Selection on X-Linked Genes during Speciation in the Drosophila Athabasca Complex

We present the results of a restriction site survey of variation at five loci in Drosophila athabasca, complimenting a previous study of the period locus. There is considerably greater differentiation between the three semispecies of D. athabasca at the period locus and two other X-linked genes (no-on-transient-A and E74A) than at three autosomal genes (Xdh, Adh and RC98). Using a modification of the HKA test, which uses fixed differences between the semispecies and a test based on differences in Fst among loci, we show that the greater differentiation of the X-linked loci compared with the autosomal loci is inconsistent with a neutral model of molecular evolution. We explore several evolutionary scenarios by computer simulation, including differential migration of X and autosomal genes, very low levels of migration among the semispecies, selective sweeps, and background selection, and conclude that X-linked selective sweeps in at least two of the semispecies are the best explanation for the data. This evidence that natural selection acted on the X-chromosome suggests that another X-linked trait, mating song differences among the semispecies, may have been the target of selection.     

ANTAGONISTIC PLEIOTROPY: AN INTERSPECIFIC DROSOPHILA COMPARISON

Wild-type flies of 12 Drosophila species and semispecies were examined to determine whether correlation patterns between early- and late-life fitness characters predicted for individuals within a population by the antagonistic-pleiotropy hypothesis are reflected in comparisons of related species and semispecies that are known to differ in lifespan. Our goal was to determine whether the hypothesis is relevant to the evolution of life-history differences beyond the population level. Two fitness traits, egg production and percentage mating success, were observed at three ages: onset of reproductive age, one week later, and one month later. Age-dependent patterns of these traits do not consistently conform to predictions of the hypothesis. Species or semispecies that show reproductive vitality early in life need not be short-lived, and long lifespan need not be accompanied by a cost in early reproductive vitality, as measured by mating success and egg production. The two fitness traits can show different age-dependent patterns in the same species or semispecies. Potential explanations for the frequent inconsistency of the data with predictions of the hypothesis are discussed. Results support the idea that the hypothesis is only relevant to the evolution of life-history differences among individuals in the same breeding population confronted by the same environmental constraints.     

Analysis of gene flow in North American face fly (Musca autumnalis) populations

Abstract. Vertical polyacrylamide gel electrophoresis was used to resolve enzymes at ten putative loci in face flies Musca autumnalis De Geer, a colonizing, Palaearctic species established in North America for at least 210 generations. Flies were sampled in 1991 from six locations in Iowa, two in Maryland, two in Minnesota, two in Tennessee, three in New York, and three in Missouri. Nondirectional temporal variation in gene frequencies over a 4-year interval was detected at farms in central Iowa. Heterogeneity in allele frequencies was detected among farms in Iowa, Maryland, New York and Minnesota but not in Tennessee and Missouri. There were no consistent departures from random mating. Partitioning variances in gene frequencies showed that 58% of the variation occurred in populations among states and 43% between populations within states. Mean reproducing immigrants per population per generation was estimated to be eighteen flies. No regional genetic differentiation was detected, and there were no barriers to gene flow within or among diverse populations in the six states. Earlier data bearing on gene flow among and between Nearctic and Palearctic face fly samples were analysed and significant differentiation was not detected.     

Evidence for gene conversion between the phosphoglycerate kinase genes of Trypanosoma brucei

Trypanosoma brucei contains a tandem array of three genes for phosphoglycerate kinase (PGKase), genes A, B and C, each coding for a different protein. We have compared allelic variants of this gene array and find evidence for gene conversion between the three genes. Near the 3' end, the different alleles and gene B contain a variable sequence that is similar to the corresponding sequence in either gene A or gene C. This sequence is flanked by glycine triplets that are conserved in all PGKases from bacteria to mammals. The triplets are encoded by (GGT)n, resulting in sequences that resemble the recombination-promoting chi-sites of Escherichia coli. Upstream of the variable sequence, there is an area of 800 base-pairs in which genes A, B and C are highly homologous; in all three genes this region ends with a sharp boundary at which gene B again shows segmental homology with both genes A and C. These results suggest that repeated gene conversion events partially erase the differences between genes A, B and C that arise in evolution and suggest that chi-like sequences may act as recombinational hotspots in protozoa such as T. brucei.     

