase I and anaphase II stages. This is the first time that a dicentric B isochromosome has been described. Based on these results, two alternative models for the origin of isochromosomes are postulated.
authors.affiliation: Laboratorio de Genética Evolutiva, Facultad de Ciencias Exactas, Químicas y Naturales, Universidad Nacional de Misiones, Félix de Azara 1552 (3300) Posadas, Argentina.
authors.forename: M L
authors.initials: ML
authors.lastname: Grieco
authors.forename: C J
authors.initials: CJ
authors.lastname: Bidau
mesh_terms: D000818:Animals; D002875:Chromosomes; D005260:Female; D006110:Grasshoppers / Q000235:genetics*; D007621:Karyotyping; D008297:Male; D008540:Meiosis; D013573:Synaptonemal Complex