mauritiana

Gene flow mediates the role of sex chromosome meiotic drive during complex speciation

During speciation, sex chromosomes often accumulate interspecific genetic incompatibilities faster than the rest of the genome. The drive theory posits that sex chromosomes are susceptible to recurrent bouts of meiotic drive and suppression, causing the evolutionary build-up of divergent cryptic sex-linked drive systems and, incidentally, genetic incompatibilities. To assess the role of drive during speciation, we combine high-resolution genetic mapping of X-linked hybrid male sterility with population genomics analyses of divergence and recent gene flow between the fruitfly species, Drosophila mauritiana and D. simulans. Our findings reveal a high density of genetic incompatibilities and a corresponding dearth of gene flow on the X chromosome. Surprisingly, we find that a known drive element recently migrated between species and, rather than contributing to interspecific divergence, caused a strong reduction in local sequence divergence, undermining the evolution of hybrid sterility. Gene flow can therefore mediate the effects of selfish genetic elements during speciation.

PHYLOGENY AND EVOLUTION OF PARTHENOGENETIC WEEVILS OF THE ARAMIGUS TESSELLATUS SPECIES COMPLEX (COLEOPTERA: CURCULIONIDAE: NAUPACTINI): EVIDENCE FROM MITOCHONDRIAL DNA SEQUENCES

Molecular-phylogenetic studies of parthenogenetic animals have been a valuable recent addition to the literature on the evolutionary biology of sex. By illuminating the origins and ages of parthenogenetic lineages, such studies can help to define the temporal scale at which selection acts against parthenogenetic lineages, as well as provide an essential framework for further study. Although parthenogenetic weevils have played an important role in cytogenetic and protein-electrophoretic studies of parthenogenesis, they have not previously been subjects of DNA-based molecular-phylogenetic study. A mitochondrial DNA study of Aramigus tessellatus, a species complex of weevils native to South America, indentified 12 distinct (1-9% divergent) maternal lineages, of which 2 represent sexual populations, while at least 9 represent parthenogenetic lineages. These lineages partially correspond to lineages previously recognized by morphological differences. Phylogenetic analysis found 14 most parsimonious trees, according to which parthenogenesis appears to have arisen 3-7 times. There is a monophyletic group of lineages (the "brown clade"), having up to 4.5% sequence divergence within it, which may be primitively parthenogenetic and over 2 million years old.

EXPERIMENTAL EVIDENCE FOR MITOCHONDRIAL DNA INTROGRESSION BETWEEN DROSOPHILA SPECIES

Differential introgression of mitochondrial genomes has been used to explain the occurrence in some species of individuals bearing mtDNA from a related species. This situation has been observed for Drosophila mauritiana (endemic to Mauritius) where a high proportion of individuals (88%) carries an mtDNA also found in D. simulans populations from Madagascar and Réunion. Using these two species, experiments were carried out to test for differential mtDNA introgression. A single virgin female from one species (initial frequency 0.03) was introduced into a population of the other. D. simulans mtDNA can, within three generations, almost entirely displace (frequency up to 0.80) D. mauritiana mtDNA. Hybrid male sterility probably curtails to a large degree parallel introgression of nuclear genes. The progress of cytoplasmic introgression is dependent on the degree of inbreeding of the recipient D. mauritiana strains. In reciprocal experiments, introgression was much less likely: few D. mauritiana migrant females are inseminated and their mtDNA frequency always remains very low. The results of these experiments support the hypothesis that a selective advantage of hybrids (probably at the nuclear level) has promoted mtDNA transfer from D. simulans Madagascar or Reunion populations into D. mauritiana through introgressive hybridization.