Rapid sexual and genomic isolation in sympatric Drosophila without reproductive character displacement

The rapid evolution of sexual isolation in sympatry has long been associated with reinforcement (i.e., selection to avoid maladaptive hybridization). However, there are many species pairs in sympatry that have evolved rapid sexual isolation without known costs to hybridization. A major unresolved question is what evolutionary processes are involved in driving rapid speciation in such cases. Here, we focus on one such system; the Drosophila athabasca species complex, which is composed of three partially sympatric and interfertile semispecies: WN, EA, and EB. To study speciation in this species complex, we assayed sexual and genomic isolation within and between these semispecies in both sympatric and allopatric populations. First, we found no evidence of reproductive character displacement (RCD) in sympatric zones compared to distant allopatry. Instead, semispecies were virtually completely sexually isolated from each other across their entire ranges. Moreover, using spatial approaches and coalescent demographic simulations, we detected either zero or only weak heterospecific gene flow in sympatry. In contrast, within each semispecies we found only random mating and little population genetic structure, except between highly geographically distant populations. Finally, we determined that speciation in this system is at least an order of magnitude older than previously assumed, with WN diverging first, around 200K years ago, and EA and EB diverging 100K years ago. In total, these results suggest that these semispecies should be given full species status and we adopt new nomenclature: WN—D. athabasca, EA—D. mahican, and EB—D. lenape. While the lack of RCD in sympatry and interfertility do not support reinforcement, we discuss what additional evidence is needed to further decipher the mechanisms that caused rapid speciation in this species complex.     

DNA sequence variation and latitudinal associations in hsp23, hsp26 and hsp27 from natural populations of Drosophila melanogaster

Heat shock genes are considered to be likely candidate genes for environmental stress resistance. Nucleotide variation in the coding sequence of the small heat shock genes (hsps) hsp26 and hsp27 from Drosophila melanogaster was studied in flies originating from the Netherlands and eastern Australia. The hsp26 gene was polymorphic for an insertion/deletion of three extra amino acids and two nonsynonymous changes in all populations. The hsp27 gene exhibited two nonsynonymous changes and three synonymous mutations. The hsp26 polymorphism showed a latitudinal cline along the east coast of Australia. This pattern was not confounded by the fact that the shsps are located in the inversion In(3 L)P which also shows a latitudinal cline in eastern Australia. A similar latitudinal cline was found for the previously described variation in hsp23, while frequencies of hsp27 alleles did not change with latitude. These findings suggest that variation at two of the shsps or closely linked loci are under selection in natural populations of D. melanogaster.     

Vertical distribution of haematophagous Diptera in temperate forests of the southeastern U.S.A.

The vertical distribution of blood‐feeding flies in two temperate forests in the southeastern U.S.A. was determined by placing 15 Centers for Disease Control and Prevention miniature light traps (12 CO₂‐baited, three unbaited controls), without lights, at three heights (1.5 m, 5.0 m, 10.0 m). More than 6550 haematophagous flies, representing 49 species in four families, were collected. Eighteen species were taken almost exclusively (90–100%) at 1.5 m or 10.0 m, and the mean number of flies per trap differed significantly with height for another six species. Five species exhibited shifts in vertical distribution between the two forests, indicating that forest structure could influence the height of host searching. Most (52.5%) mammalophilic flies were collected at 1.5 m, whereas most (56.4%) ornithophilic flies were taken at 10.0 m, suggesting that host associations influence vertical distributions. The significant differences in the composition of haematophagous fly populations among forest strata emphasize the importance of trap placement in vector surveillance and of understanding the ecological relationships of blood‐feeding flies.     

Enzyme polymorphism and species discrimination in fruit flies of the genus Dacus (Tephritidae).

Sympatric populations of D. tryoni and D. neohumeralis are difficult to completely distinguish taxonomically. Using five pigmentation characters, each of some taxonomic value, a small proportion of individuals cannot be assigned to either species nor definitely classified as hybrids. To aid in species discrimination and hybrid identification gene frequencies in natural populations were estimated at three polymorphic protein loci, an alcohol dehydrogenase (Adh), an octanol dehydrogenase (Odh) and an esterase (E-2). Samples of flies were taken from four sites spread over 1200 miles along the Australian eastern coast. Within each species allelic frequencies at each locus were largely the same at all localities. Consistent differences in gene frequencies between species occurred at all three loci, strongly supporting the hypothesis of two distinct gene pools. The Adh locus best discriminated between species with a unique allele occurring in D. neohumeralis at a frequency of 0-85. None of the loci showed complete differentiation and hence it was not possible to find a quick and easy method to distinguish the species nor to detect field hybrids. Directional selection of laboratory populations for a change in callus colour (the best pigmentation character for separating the species) indicated that at the Adh and E-2 loci frequencies of major alleles were not genetically associated with major genes for callus colour. Thus genotype determination at these loci when considered together with pigmentation characters may be valuable taxonomically for further distinguishing between the species.     

