Drosophila Melanogaster Mitochondrial DNA: Gene Organization and Evolutionary Considerations

The sequence of a 8351-nucleotide mitochondrial DNA (mtDNA) fragment has been obtained extending the knowledge of the Drosophila melanogaster mitochondrial genome to 90% of its coding region. The sequence encodes seven polypeptides, 12 tRNAs and the 3' end of the 16S rRNA and CO III genes. The gene organization is strictly conserved with respect to the Drosophila yakuba mitochondrial genome, and different from that found in mammals and Xenopus. The high A + T content of D. melanogaster mitochondrial DNA is reflected in a reiterative codon usage, with more than 90% of the codons ending in T or A, G + C rich codons being practically absent. The average level of homology between the D. melanogaster and D. yakuba sequences is very high (roughly 94%), although insertion and deletions have been detected in protein, tRNA and large ribosomal genes. The analysis of nucleotide changes reveals a similar frequency for transitions and transversions, and reflects a strong bias against G + C on both strands. The predominant type of transition is strand specific.     

Natural Selection with Nuclear and Cytoplasmic Transmission. III. Joint Analysis of Segregation and Mtdna in Drosophila Melanogaster

Despite the widespread use of mitochondrial DNA by evolutionary geneticists, relatively little effort has been spent assessing the magnitude of forces maintaining mtDNA sequence diversity. In this study the influence of cytoplasmic variation on viability in Drosophila was examined by analysis of second chromosome segregation. A factorial experiment with balancer chromosomes permitted the effects of cytoplasma and reciprocal crosses to be individually distinguished. The first test used six lines of diverse geographic origin, testing the segregation of all six second chromosomes in all six cytoplasms. The second and third tests were also factorial designs, but used flies from one population in central Pennsylvania. The fourth test was a large chain cross, using 28 lines from the same Pennsylvania population. Only the first test detected a significant nuclear-cytoplasmic effect. Restriction site variation in the mtDNA of all of these lines was assayed by Southern blotting, and statistical tests were performed in an effort to detect an influence of mtDNA type on fitness components. Posterior linear contrasts revealed an effect of mtDNA on segregation only among lines of diverse geographic origin. Within a population, no such influence was detected, even though the experiment was sufficiently large to have revealed statistical significance of a 0.5% segregation difference with a 57% probability.     

中国大陆部分地区Drosophila immigrans果蝇种群中mtDNA的遗传多态性研究

选用14种限制性内切酶对分布在中国大陆部分地区的Drosophilaimmigrans果蝇种群的线粒体DNA（mtDNA）限制性片段长度多态性（RFLP）进行了分析。在6个地理种群的46个单雌系中仅检测到11种限制性类型。表征种群内均一程度的I值平均为0．833。衡量种群间等同程度的J值平均为0．797。在整个种群中只有16．8％（Gst）的变异是由种群间变异所引起的。说明分布在中国大陆部分地区的D．immigrans果蝇的遗传组成均一程度高,遗传多态程度低,遗传变异贫乏。由UPG法分析6个种群的净遗传距离,显示了分布在秦岭华阳种群（HY）的特殊性。推测D．immigrans果蝇扩散到云南的高海拔地区可能是较晚发生的事件。并推测中国大陆的D．immigrans种群比分布在中国台湾,日本的种群原始。     

Genetic Analysis of Stellate Elements of Drosophila Melanogaster

Repeated elements are remarkably important for male meiosis and spermiogenesis in Drosophila melanogaster. Pairing of the X and Y chromosomes is mediated by the ribosomal RNA genes of the Y chromosome and X chromosome heterochromatin, spermiogenesis depends on the fertility factors of the Y chromosome. Intriguingly, a peculiar genetic system of interaction between the Y-linked crystal locus and the X-linked Stellate elements seem to be also involved in male meiosis and spermiogenesis. Deletion of the crystal element of the Y, via an interaction with the Stellate elements of the X, causes meiotic abnormalities, gamete-genotype dependent failure of sperm development (meiotic drive), and deposition of protein crystals in spermatocytes. The current hypothesis is that the meiotic abnormalities observed in cry(-) males is due to an induced overexpression of the normally repressed Ste elements. An implication of this hypothesis is that the strength of the abnormalities would depend on the amount of the Ste copies. To test this point we have genetically and cytologically examined the relationship of Ste copy number and organization to meiotic behavior in cry(-) males. We found that heterochromatic as well as euchromatic Ste repeats are functional and that the abnormality in chromosome condensation and the frequency of nondisjunction are related to Ste copy number. Moreover, we found that meiosis is disrupted after synapsis and that cry-induced meiotic drive is probably not mediated by Ste.     

