Unequal crossing-over associated with asymmetrical synapsis between nomadic elements in the Drosophila melanogaster genome

Abstract

The molecular structure of reciprocal duplications and deficiencies produced by unequal crossing-over at the white (w) locus of D. melanogaster females heterozygous for the alleles wa and wa4 was examined. A transposable, covia-like element is found at the rearrangement breakpoints. Asymmetrical pairing between 2 copies of this element, which are at least 60 kilobases apart in the parenteral chromosomes, followed by a crossover within the paired elements, may be responsible for the duplication and deficiencies observed. The frequency of these events is high compared with normal homologous exchange, implying that synaptic pairing during meiosis must be sufficiently flexible as to allow efficient recognition of sequences located in nonidentical positions on homologous chromosomes. A possible mechanism is suggested for the generation of tandem duplications in eukaryotic organisms.