NON-HOMOLOGOUS SYNAPTONEMAL COMPLEX-FORMATION IN A HETEROMORPHIC BIVALENT IN KEYACRIS-SCURRA (MORABINAE, ORTHOPTERA)

In some populations of the grasshopper Keyacris scurra, there are many individuals heterozygous for centromere position polymorphisms. From a consideration of chiasma positions these are almost certainly due to pericentric inversions. In this species, as in other grasshoppers, the deleterious effects of chiasmata within these heterozygous regions are avoided by non-homologous, straight pairing. Reconstruction of synaptonemal complexes from two adjacent pachytene nuclei in an individual heterozygous for centromere position (telo/metacentric) on the second longest (CD) bivalent by electron microscopy of serial sections allowed the identification of all the bivalents. The centromeres were identified by characteristic densestaining material. The synaptonemal complex was found to form straight through the heteromorphic region, including both centromere positions. The pairing was clearly non-homologous at these asymmetrical centromere positions, and probably therefore, in the presumably inverted region between them. This regular non-homologous pairing explains why chiasmata never form in the heterozygous region, but does not conclusively prove that the rearrangement is an inversion rather than a centric transposition. © 1978 Springer-Verlag.