Fine mapping of dominant X-linked incompatibility alleles in Drosophila hybrids

Sex chromosomes have a large effect on reproductive isolation and play an important role in hybrid inviability. In Drosophila hybrids, X-linked genes have pronounced deleterious effects on fitness in male hybrids, which have only one X chromosome. Several studies have succeeded at locating and identifying recessive X-linked alleles involved in hybrid inviability. Nonetheless, the density of dominant X-linked alleles involved in interspecific hybrid viability remains largely unknown. In this report, we study the effects of a panel of small fragments of the D. melanogaster X-chromosome carried on the D. melanogaster Y-chromosome in three kinds of hybrid males: D. melanogaster/D. santomea, D. melanogaster/D. simulans and D. melanogaster/D. mauritiana. D. santomea and D. melanogaster diverged over 10 million years ago, while D. simulans (and D. mauritiana) diverged from D. melanogaster over 3 million years ago. We find that the X-chromosome from D. melanogaster carries dominant alleles that are lethal in mel/san, mel/sim, and mel/mau hybrids, and more of these alleles are revealed in the most divergent cross. We then compare these effects on hybrid viability with two D. melanogaster intraspecific crosses. Unlike the interspecific crosses, we found no X-linked alleles that cause lethality in intraspecific crosses. Our results reveal the existence of dominant alleles on the X-chromosome of D. melanogaster which cause lethality in three different interspecific hybrids. These alleles only cause inviability in hybrid males, yet have little effect in hybrid females. This suggests that X-linked elements that cause hybrid inviability in males might not do so in hybrid females due to differing sex chromosome interactions.

Extensive paternal mtDNA leakage in natural populations of Drosophila melanogaster

Strict maternal inheritance is considered a hallmark of animal mtDNA. Although recent reports suggest that paternal leakage occurs in a broad range of species, it is still considered an exceptionally rare event. To evaluate the impact of paternal leakage on the evolution of mtDNA, it is essential to reliably estimate the frequency of paternal leakage in natural populations. Using allele-specific real-time quantitative PCR (RT-qPCR), we show that heteroplasmy is common in natural populations with at least 14% of the individuals carrying multiple mitochondrial haplotypes. However, the average frequency of the minor mtDNA haplotype is low (0.8%), which suggests that this pervasive heteroplasmy has not been noticed before due to a lack of power in sequencing surveys. Based on the distribution of mtDNA haplotypes in the offspring of heteroplasmic mothers, we found no evidence for strong selection against one of the haplotypes. We estimated that the rate of paternal leakage is 6% and that at least 100 generations are required for complete sorting of mtDNA haplotypes. Despite the high proportion of heteroplasmic individuals in natural populations, we found no evidence for recombination between mtDNA molecules, suggesting that either recombination is rare or recombinant haplotypes are counter-selected. Our results indicate that evolutionary studies using mtDNA as a marker might be biased by paternal leakage in this species.

Paternal transmission of mitochondrial DNA as an integral part of mitochondrial inheritance in metapopulations of Drosophila simulans

Maternal inheritance is one of the hallmarks of animal mitochondrial DNA (mtDNA) and central to its success as a molecular marker. This mode of inheritance and subsequent lack of heterologous recombination allows us to retrace evolutionary relationships unambiguously down the matriline and without the confounding effects of recombinant genetic information. Accumulating evidence of biparental inheritance of mtDNA (paternal leakage), however, challenges our current understanding of how this molecule is inherited. Here, using Drosophila simulans collected from an East African metapopulation exhibiting recurring mitochondrial heteroplasmy, we conducted single fly matings and screened F1 offspring for the presence of paternal mtDNA using allele-specific PCR assays (AS-PCR). In all, 27 out of 4092 offspring were identified as harboring paternal mtDNA, suggesting a frequency of 0.66% paternal leakage in this species. Our findings strongly suggest that recurring mtDNA heteroplasmy as observed in natural populations of Drosophila simulans is most likely caused by repeated paternal leakage. Our findings further suggest that this phenomenon to potentially be an integral part of mtDNA inheritance in these populations and consequently of significance for mtDNA as a molecular marker.

Genome sequencing reveals complex speciation in the Drosophila simulans clade

The three species of the Drosophila simulans clade—the cosmopolitan species, D. simulans, and the two island endemic species, D. mauritiana and D. sechellia—are important models in speciation genetics, but some details of their phylogenetic and speciation history remain unresolved. The order and timing of speciation are disputed, and the existence, magnitude, and timing of gene flow among the three species remain unclear. Here we report on the analysis of a whole-genome four-species sequence alignment that includes all three D. simulans clade species as well as the D. melanogaster reference sequence. The alignment comprises novel, paired short-read sequence data from a single highly inbred line each from D. simulans, D. mauritiana, and D. sechellia. We are unable to reject a species phylogeny with a basal polytomy; the estimated age of the polytomy is 242,000 yr before the present. However, we also find that up to 4.6% of autosomal and 2.2% of X-linked regions have evolutionary histories consistent with recent gene flow between the mainland species (D. simulans) and the two island endemic species (D. mauritiana and D. sechellia). Our findings thus show that gene flow has occurred throughout the genomes of the D. simulans clade species despite considerable geographic, ecological, and intrinsic reproductive isolation. Last, our analysis of lineage-specific changes confirms that the D. sechellia genome has experienced a significant excess of slightly deleterious changes and a dearth of presumed favorable changes. The relatively reduced efficacy of natural selection in D. sechellia is consistent with its derived, persistently reduced historical effective population size.

