Interchromosomal effects of autosomal inversions in Drosophila melanogaster

Comparative studies of P- and hobo-element activity, fitness components and recombination parameters in two natural populations of Drosophila melanogaster in Moldova

The genomic frequency of crossovers, fitness components, and activity of P- and hobo-transposable elements were studied in two natural populations of Drosophila melanogaster from regions with different pesticide pressure (forest and orchard populations). A significant increase in rf (recombination frequency) in the orchard population (as compared with the forest population) has been found within segments al-b, b-cn, cn-vg (chromosome 2) and for some chromosome 3 segments, th-st, st-cu, cu-sr, sr-e. None of the populations exhibited P-properties. However, the forest population showed weak hobo activity. Both populations contained probably defective hobo copies. With respect to average fitness (as estimated using a compound-autosome strain) and fecundity, the forest population was superior to the orchard one, whereas the latter had a higher locomotor activity and resistance to etherization. The difference in rf between the populations may be due to differences in their environmental conditions. Selection for pesticide resistance may favor the propagation of genotypes with higher rf, thus, possibly, ensuring release of adaptively significant genetic variation.

Recombination and meiosis

Although exchanges between sister chromatids are common in mitotic cells, those involving homologous chromosomes are rare. Since recombination between homologues is one of the functions of meiosis, it follows that one aspect of the differentiation of the meiocyte involves the synthesis of proteins or enzymes which facilitate synapsis and exchange. Mutants are known which seem to have constitutive levels of mitotic recombination between homologues, and these may be defective in the mechanism which normally represses mitotic recombination. It has been proposed that one component of the synaptonemal complex (s.c.) is a filamentous pairing protein with DNA binding sites which are base sequence specific. Synapsis occurs because the distribution of these sequences is the same in homologues. When only non-homologous chromosomes are present, as in haploid meiosis, only weak pairing can occur, since the base sequences are largely out of register. Although certain features of recombination at the molecular level are known, none of the models so far proposed suggest an explanation for interference between crossovers. It is suggested that interference may depend on the presence of a limited amount of another DNA binding protein which is specifically located within the s.c. A crossover between naked DNA molecules is initially a weak structure, which must be later converted into a visible and mechanically strong chiasma. It is assumed that this stabilization of a crossover is achieved by the DNA binding protein, which can diffuse freely within the s.c. and bind cooperatively to any recombinant DNA molecules within it. Depletion of the binding protein within the vicinity of a crossover makes it unlikely that the second crossover can be formed nearby.