Loredana Nigro

The effect of heteroplasmy on cytoplasmic incompatibility in transplasmic lines of Drosophila simulans showing a complete replacement of the mitochondrial DNA

Unidirectional cytoplasmic incompatibility has been observed in crosses between different strains of Drosophila simulans when the males, but not the females, involved in the crosses are infected with intracellular rickettsia-like microorganisms. In contrast to what is known about this system an unexpected partial incompatibility has been observed between two infected strains. Transplasmic lines have been constructed in the laboratory to investigate this phenomenon. The injection of a foreign infected cytoplasm into a strain which apparently shows the same kind of bacterial infection causes an incompatibility in crosses between injected heteroplasmic males, but not injected females, of the same strain. These findings suggest that several crossing types exist within the Drosophila simulans incompatibility system. The complete replacement of the original mitochondrial DNA observed in some of the transplasmic lines made it possible to analyse and exclude any link between incompatibility and mitochondrial genomes.

Mitochondrial DNA sequence divergence in theMelanogaster and oriental species subgroups ofDrosophila

SummaryThe nucleotide sequence of a segment of the mitochondrial DNA from threeDrosophila species (D. erecta, D. eugracilis, andD. takahashii), belonging to different subgroups of themelanogaster group has been determined. The segment encompasses three complete tRNA genes (tRNAtrp, tRNAcys, and tRNAtyr) and portions of two protein-coding genes: the subunit 2 of the NADH dehydrogenase (ND2) and the subunit 1 of the cytochrome oxidase (COI). Comparisons also involve homologous sequences already known for four otherDrosophila species of themelanogaster group. Length differences were confined in the intergenic region where a long stretch of AT repeats was observed in one of the species analyzed. The three tRNA genes exhibit very different evolutionary rates, the most slowly evolving one, tRNAtyr, is adjacent to the 5′ end of COI; tRNAs in similar positions have been previously shown to evolve slowly because they are probably involved in transcript processing. Although the rate of synonymous substitutions was very similar between ND2 and COI genes there were strong discrepancies between them in terms of the number of nonsynonymous substitutions. Differences have also been found in G+C content of the genes, which are likely to be linked to different selective pressures. There is a reduction in G+C content in the region where selective constraints are reduced. This suggests the existence of different levels of constraints along the sequenced segment. An overall analysis of the types of substitutions showed a decrease in A+T content during the course of evolution of the species.

Nuclear background affects frequency dynamics of mitochondrial DNA variants in Drosophila simulans.

Transplasmic lines, in which the original mitochondrial DNA (mtDNA) had been completely replaced by the mtDNA of the donor, were constructed to test for selection on mtDNA variants and to investigate the effects of nuclear-cytoplasmic interactions in Drosophila simulans. These lines were used to set up 12 experimental populations putting in competition two mtDNA variants with two different starting frequencies. Population cages were maintained for 20 generations and periodically sampled to monitor haplotype frequencies. Evidence is provided for a complete reversal of the forces acting on the frequency dynamics of the mtDNA variants according to the lines used to establish the population cages. In the cages where the competing lines had both the original nuclear and mtDNA, the sill type mtDNA variant seemed to be favoured but the situation was reversed when this variant was put in a different nuclear background.

Esterase-6 allozymes: Biochemical studies of two common and one rare variant inDrosophila melanogaster

The biochemical properties of three allozymes coded by theEst-6 locus, two common forms (EST-6S and EST-6F) and one rare form (EST-6VF), were studied. The results show the existence of differences in isoelectric point, activity, activation energy, Km, and temperature coefficient among the three variants, especially between the two common forms and the one rare form. The specific activity of the rare enzymatic variant seems to be less affected by temperature variation. The possible significance of these findings in relation to the mechanism of reproduction is briefly discussed.

Evolution of the mitochondrial ribosomal RNA in the oriental species subgroups of Drosophila

Abstract The nucleotide sequence of two segments of the mtDNA molecule of different species of Drosophila containing a portion of the large rRNA (16S) and a portion of the small rRNA (23S) has been determined. The polymerase chain reaction (PCR) technique with “universal” primers has been used to amplify the segments for direct sequencing. The species analysed belong to four different subgroups of the melanogaster species group. The phylogenetic relationships of the species have been reconstructed by using the neighbour-joining algorithm and the significance of the inferred phylogeny tested by bootstrap method.