Inter-island divergence within Drosophila mauritiana, a species of the D. simulans complex: Past history and/or speciation in progress?

Speciation with gene flow may be more common than generally thought, which makes detailed understanding of the extent and pattern of genetic divergence between geographically isolated populations useful. Species of the Drosophila simulans complex provide a good model for speciation and evolutionary studies, and hence understanding their population genetic structure will increase our understanding of the context in which speciation has occurred. Here, we describe genetic diversity and genetic differentiation of two distant populations of D. mauritiana (Mauritius and Rodrigues Islands) at mitochondrial and nuclear loci. We surveyed the two populations for their mitochondrial haplotypes, eight nuclear genes and 18 microsatellite loci. A new mitochondrial type is fixed in the Rodrigues population of D. mauritiana. The two populations are highly differentiated, their divergence appears relatively ancient (100,000 years) compared to the origin of the species, around 0.25MYA, and they exhibit very limited gene flow. However, they have similar levels of divergence from their sibling, D. simulans. Both nuclear genes and microsatellites revealed contrasting demographic histories between the two populations, expansion for the Mauritius population and stable population size for the Rodrigues Island population. The discovery of pronounced geographic structure within D. mauritiana combined to genetic structuring and low gene flow between the two island populations illuminates the evolutionary history of the species and clearly merits further attention in the broad context of speciation.

Tissue segregation of mitochondrial haplotypes in heteroplasmic Hawaiian bees: implications for DNA barcoding

The issue of mitochondrial heteroplasmy has been cited as a theoretical problem for DNA barcoding but is only beginning to be examined in natural systems. We sequenced multiple DNA extractions from 20 individuals of four Hawaiian Hylaeus bee species known to be heteroplasmic. All species showed strong differences at polymorphic sites between abdominal and muscle tissue in most individuals, and only two individuals had no obvious segregation. Two specimens produced completely clean sequences from abdominal DNA. The fact that these differences are clearly visible by direct sequencing indicates that substantial intra-individual mtDNA diversity may be overlooked when DNA is taken from small tissue fragments. At the same time, differences in haplotype distribution among individuals may result in incorrect recognition of cryptic species. Because DNA barcoding studies typically use only a small fragment of an organism, they are particularly vulnerable to sequencing bias where heteroplasmy and haplotype segregation are present. It is important to anticipate this possibility prior to undertaking large-scale barcoding projects to reduce the likelihood of haplotype segregation confounding the results.

All-male asexuality: origin and maintenance of androgenesis in the Asian clam Corbicula

Androgenesis is a rare form of asexual male reproduction found in disparate taxa across the Tree of Life. Phylogenetic analyses of mitochondrial genes suggest that androgenesis has arisen repeatedly in the Asian clam genus Corbicula. Two of these androgenetic species have been introduced to North America. Multiple lines of genetic evidence suggest that although nuclear recombination between these two species is rare, mitochondrial genome capture is a frequent consequence of androgenetic parasitism of heterospecific eggs. Egg parasitism may also rarely result in partial nuclear genome capture between closely related species of Corbicula, which provides a mechanism for the otherwise clonal species to avoid the deleterious effects of asexuality. Egg parasitism among congeners may explain why androgenesis has been maintained in Corbicula after fixation and has not yet led to population extinction. This mechanism also provides an explanation for the apparent multiple origins of androgenesis in Corbicula as seen on the mitochondrial DNA phylogeny. We suggest that a single androgenetic lineage may have repeatedly captured mitochondrial genomes (as well as portions of nuclear genomes) from various sexual species, resulting in several distinct androgenetic species with distantly related mtDNA genomes and divergent morphologies.

