Variation in Y Chromosome Segregation in Natural Populations of Drosophila melanogaster

Functional variation among Y chromosomes in natural populations of Drosophila melanogaster was assayed by a segregation study. A total of 36 Y chromosomes was extracted and ten generations of replacement backcrossing yielded stocks with Y chromosomes in two different genetic backgrounds. Eleven of the Y chromosomes were from diverse geographic origins, and the remaining 25 were from locally captured flies. Segregation of sexes in adult offspring was scored for the four possible crosses among the two backgrounds with each Y chromosome. Although the design confounds meiotic drive and effects on viability, statistical partitioning of these effects reveals significant variation among lines in Y chromosome segregation. Results are discussed in regards to models of Y-linked segregation and viability effects, which suggest that Y-linked adaptive polymorphism is unlikely.     

The Genetic Basis of Resistance to Diazinon in Natural Populations of Drosophila melanogaster

Isofemale strains of Drosophila melanogaster were established from single inseminated females collected from populations along the east coast of Australia. Strains were tested for resistance to the organophosphorus insecticide diazinon at larval and/or adult stages of the life cycle. Considerable phenotypic variation was observed within and between population samples but there was no association between collection site of a sample and resistance status. Adult and larval resistance levels were uncorrelated. Resistance levels in adults were low (2-fold) and polygenically based. Larval resistance levels, due to single genes (or gene complexes) on chromosomes II and III, were significant (15-fold). Evidence indicates that the gene on chromosome II is Cyp6g1.     

Quantitative Trait Loci for Lipid Content in Drosophila melanogaster

Recombinant inbred lines derived from a natural population were used to investigate natural genetic variation for lipid abundance, protein abundance, and weight of Drosophila melanogaster. Females were heavier and contained more lipid and soluble protein than males. Lipid and protein abundance were genetically correlated with female weight, but male weight was not correlated with lipid or protein. Lipid and protein abundance were genetically correlated in males, but not in females. Quantitative trait loci (QTLs) for weight and protein abundance were predominantly on the X chromosome, whereas QTLs for lipid abundance were found on the second and third chromosomes. QTLs for lipid proportion (lipid abundance normalized by weight or protein abundance) were present on all chromosomes; a lipid proportion QTL on the third chromosome correlated with a QTL for starvation resistance observed in a previous study using the same set of recombinant inbred lines, suggesting that it might underlie both traits. Candidate genes are discussed in relationship to lipid abundance, lipid proportion, and starvation resistance.     

Metallothionein Gene Duplications and Metal Tolerance in Natural Populations of Drosophila melanogaster

A search for duplications of the Drosophila melanogaster metallothionein gene (Mtn) yielded numerous examples of this type of chromosomal rearrangement. These duplications are distributed widely--we found them in samples from four continents, and they are functional--larvae carrying Mtn duplications produce more Mtn RNA and tolerate increased cadmium and copper concentrations. Six different duplication types were characterized by restriction-enzyme analyses using probes from the Mtn region. The restriction maps show that in four cases the sequences, ranging in size between 2.2 and 6.0 kb, are arranged as direct, tandem repeats; in two other cases, this basic pattern is modified by the insertion of a putative transposable element into one of the repeated units. Duplications of the D. melanogaster metallothionein gene such as those that we found in natural populations may represent early stages in the evolution of a gene family.     

Tirant Stealthily Invaded Natural Drosophila melanogaster Populations during the Last Century

It was long thought that solely three different transposable elements (TEs)—the I-element, the P-element, and hobo—invaded natural Drosophila melanogaster populations within the last century. By sequencing the “living fossils” of Drosophila research, that is, D. melanogaster strains sampled from natural populations at different time points, we show that a fourth TE, Tirant, invaded D. melanogaster populations during the past century. Tirant likely spread in D. melanogaster populations around 1938, followed by the I-element, hobo, and, lastly, the P-element. In addition to the recent insertions of the canonical Tirant, D. melanogaster strains harbor degraded Tirant sequences in the heterochromatin which are likely due to an ancient invasion, likely predating the split of D. melanogaster and D. simulans. These degraded insertions produce distinct piRNAs that were unable to prevent the novel Tirant invasion. In contrast to the I-element, P-element, and hobo, we did not find that Tirant induces any hybrid dysgenesis symptoms. This absence of apparent phenotypic effects may explain the late discovery of the Tirant invasion. Recent Tirant insertions were found in all investigated natural populations. Populations from Tasmania carry distinct Tirant sequences, likely due to a founder effect. By investigating the TE composition of natural populations and strains sampled at different time points, insertion site polymorphisms, piRNAs, and phenotypic effects, we provide a comprehensive study of a natural TE invasion.     