Mutations in the acetylcholinesterase gene of Bactrocera dorsalis associated with resistance to organophosphorus insecticides

Mutations in the gene encoding the enzyme acetylcholinesterase (AChE) of the oriental fruit fly, Bactrocera dorsalis, associated with resistance to an organophosphorus insecticide have been characterized. Three point mutations producing nonsynonymous changes in the predicted amino acid sequence of the product of the B. dorsalis ace gene in resistant vs. susceptible flies have been identified. One of these changes is unique to B. dorsalis while the other two occur at sites that are identical to mutations previously described for another Bactrocera species. Although the precise role of the third mutation is not clearly established, the independent origin of two identical alterations in these two species strongly supports the idea proposed previously that molecular changes associated with insecticide resistance in key genes and enzymes such as AChE are largely constrained to a limited number of sites. The results obtained here also suggest that the widespread use of organophosphorus insecticides will likely lead to a predictable acquisition of resistance in wild populations of B. dorsalis as well as other pest species. For surveys of B. dorsalis populations that may develop resistance, diagnostic tests using PCR-RFLP based methods for detecting the presence of all three mutations in individual flies are described.     

Boolean implication networks derived from large scale,whole genome microarray datasets

We describe a method for extracting Boolean implications (if-then relationships) in very large amounts of gene expression microarray data. A meta-analysis of data from thousands of microarrays for humans, mice, and fruit flies finds millions of implication relationships between genes that would be missed by other methods. These relationships capture gender differences, tissue differences, development, and differentiation. New relationships are discovered that are preserved across all three species.     

Seasonal variation of the capture and field distribution of sterile Anastrepha ludens in Tamaulipas,Mexico

Anastrepha ludens (Loew) (Diptera: Tephritidae) is one of the most important citrus pest in Mexico. The sterile insect technique (SIT) is used against pest populations of fruit flies for suppression, eradication, containment and prevention to reduce damages in fruit‐growing areas. In this study, we analyzed the seasonal variation of captures and field distribution of sterile A. ludens released in different seasons of the year in north‐eastern Mexico. Chilled releases were conducted by air at constant densities per ha on a citrus area for a period of 32 weeks that included the coldest and warmest seasons that is winter, spring and summer. Multilure traps baited with torula yeast pellets were used to capture sterile flies. Fly capture data were compared over the three seasons and correlated with climate. The lowest number of captures of the sterile insect occurred in the summer and the highest in winter and spring. High and low temperatures were negatively correlated with fly captures. Field distribution was also negatively correlated with high temperatures in summer, but no relationships were observed in winter and spring. No relationships were observed between rainfall with capture and field distribution of sterile flies. These results indicate that summer is a season involving agro‐ecological and environmental constraints for the capture and field distribution of sterile flies. This study may be useful for enhancing release strategies and optimizing economic resources in north‐eastern Mexico. Further research on the behaviour of sterile flies under stressful environments is suggested.     

Comparisons of the polypeptide chains of globins

Summary Vertebrate myoglobins and hemoglobins each consist of units containing a heme group and an associated polypeptide chain. The polypeptide chains form an homologous series and can be compared with each other to measure amino acid differences and minimum base differences per codon. These differences are the result of mutations which have been incorporated during divergent evolution from a common ancestral gene. Each such base replacement is termed an evolutionary event. Each amino acid replacement is the result of one or more evolutionary events. However there can be only one amino acid difference between two sites. The minimum mutational difference between two sites is perceptible only as one, two or three base differences per codon, but there may be more evolutionary events than base differences, because of revertants and multiplicity of base replacements at the same site. When all three bases in a codon are changed, the result is recognizable in only about five per cent of cases. Therefore such recognizable three-base changes indicate a large number of evolutionary events.     

Differentiation in morphology and habitat partitioning of genetically characterized Gammarus fossarum forms (Amphipoda) across a contact zone

Evolutionary processes do not affect all character types equally. Depending on environmental effects, morphological and ecological evolution may be uncoupled from molecular evolution. A natural contact zone between two forms of the amphipod Gammarus fossarum was used to assess the levels of differentiation in genetic, morphological and habitat characters. About 50 individuals per population were analysed for six enzyme loci in 72 populations and four size-standardized morphometric traits in a subset of 32 populations. Nine environmental parameters per population were used to characterize the habitat. All three character sets revealed significant divergences between G. fossarum forms. The eastern form A of G. fossarum differs from the western form B by longer setae at the second antennae (males) and longer exopodits at die third uropods (males and females). The variance partitioning within populations, among populations and between forms, was compared within the three character sets. The relative variance component between forms in genetic characters appeared to be 10-fold greater in comparison to morphological traits, indicating morphological stasis during speciation. The inter-form environmental differentiation was shown to be low, but consistent between pure populations. The eastern form A seems to prefer higher altitudes, a substrate type of gravel, stones and leaves, shading, and water with higher conductivity and pH man the western form B.     