Evidence for paternal leakage in hybrid periodical cicadas (Hemiptera: Magicicada spp.)

Mitochondrial inheritance is generally assumed to be maternal. However, there is increasing evidence of exceptions to this rule, especially in hybrid crosses. In these cases, mitochondria are also inherited paternally, so “paternal leakage” of mitochondria occurs. It is important to understand these exceptions better, since they potentially complicate or invalidate studies that make use of mitochondrial markers. We surveyed F₁ offspring of experimental hybrid crosses of the 17-year periodical cicadas Magicicada septendecim, M. septendecula, and M. cassini for the presence of paternal mitochondrial markers at various times during development (1-day eggs; 3-, 6-, 9-week eggs; 16-month old 1^st and 2^nd instar nymphs). We found evidence of paternal leakage in both reciprocal hybrid crosses in all of these samples. The relative difficulty of detecting paternal mtDNA in the youngest eggs and ease of detecting leakage in older eggs and in nymphs suggests that paternal mitochondria proliferate as the eggs develop. Our data support recent theoretical predictions that paternal leakage may be more common than previously estimated.

Sympatric Drosophila simulans flies with distinct mtDNA show age related differences in mitochondrial metabolism

The primary causes of age-related changes in mitochondrial metabolism are not known. The goal of this study is to document the influence of naturally occurring mtDNA variation on age dependent changes in mitochondrial respiration, hydrogen peroxide (H(2)O(2)) generation and antioxidant defenses in the fly Drosophila simulans. Possible changes include an increase in rates of reactive oxygen species production with age and/or an age dependent decrease in antioxidant response. For this study we have used flies harboring distinct siII and siIII mtDNA types. Previously we have shown that males harboring siII mtDNA had higher rates of mitochondrial H(2)O(2) production from complex III at 11d compared to males with the siIII mtDNA type. Here, we corroborate those results and show that Drosophila harboring the siII and siIII mtDNA types exhibit significantly different patterns of pro-oxidant and antioxidant activities as they age. Flies harboring siII mtDNA had higher rates of mitochondrial H(2)O(2) production and manganese superoxide dismutase activity at 11 and 18d of age than siIII mtDNA harboring flies. Copper-zinc superoxide dismutase activity increased from 11 to 25d in siII flies while the accumulation of oxidized glutathione did not change between 11 and 25d. In contrast, siIII harboring flies showed an age dependent increase in H(2)O(2) production, reaching higher production rates on day 25 than that observed in siII flies. Copper-zinc superoxide dismutase activities did not change between 11 and 25d while the oxidized glutathione accumulation increased with age. The results show antioxidant levels correlate with pro-oxidant levels in siII but not siIII flies. These results demonstrate our ability to correlate mtDNA variation with differences in whole mitochondrial physiology and individual complex biochemistry.

Sympatric Drosophila simulans flies with distinct mtDNA show difference in mitochondrial respiration and electron transport

The role of mitochondrial DNA (mtDNA) in mitochondrial metabolism is understudied yet humans harboring specific mtDNA types age at dissimilar rates, are unequally susceptible to various diseases, and differentially adapt to various environmental conditions. This study compares mitochondrial respiration, proton leak and electron transport of Drosophila simulans males with distinct mtDNA haplogroups (siII and -III) that were collected in sympatry in Kenya. Despite the large divergence among haplogroups there is very low intrahaplogroup variation and no correlated variation in the nuclear genome has been detected. We show that repeatable bioenergetic differences exist between 11d old males harboring siII and siIII mtDNA. Males with siIII mtDNA showed higher (i) state 3 respiration rates from isolated mitochondria for both complex I and complex III based substrates, and (ii) complex IV (cytochrome c oxidase) activity. Males harboring siIII mtDNA had lower (i) hydrogen peroxide formation by both complexes I and III, (ii) proton leak from isolated mitochondria, (iii) mitochondrial ATPase activity, and (iv) mitochondrial cytochrome content. In combination, the results suggest that mitochondria isolated from siIII mtDNA harboring males have more efficient metabolism than siII mtDNA harboring males.