Est-6 in Drosophila melanogaster: effects of genetic composition, temperature and oviposition period on fitness in laboratory populations polymorphic for rare alleles

The Esterase-6 locus in Drosophila melanogaster is known to be involved both in the metabolism of esters and in sperm use and remating of females. Several studies have also produced evidence for selective differences between the allozymic variants at this locus. In this paper we have studied the selection differences when two rare alleles (Est-6 VS and Est-6 VF) are competing on the one hand, and when a common allele (Est-6 F) is in competition against the rare allele Est-6 VF on the other. Isogenic lines with homogeneous background were used.In general in the first experiment we found a heterozygous advantage and in the latter strong selection against the rare allele. There was no evidence for any frequency dependent selection. The selective differences were also studied with respect to three different input gene frequencies, three temperatures and in two different periods of oviposition. All the factors were found to influence both the total number of flies emerging and their genotypic composition. There was also interaction between the selective effects of these factors.

Selection in highly inbred populations of Drosophila melanogaster polymorphic at a single locus 1

The polymorphic Esterase-6 system in Drosophila melanogaster has been studied in laboratory populations made up from highly inbred lines. The Est-6S and Est-6F homozygous lines were obtained after 100 generations of inbreeding, selecting heterozygotes at each generation. The level of homogeneity in the genetic background of the lines has been estimated by computer simulation of the inbreeding process and confirmed by electrophoresis. The experimental populations were cultured on a control medium and on media containing different concentrations of naturally occurring esters known to be rapidly utilised by Esterase-6. The results indicate significant genotype-environment interactions. The high level of genetic homogeneity of the lines and the specific pressure applied to the locus analysed suggest that the fitness effects observed could be reasonably attributed to Est-6 allozymes.

Occurrence of a rare allele at the Est-6 locus in natural populations of Drosophila melanogaster

Abstract An attempt to measure the extent of the variation for null-activity alleles at the Esterase-6 (Est-6) locus in Drosophila melanogaster is reported. In the method a laboratory stock was used carrying a third chromosome with an Est-6O allele, balanced with an Est-6S -ultrabithorax (Ubx) chromosome. The results suggest that Est-6 null alleles are present at polymorphic frequency in some Italian populations.

Genetics and Biochemistry of Esterase-6 in Drosophila Melanogaster

Esterase-6 (Est-6) locus in Drosophila melanogaster maps on the third chromosome in the position 36.8. It encodes for a carboxylesterase (M.W. about 90,000) (Danford and Beardmore, 1979), the function of which is still unknown, but which is presumably involved in the reproductive physiology. Histochemical evidence suggests the presence of high esterase-6 activity in the ejaculatory bulb of the adult. Moreover it has been proposed that the Est-6 locus might represent an important determinant of the individual fitness (Aslund and Rasmuson, 1978).

Esterase-6 polymorphism inDrosophila melanogaster:Effects of temperature and methyl malonate on genotypic trajectories in polymorphic populations set up with highly inbred lines

It is generally difficult to identify possible effects of selection at a specific locus because of the heterogeneity of the genetic background. Geographical patterns ofEst-6 gene frequencies suggest that there is selection at this locus but selection on loci closely linked to it cannot be excluded. Differences in catalytic properties between allozymes have been shownin vitro; further, several laboratory studies have shown apparent fitness differences between allozymes. Our study used inbred lines highly homogeneous in the genetic background. Four populations were set up fromEst-6s andEst-6F homozygous females inseminated by males of the same genotype at each combination of three factors: temperature (18 and 25°C); methyl malonate (presence or absence); input gene frequencies [p(S) = 0.2 and 0.8]. The populations were sampled periodically for about 28 generations. Methyl malonate was chosen to exert pressure in the enzymatic function of esterase-6. Statistical analyses show that: there are no sex differences; gene frequencies change from input values to those of the first sampling, when only individuals of the first generation are present at 18oC or individuals of the second generation just begin to appear at 25°C; gene frequencies do not change thereafter and Hardy-Weinberg equilibrium is established. The changes in gene frequencies observed in the first generations suggest thatEst-6 can under certain conditions be a target of selection. Such conditions may not, however, occur in natural populations.