Influence of three resistance sources in winter wheat derived from TAM 107 on yield response to Russian wheat aphid

A study to determine yield response to the Russian wheat aphid, Diuraphis noxia (Mordvilko), was conducted during the 1997-1998 and 1998-1999 growing seasons at three eastern Colorado locations, Akron, Fort Collins, and Lamar, with three wheat lines containing either Russian wheat aphid-resistant Dn4 gene, Dn6 gene, or resistance derived from PI 222668, and TAM 107 as the susceptible control. Russian wheat aphids per tiller were greater on TAM 107 than the resistant wheat lines at the 10x infestation level at Fort Collins and Akron in 1999. Yield, seed weight, and number of seeds per spike for each wheat line were somewhat affected by Russian wheat aphid per tiller mainly at Fort Collins. The infested resistant wheat lines harbored fewer Russian wheat aphids and yielded more than the infested susceptible wheat lines. Wheat lines containing the Dn4, Dn6, and PI 222668 genes contain different levels of antibiosis or antixenosis and tolerance. Although differences existed among sites and resistance, there is a benefit to planting resistant wheat when there is a potential for Russian wheat aphid infestations.     

Spatial diversity in mitochondrial cytochrome c oxidase in house flies

DNA sequence analysis at mitochondrial gene COI was surveyed in 293 house flies, Musca domestica Linneaus (Diptera: Muscidae), in 29 populations from North, Central and South America, Europe, Asia, Africa, and the Western Pacific. Nei's gene diversity index (H(S)) was 0.47, the chance that two randomly chosen flies have different COI haplotypes. Haplotype diversity was greater in the Old World (H(S) = 0.58) than the New World (H(S) = 0.31). The hierarchical partition of the total diversity indicated substantial differentiation at all levels (G(ST) = 0.30), and highly structured populations. All pairwise estimates of gene flow between zoogeographical regions were less than 0.70 reproducing females per generation. The results are compared to those of a similar study based on the single-strand conformation polymorphism method. Probable colonization scenarios for house flies into the New World are discussed and it is concluded that house flies are a recent addition to the fauna of the Western Hemisphere.     

Effectiveness of protein baits on melon fly and oriental fruit fly (Diptera: Tephritidae): attraction and feeding

Attraction and feeding responses of oriental fruit fly, Bactrocera dorsalis (Hendel), and melon fly, Bactrocera cucurbitae (Coquillett), were determined for different protein baits. In separate choice attraction assays for each species, significantly more flies arrived at stations with bait than water, but no differences existed among baits of GF-120 Fruit Fly Bait, GF-120 NF Naturalyte Fruit Fly Bait, Provesta 621 autolyzed yeast extract, and Mazoferm E802. In comparison with B. dorsalis, B. cucurbitae had 2.8 times more responders and a 4.8 times better discrimination between baits and water. In a second attraction assay with only B. dorsalis, volume of bait was negatively correlated to numbers of flies alighting on the bait. Feeding assays for both species demonstrated that time spent feeding and duration on a leaf were both significantly affected by bait type. B. dorsalis fed the longest on Provesta 621, with significantly less feeding on the other baits, and with all baits resulting in more feeding than water. The longest feeding times for B. cucurbitae resulted with Mazoferm E802 and Provesta 621, and all baits except GF-120 NF resulted in eliciting a significantly longer feeding duration than water. In separate toxicology assays for each species, significantly higher mortality resulted from bait formulations containing spinosad compared with blank baits, but no differences existed between GF-120 and GF-120 NF formulations. The differences are discussed between the two Bactrocera species primarily in regard to bait preference, extent of response, and previous work on laboratory flies.     

Polymorphic HSRs in chromosome 1 of the two semispecies Mus musculus musculus and M. m. domesticus have a common origin in an ancestral population

HSRs (homogeneously staining regions) are the cytological correlates of DNA amplification. In the house mouse, Mus musculus, many populations are polymorphic for the presence or absence of HSRs on chromosome 1. In the semispecies M. m. domesticus the amplified DNA is present within one HSR, whereas in M. m. musculus chromosomes 1 with two HSRs are found. Hybridization of HSR-specific probes to Southern blots of HSR-carrying genomic DNAs from different localities and semispecies revealed similar complex band patterns. the remaining variation is restricted to sequences with a low degree of amplification. Variation is higher between semispecies than within one semispecies. It is assumed that HSRs are derived from one original amplification event and that unequal recombination is the mechanism underlying the length variation of HSRs present today in both semispecies. Evidence from G-banding and in situ hybridization shows that the two HSRs of M. m. musculus originated from a single HSR by means of a paracentric inversion, where one break-point was located within the single HSR and the second outside the HSR. As a consequence of the paracentric inversion the two HSRs of M. m. musculus are permanently linked together. Since exchange of genes between the two semispecies is restricted to a narrow hybrid zone the amplification that gave rise to the HSR most probably occurred prior to the divergence into the semispecies M. m. domesticus and M. m. musculus about 1 million years ago.by D. Schweizer     