Wolbachia in the Asian rice gall midge, Orseolia oryzae (Wood-Mason): correlation between host mitotypes and infection status

Using a PCR-based method, we detected Wolbachia in the Asian rice gall midge. Furthermore, results showed that all females across all biotypes are infected with Wolbachia. However, all male flies are not infected and show different infection frequency in different biotypes. We have also identified three mitotypes, in the rice gall midge, based on DraI restriction pattern of a portion of the 12S rRNA gene that was PCR amplified using primers specific to this gene. All the females and infected male flies had type 1 mtDNA while uninfected males showed only type 2 or 3 mtDNA. Inheritance patterns of mtDNA revealed the existence of a correlation between mtDNA type and Wolbachia infection in the Asian rice gall midge. Evidence for paternal inheritance of mtDNA in Wolbachia-free gall midge is also presented.

Myopia, intelligence, and the expanding human neocortex: behavioral influences and evolutionary implications

The first two parts of this monograph document that areas of the human neocortex heavily used to cope with a complex, language-driven society have been expanding rapidly and suggest strongly that this is linked with the huge upsurge that's occurred in myopia, and with the large gradual 20th-century increase in measured intelligence. Part III proposes mechanisms capable of supporting such rapid changes, without violating the basic precepts of Darwin's thinking. Part IV discusses the social and evolutionary ramifications of our apparent proclivity for rapid, progressive, adaptive neocortical change, and suggests areas for productive research.

Paternal products and by-products in Drosophila development

Tails of fertilizing spermatozoa persist throughout embryogenesis in Drosophila species and can be observed within the midguts of larvae after hatching. Throughout development, sperm proteins slowly diffuse or are stripped from the giant sperm tail residing within the embryo's anterior end. The shape and position of the sperm within the embryo are regulated such that, during organ formation, the unused portion of the sperm is enveloped by the developing midgut. This persistent, paternally derived structure is composed of the sperm's mitochondrial derivatives and appears to be defecated by the larva soon after hatching. These complex sperm-egg interactions may represent mechanisms to avoid intragenomic conflict by ensuring strictly maternal inheritance of mitochondrial DNA (mtDNA).

Mitochondrial DNA polymorphism in a black fly, Simulium vittatum (Diptera: Simuliidae)

Mitochondrial DNA (mtDNA) was extracted from pooled field-collected samples representing six species of black flies (Cnephia dacotensis, Simulium bivittaum, S. johansenni, S. luggeri, S. piperi, S. vittatum) and compared by restriction fragment length polymorphism (RFLP) analysis. Morphospecies were molecularly distinct, with few shared restriction fragments. Eleven populations of S. vittatum were found that appeared to be homogeneous for a single mitochondrial haplotype. Ten other populations of S. vittatum showed extensive mitochondrial heterogeneity. In part, these samples contained mixtures of two cytologically recognized siblings: IIIL-1 and IS-7. About 70% of the mitochondrial genome of a population pure for sibling IIIL-1 was cloned as five HindIII fragments, which were used as hybridization probes to examine individual black flies. Thirteen mtDNA haplotypes involving permutations of 10 HindIII restriction sites were identified in individual black flies examined from 26 populations. DNA from 168 larvae cut with both EcoR1 and HindIII revealed five additional haplotypes. One HindIII haplotype was present in 84% of 390 larvae examined and predominated in every population examined from New York to California and in both the IIIL-1 and IS-7 siblings. Nebraska populations had individuals with nearly all known haplotypes. The most common haplotype was usually the only form present in warm, silty streams with organic enrichment. Rarer haplotypes were found in cool, spring-fed streams but without clear geographic or phylogenetic components.

The fate of paternal mitochondrial DNA in developing female mussels, Mytilus edulis: implications for the mechanism of doubly uniparental inheritance of mitochondrial DNA

Species of the marine mussel family Mytilidae have two types of mitochondrial DNA: one that is transmitted from the mother to both female and male offspring (the F type) and one that is transmitted from the father to sons only (the M type). By using pair matings that produce only female offspring or a mixture of female and male offspring and a pair of oligonucleotide primers that amplify part of the COIII gene of the M but not the F mitochondrial genome, we demonstrate that both male and female embryos receive M mtDNA through the sperm and that within 24 hr after fertilization the M mtDNA is eliminated or is drastically reduced in female embryos but maintained in male embryos. These observations are important for understanding the relationship between mtDNA transmission and sex determination in species with doubly uniparental inheritance of mitochondrial DNA.

