Pericentric inversion in natural populations of Oligoryzomys nigripes (Rodentia : Sigmodontinae)

We analysed polymorphism for pericentric inversion in chromosome 3 of Oligo ryzomys nigripes (Rodentia: Sigmodontinae) in several populations in Brazil and examined the meiotic behaviour of this chromosome in heterozygotes. We observed an orderly pairing of all chromosomes at pachytene in heterozygot es for the inverted chromosome 3. No indication of meiotic arrest and germ- cell death was found. Electron microscopy of synaptonemal complexes and con ventional meiotic analysis indicated strictly nonhomologous synapsis and cr ossing-over suppression in the inverted region in the heterozygotes, which prevent the formation of unbalanced gametes. Thus, the pericentric inversio n in chromosome 3 does not apparently result in any selective disadvantages in heterozygous carriers. In the majority of the populations studied, the frequencies of acrocentric homozygotes, metacentric homozygotes, and hetero zygotes were in Hardy-Weinberg equilibrium. However, in some populations, w e detected an excess of heterozygotes and a deficiency of acrocentric homoz ygotes.