THE GEOGRAPHIC PATTERN OF GENETIC DIFFERENTIATION BETWEEN HOST ASSOCIATED POPULATIONS OF RHAGOLETIS POMONELLA (DIPTERA: TEPHRITIDAE) IN THE EASTERN UNITED STATES AND CANADA

True fruit flies in the Rhagoletis pomonella sibling species group are at the center of a long-standing debate concerning modes of speciation. The allopatric separation of populations is widely thought to be a prerequisite for speciation in sexually reproducing animals. However, speciation in the R. pomonella group appears to have occurred sympatrically as a consequence of these flies shifting and adapting to new host plants. The sympatric shift of R. pomonella from its native host hawthorn to introduced domestic apple, which occurred approximately 150 years ago, provides a test of whether host specialization is sufficient to allow populations to differentiate in the absence of geographic barriers to gene flow. We report the results of a geographic study of allozyme variation for hawthorn and apple infesting populations of R. pomonella across the eastern United States and Canada. Six loci consistently show significant allele frequency differences at paired apple and hawthorn sites. These six loci map to three different regions of the genome, and linkage disequilibrium exists between non-allelic genes within each of these regions. Allele frequencies for five of the six loci displaying host associated differences also co-vary significantly with latitude. Inter-host divergence is, therefore, superimposed on north-south clinal patterns of intra-host variation such that the magnitude of genetic divergence between hawthorn and apple flies is a function of latitude. The findings suggest that partially reproductively isolated “host races” can evolve in sympatry as a consequence of R. pomonella infesting new host plants. Host recognition and host associated developmental traits are discussed as important factors differentiating apple and hawthorn flies.     

Genetic delineation of sibling species of the pest fruit fly Bactocera (Diptera: Tephritidae) using microsatellites

Using a large set of microsatellites, the genetic relationships between three closely related Australian fruit fly species, Bactrocera tryoni (Froggatt), B. neohumeralis (Hardy) and B. aquilonis(May) were investigated. Bactrocera tryoni and B. neohumeralis are sympatric, while B. aquilonisis allopatric to both. The sympatric species, B. tryoni and B. neohumeralis, were found to be genetically distinct. It is likely that despite differences in mating time between these two species, some gene flow still occurs. In contrast, the sibling species B. tryoni and B. aquilonis were found to be closely related, despite allopatry. The level of genetic divergence was similar to that found within eastern Australian populations of B. tryoni. Consideration of all available genetic data suggests that this similarity is not due to recent (i.e. within the last 30 years) displacement of B. aquilonis by B. tryoni from the B. aquilonis region (north-western Australia). Instead the data suggests that, at least in the areas sampled, asymmetrical hybridization may have occurred over a longer timescale.     

Mitochondrial phylogeography of the moor frog, Rana arvalis

The moor frog Rana arvalis is a lowland species with a broad Eurasiatic distribution, from arctic tundra through forest to the steppe zone. Its present-day range suggests that glacial refugia of this frog were located outside southern European peninsulas. We studied the species-wide phylogeographical pattern using sequence variation in a 682 base pairs fragment of mtDNA cytochrome b gene; 223 individuals from 73 localities were analysed. Two main clades, A and B, differing by c. 3.6% sequence divergence were detected. The A clade is further subdivided into two subclades, AI and AII differing by 1.0%. All three lineages are present in the Carpathian Basin (CB), whereas the rest of the species range, including huge expanses of Eurasian lowlands, are inhabited solely by the AI lineage. We infer that AII and B lineages survived several glacial cycles in the CB but did not expand, at least in the present interglacial, to the north. The geographical distribution and genealogical relationships between haplotypes from the AI lineage indicate that this group had two glacial refugia, one located in the eastern part of the CB and the other probably in southern Russia. Populations from both refugia contributed to the colonization of the western part of the range, whereas the eastern part was colonized from the eastern refugium only. The effective population size as evidenced by theta(ML) is an order of magnitude higher in the AI lineage than in the AII and B lineages. Demographic expansion was detected in all three lineages.