Nonrandom partition of mitochondria in heteroplasmic Drosophila

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 siI, siII or siIII mtDNA type, respectively, and two strains of Drosophila mauritiana carrying the maI or maII mtDNA type. Progeny of eggs from all combinations of injection were analysed. The maII or siI 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 siII, siIII or maI 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, siII 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: siII has the most advantage, then siIII and maI, and finally siI and maII. In the siII/maII heteroplasmy case, the loss of maII was more rapid at a high temperature.

Sex-biased heteroplasmy and mitochondrial DNA inheritance in the mussel Mytilus galloprovincialis Lmk

An exceptional mode of mtDNA inheritance involving separate maternal and paternal transmission routes has been reported recently in the mussel Mytilus edulis. This mode of inheritance provides an explanation for the high levels of heteroplasmy for two highly diverged genomes observed in males of this species. Here we provide evidence for a similar pattern of heteroplasmy in Atlantic and Mediterranean forms of the related mussel M. galloprovincialis. The results support the hypothesis that this mode of mtDNA inheritance has an ancient origin. In addition, the detection of some heteroplasmic females suggests preferential, rather than exclusive, transmission within male and female lines of descent. We also present evidence that the two highly diverged genomes display a parallel split between the Atlantic and Mediterranean forms, consistent with neutral evolution.

Transfer of paternal mitochondrial DNA during fertilization of honeybee (Apis mellifera L.) eggs

Strict maternal inheritance of mitochondrial (mt) DNA is believed to be the rule in most eukaryotic organisms because of exclusion of paternal mitochondria from the egg cytoplasm during fertilization. In honeybees, polyspermic fertilization occurs, and many spermatozoa, including their mitochondria-rich flagellum, can completely penetrate the egg, thus allowing for a possibly high paternal leakage. In order to identify paternal mtDNA in honeybee eggs, restriction fragment length polymorphisms (RFLP) of different subspecies were used. Total DNA extracts of different developmental stages of an Apis mellifera carnica x Apis mellifera capensis hybrid brood were tested with a radioactively-labelled diagnostic mtDNA probe. Densitograms of autoradiographs indicated that the male contribution represents up to 27% of the total mitochondrial DNA in the fertilized eggs 12 h after oviposition. In subsequent developmental stages the portion of paternal mtDNA slowly decreased until hatching of the larvae when only traces were found. Although rapid disintegration of paternal mtDNA does not occur, the initially high paternal mitochondrial contribution is not maintained in the adult animal.

Direct evidence for extensive paternal mitochondrial DNA inheritance in the marine mussel Mytilus

Inheritance of mitochondrial DNA in animals was thought to be strictly maternal. Recently, evidence for incidental paternal mtDNA leakage was obtained in hybrid crosses of Drosophila and mice. In mice, the frequency of paternal mtDNA contributions was estimated at 10(-4), compared with maternal contributions. The common occurrence in the marine mussel Mytilus of heteroplasmic individuals with two or more types of highly diverged mtDNA molecules was interpreted as strong evidence for biparental mtDNA inheritance by some, but not by others. We report here results from pair-matings involving two species of mussels, Mytilus edulis and Mytilus trossulus. Extensive contribution of paternal mtDNA, amounting to several orders of magnitude higher than that inferred for Drosophila or mice, was observed in both intra- and interspecific crosses.

Nucleus-driven mutations of human mitochondrial DNA

Neuromuscular disorders due to abnormalities of mitochondrial energy supply have become an important area of human pathology. In particular, lesions of the mitochondrial genome (mtDNA), a small extra-nuclear chromosome which encodes 13 subunits of the respiratory chain complexes, are responsible for a steadily increasing number of neuromuscular syndromes. In addition to sporadic or maternally-inherited mutations, either qualitative or quantitative abnormalities of mtDNA can be transmitted as Mendelian traits, leading to well-defined mitochondrial encephalomyopathies. The latter are presumably caused by mutations in still unknown nucleus-encoded genes which deleteriously interact with the mitochondrial genome. These observations are of importance from both clinical and theoretical points of view, because they are the first examples of diseases produced by abnormalities of the nuclear control over mitochondrial biogenesis.