A Genetic Analysis of Male-Predominant Pheromones in Drosophila Melanogaster

Chemical signals from males play an important role in stimulating Drosophila melanogaster females to mate, and male-predominant pheromones may influence a female's choice of mates. Male-predominant pheromones also inhibit courtship, thereby functioning as antiaphrodisiacs. Interstrain variation in the ratio of two male-predominant pheromones (7-tricosene and 7-pentacosene) has been reported, but the genetic basis for this potentially important variation has not been examined. In a series of crosses between strains that differ radically in the amounts of 7-tricosene and 7-pentacosene, we have identified both X-linked and autosomal contributions to interstrain variation in the amounts of these compounds. The X-linked loci act as enhancers for production of the compound predominant in the strain from which the X chromosome originated. Autosomal factors for each of the two compounds appear to segregate as high vs. low, with incomplete dominance of high 7-tricosene over low, and low 7-pentacosene over high. A significant negative correlation between the quantities of 7-pentacosene and 7-tricosene in the F2 and backcross progeny, but not in the F1s or parentals, indicates linkage between autosomal loci regulating the expression of each compound. However, the phenotypic distributions of the backcross progeny indicate that additional unlinked loci are also directly involved in the production of these two hydrocarbons.     

Polygenic Mutation in Drosophila Melanogaster: Genetic Analysis of Selection Lines

We have conducted genetic analyses of 12 long-term selection lines of Drosophila melanogaster derived from a highly inbred base population, containing new mutations affecting abdominal and sternopleural bristle number. Biometric analysis of the number of effective factors differentiating the selected lines from the base inbred indicated that with the exception of the three lines selected for increased number of abdominal bristles, three or more mutations contributed to the responses of the selection lines. Analysis of the chromosomal distribution of effects revealed that mutations affecting abdominal bristle number occurred on all three major chromosomes. In addition, Y-linked mutations with effects ranging from one to three bristles occurred in all three lines selected for decreased number of abdominal bristles, as well as in one line selected for increased abdominal bristle number. Mutations affecting sternopleural bristle number were mainly on the X and third chromosomes. One abdominal and one sternopleural selection line showed evidence of a segregating lethal with large effects on bristle number. As an indirect test for allelism of mutations occurring in different selection lines, the three lines selected in the same direction for the same trait were crossed in all possible combinations, and selection continued from the F(2) hybrids. Responses of the hybrid lines usually did not exceed those of the most extreme parental lines, indicating that the responses of the parental lines may have been partly due to mutations at the same loci, although other interpretations are possible.     

A Genetic Analysis of the Stoned Locus and Its Interaction with Dunce, Shibire and Suppressor of Stoned Variants of Drosophila Melanogaster

The genetic complementation patterns of both behavioral and lethal alleles at the stoned locus have been characterized. Mosaic analysis of a stoned lethal allele suggests that stoned functions either in the nervous system or in both the nervous system and musculature, but is not required for gross neural development. The behavioral alleles stn(ts) and stn(C), appear to be defective in a diametrically opposite sense, show interallelic complementation, and indicate distinct roles for the stoned gene product in the visual system and in motor coordination. A number of other neurological mutations have been investigated for their possible interaction with the viable stoned alleles. Mutations at two loci, dunce and shibire, act synergistically with the stn(ts) mutations to cause lethality, but fail to interact with stn(C). A third variant (Suppressor of stoned) has been identified which can suppress the debilitation associated with the stn(ts) mutations. These data, together with a previously identified interaction between the stn(ts) and tan mutants, indicate a central role for the stoned gene product in neuronal function, and suggests that the stoned gene product interacts, either directly or indirectly, with the neural cAMP second messenger system, with the synaptic membrane recycling pathway via dynamin, and with biogenic amine metabolism.     

Risk of Multiple Sclerosis: Analysis of Interactions between Variants of Nuclear and Mitochondrial Genomes

Molecular Biology - There is increasing evidence that the interaction of the mitochondrial and nuclear genomes substantially affects the risk of neurodegenerative diseases. The role of mitonuclear...     

