Maternally inherited bacteria of the genus Wolbachia are responsible for reproductive incompatibilities between strains of Drosophila simulans. Such incompatibilities are known in several types of crosses, including both directions of crossing between two types of infected strains, 'R' and 'S'. To determine whether the bidirectional incompatibility between R and S flies is due only to differences between their, bacteria, flies from an uninfected strain have been experimentally infected with bacteria associated with each type. The incompatibility properties of experimental strains are close to those of original strains harbouring the same bacteria and therefore independent of nuclear background. Backcross experiments, however, show that the infection level of a strain depends on the nature of paternal ancestors. This is not explained by nuclear effects but is possibly the result of an interaction between the infection levels of both parents, in which the infection level of S strains is an equilibrium between a tendency for females to produce weakly infected offspring and selection of more infected eggs by sperm from infected males.

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The genetics of two esterase loci active in autogenous adults of the mosquito Culex pipiens pipiens L. has been studied by means of starch gel electrophoresis. Three alleles at the Est-1 locus and eight at the Est-2 locus are described. Both loci have a null allele. Active alleles are codominant and there is no hybrid enzyme in heterozygotes. The Est-1 locus codes esterases preferentially hydrolyzing alpha-naphthylacetate and the Est-2 locus esterases preferentially hydrolyzing beta-naphthylacetate. Strains homozygous for both loci were selected. Linkage studies of the two loci have shown that they are not sex linked but are linked to each other, the crossover frequency being 8.6%. The polymorphism of two laboratory and two natural populations is described for both loci. Phenotypic distributions are in good agreement with Hardy-Weinberg expectations.

In order to understand the status of heteroplasmy and its evolution within the Drosophila melanogaster subgroup, cytoplasm microinjections between eggs were performed involving three lineages of Drosophila simulans, carrying the siT, sill or silil mtDNA type, respectively, and two strains of Drosophila mauritiana carrying the maT or mall mtDNA type. Progeny of eggs from all combinations of injection were analysed. The mall or sil molecules, when provided by the donor, were never detected in the offspring of the hosts, whatever the host's mitochondrial type. Heteroplasmic flies were detected when sill, suIT or mal mitochondria were injected into any of the other cytoplasms. In the majority of cases the percentage of foreign mtDNA increased over generations, leading to a complete replacement of the endogenous mtDNA. In most cases, sill was prevalent. The stochastic processes involved in the evolution of heteroplasmic states are strongly affected by selective values of the different mtDNA types, with a clear hierarchy among them: sill has the most advantage, then sill! and maT, and finally siT and mall. In the sill/mall heteroplasmy case, the loss of mall was more rapid at a high temperature.