An improved test for Africanized honeybee mitochondrial DNA

Mitochondrial DNA derived from Apis mellifera scutellata, the ancestor of the Africanized bees of the New World, lacks a BglII restriction site found in other types of honeybee. We present primers allowing amplification of a 485-bp section of the cytochrome b gene containing this site, using the polymerase chain reaction. Digestion of the amplified product with BglII yields contrasting patterns between Africanized and other honeybees.

Mitochondrial DNA heteroplasmy maintained in natural populations of Drosophila simulans in Réunion

Mitochondrial DNA (mtDNA) variation in Drosophila simulans was studied to determine whether the cytoplasmic state of mtDNA heteroplasmy persists in natural populations in Réunion. For this purpose, 172 isofemale lines, newly collected from two local populations, were examined, among which three types of mtDNA (siII, siIII and siIII') were found, based on the Hpa II restriction pattern. Ten of the lines were heteroplasmic for a combination of siII and siIII, as determined by autoradiography. The same type of heteroplasmy had been noted in one of the two local populations 8 years before (Satta et al. 1988). The present results suggest that the heteroplasmic state occurs recurrently in natural populations of D. simulans in Réunion.

Heteroplasmy suggests limited biparental inheritance of Mytilus mitochondrial DNA

Strict maternal inheritance of mitochondrial DNA is commonly observed in animals. There is usually only one mitochondrial DNA population (homoplasmy) within an individual. Mussels of the Mytilus edulis species group appear to be exceptions in both respects. Of 150 Mytilus individuals examined, 85 were heteroplasmic. Mitochondrial DNA types within heteroplasmic individuals differed greatly; in one comparison, the inferred sequence difference was 20 +/- 5 percent. Homoplasmic individuals with mitochondrial DNA similar to the heteroplasmic mitochondrial DNA types were found. These observations are best explained by the hypothesis that biparental inheritance of mitochondrial DNA can occur in Mytilus.

The generation of transplasmic Drosophila simulans by cytoplasmic injection: effects of segregation and selection on the perpetuation of mitochondrial DNA heteroplasmy

Experimental transplasmic Drosophila simulans were obtained through cytoplasm microinjection between eggs carrying different mitochondrial genomes. These genomes (siII and siIII) show a 1.5% difference in their sequences. They produced a large number of heteroplasmic flies in their F1 progeny and several flies were still heteroplasmic at the eighth generation. The distribution of frequencies of mitochondrial genotypes in the offspring of heteroplasmic females suggests that the stochastic processes involved in the evolution of experimental heteroplasmy of multiple nucleotide sites are very similar to those previously described for spontaneous length heteroplasmy. In addition, the siII genome has a noticeable advantage over the siIII genome in both directions of injection. This advantage is estimated at 58% per fly generation and 5% per cell generation.

Intraspecific genetic variability in mitochondrial DNA of the screwworm fly (Cochliomyia hominivorax)

Mitochondrial DNA variability has been analyzed in the primary screwworm fly (Cochliomyia hominivorax) using restriction endonuclease fragment patterns and restriction site mapping. A total of 30 different screwworm lines originating from Texas to Costa Rica and the Island of Jamaica was examined using 15 restriction endonucleases. Eleven of the restriction enzymes revealed polymorphism and yielded 16 mitochondrial genotypes or haplotypes. Two of the haplotypes were widely distributed, haplotype 1 being found scattered across southern Mexico and haplotype 2 along the west coast of Mexico. Haplotype 1 also appeared paired with several other haplotypes in mixed lines that were most likely the result of collecting an egg mass to which more than one female had contributed or to some form of contamination by haplotype 1 after introduction into the laboratory. These lines became fixed before single insects were examined and thus it is impossible to rule out heteroplasmy. The other 14 haplotypes were found in only a single locale and 12 of these were found in only one line. The average sequence diversity among 27 mainland lines was about 0.5%. The two Jamaican lines and one east coast mainland line differed from the others by greater than 2%. The pattern of geographical distribution, a small number of apparently recurring haplotypes and a substantial number (75%) of the haplotypes unique, bears similarities to patterns observed in other insects such as Drosophila. The high frequency of unique genotypes in southern Mexico suggests a population with a very reduced gene flow, which may have had a positive effect on the sterile male release control program.