Genotype-by-Environment and Epistatic Interactions in Drosophila Melanogaster: The Effects of Gpdh Allozymes, Genetic Background and Rearing Temperature on Larval Developmental Time and Viability

The possible role of temperature as a component of natural selection generating the latitudinal clines in Gpdh allele frequencies in natural populations of Drosophila melanogaster was examined. Effects of rearing temperature (16 degrees, 22 degrees and 29 degrees) and of Gpdh allozymes (S and F) on larval developmental time and viability were measured. Eight genetic backgrounds from each of three populations (continents) were used to assess the generality of any effects. Analyses of variance indicated significant temperature effects and allozyme-by-genetic background interaction effects for both characters. Viability showed significant genetic background effects, as well as significant temperature-by-allozyme and temperature-by-allozyme-by-population interactions. In general, the S/S genotype was significantly lower in viability than the F/F and F/S genotypes at extreme temperatures (16 degrees and 29 degrees), with no significant differences at 22 degrees. However, each population had a slightly different pattern of viability associated with temperature, and only the Australian population showed a pattern that could contribute to the observed cline formation. Although the same two interactions were not significant for developmental time, examination of the means showed that the S/S genotype had a slightly faster rate of development at 16 degrees than the F/F genotype in all populations (by an average of 0.25 day or 1.1%). The low temperature effect on developmental time is consistent with the clines observed in nature, with the S allele increasing in frequency with higher latitudes. The results for both viability and developmental time are consistent with the interpretation of Gpdh as a minor polygene affecting physiological phenotypes, as indicated by previous work with adult flight metabolism. Finally, it is proposed that the temperature-dependent antagonistic effects of the allozymes on viability vs. developmental time and flight metabolism may be the underlying force giving rise to the worldwide polymorphism.     

The Effect of Sequence Homozygosity on the Frequency of X-Chromosomal Exchange in Drosophila Melanogaster Females

The repair of mismatched heteroduplex DNA has been implicated in the normal resolution of meiotic exchange events. Although sequence microheterogeneity over defined intervals of homologous chromosomes has been correlated with local effects on recombination, this correlation has not previously been extended to effects on chromosomal levels of exchange. In order to determine the role of microheterogeneity in normal exchange between homologs, a system was devised for monitoring exchange between isogenic X chromosomes. Lack of microheterogeneity did not significantly alter the frequency of exchange along the isogenic X chromosomes relative to controls or to previously reported values. There were, however, characteristic levels of exchange intrinsic to the cloned X chromosomes in each of the lines tested.     

Comparative Genomics of Mitochondrial DNA in Drosophila simulans

The current study compares the nucleotide variation among 22 complete mitochondrial genomes of the three distinct Drosophila simulans haplotypes with intron 1 of the alcohol dehydrogenase-related locus. This is the first study to investigate the sequence variation of multiple complete mitochondrial genomes within distinct mitochondrial haplotypes of a single species. Patterns of variation suggest distinct forces are influencing the evolution of mitochondrial DNA (mtDNA) and autosomal DNA in D. simulans. First, there is little variation within each mtDNA haplotype but strong differentiation among them. In contrast, there is no support for differentiation of the mitochondrial haplotypes at the autosomal locus. Second, there is a significant deficiency of mitochondrial variation in each haplotype relative to the autosomal locus. Third, the ratio of nonsynonymous to synonymous substitutions is not equal in all branches of the well-resolved phylogeny. There is an excess of nonsynonymous substitutions relative to synonymous substitutions within each D. simulans haplotype. This result is similar to that previously observed within the mtDNA of distinct species. A single evolutionary force may be causally linked to the observed patterns of mtDNA variation—a rickettsia-like microorganism, Wolbachia pipientis, which is known to directly influence mitochondrial evolution but have a less direct influence on autosomal loci. Received: 16 September 1999 / Accepted: 14 March 2000     

Incomplete Maternal Transmission of Mitochondrial DNA in Drosophila

The possibility of incomplete maternal transmission of mitochondrial DNA (mtDNA) in Drosophila, previously suggested by the presence of heteroplasmy, was examined by intra- and interspecific backcrosses of Drosophila simulans and its closest relative, Drosophila mauritiana. mtDNAs of offspring in these crosses were characterized by Southern hybridization with two alpha-32P-labeled probes that are specific to paternal mtDNAs. This method could detect as little as 0.03% paternal mtDNA, if present, in a sample. Among 331 lines that had been backcrossed for ten generations, four lines from the interspecific cross D. simulans (female) x D. mauritiana (male) showed clear evidence for paternal leakage of mtDNA. In three of these the maternal type was completely replaced while the fourth was heteroplasmic. Since in this experiment the total number of fertilization is known to be 331 x 10 = 3310, the proportion of paternal mtDNA per fertilization was estimated as about 0.1%. The mechanisms and evolutionary significance for paternal leakage are discussed in light of this finding.