Natural Variation in Odorant Recognition Among Odorant-Binding Proteins in Drosophila melanogaster

Chemical recognition is essential for survival and reproduction. Adaptive evolution has resulted in diverse chemoreceptor families, in which polymorphisms contribute to individual variation in chemosensation. To gain insights into the genetic determinants of individual variation in odorant recognition, we measured olfactory responses to two structurally similar odorants in a population of wild-derived inbred lines of Drosophila melanogaster. Odorant-binding proteins (OBPs) are the first components of the insect olfactory system to encounter odorants. Previously four single-nucleotide polymorphisms (SNPs) in the Obp99 group were associated with variation in olfactory responses to benzaldehyde. Here, we identify six different SNPs that are associated with variation in responses to a structurally similar odorant, acetophenone, in the same Obp genes. Five SNPs are in coding regions of Obp99b and Obp99d and one SNP is in the 3′-untranslated region of Obp99a (A610G). We found that the 610G allele is associated with higher response scores to acetophenone than the 610A allele, but with lower expression of Obp99a, suggesting that binding of acetophenone to Opb99a might limit rather than facilitate access to odorant receptors. Our results show that overlapping sets of OBPs contribute to odorant recognition for structurally similar odorants, but that different SNPs are associated with odorant-specific individual variation. Thus, dual olfactory recognition where OBPs regulate odorant access to receptors may enhance olfactory discrimination.ADAPTIVE evolution in diverse chemical environments has resulted in large multigene chemoreceptor families, including odorant-binding protein (Obp) genes, odorant receptor (Or) genes, and gustatory receptor (Gr) genes (Hekmat-Scafe et al. 2002; Robertson et al. 2003; Nozawa and Nei 2007; Nei et al. 2008; Su et al. 2009). Polymorphisms in these chemoreceptor genes contribute to individual variation in chemosensory behavior (Keller et al. 2007; Wang et al. 2007). At the same time, combinatorial recognition of odorants may contribute functional redundancy, which allows individual variation without compromising overall olfactory ability. This may be the reason why segregating null alleles of chemoreceptor genes can be maintained within a population (Takahashi and Takano-Shimizu 2005; Wang et al. 2007). Drosophila melanogaster presents a favorable model for investigating the genetic basis of individual variation in olfactory discrimination, because the genome can be manipulated readily. Furthermore, flies can be inbred, which enables repeated behavioral measurements on identical genotypes under controlled environmental conditions. In addition, both the olfactory and the gustatory systems of Drosophila have been well characterized (Su et al. 2009; Yarmolinsky et al. 2009). Convergent projections of olfactory neurons expressing distinct odorant receptors have been mapped to specific glomeruli in the antennal lobe (Gao et al. 2000; Vosshall et al. 2000), and detailed electrophysiological studies on transgenic flies have identified molecular response profiles of a large fraction of the odorant receptor repertoire (de Bruyne et al. 2001). Surprisingly, however, behavioral responses to odorants do not necessarily conform to predictions based on electrophysiological response profiles (Keller and Vosshall 2007).Whereas Drosophila odorant receptors have been studied extensively, less is known about the function of odorant-binding proteins (OBPs) in mediating odor recognition and olfactory discrimination. OBPs are secreted by support cells in olfactory sensilla into the aqueous perilymph that surrounds olfactory dendrites and are thought to facilitate solubilization and transport of hydrophobic odorants, thereby either promoting or limiting access of odorants to odorant receptors (Steinbrecht 1998). For example, the pheromone-binding protein of the silk moth, Bombyx mori, binds and releases bombykol in a pH-dependent manner at the membrane interface (Wojtasek and Leal 1999; Sakurai et al. 2004). In D. melanogaster, an OBP, Lush, is essential for activation of the Or67d receptor by the pheromone cis-vaccenyl acetate in trichoid sensilla of the Drosophila third antennal segment (Ha and Smith 2006; Kurtovic et al. 2007). Binding of the pheromone causes a conformational transition in Lush, which enables this OBP to activate the Or67d receptor (Laughlin et al. 2008). Lush also interacts with short chain alcohols (Kim et al. 1998), but recognition of alcohols by Lush does not involve a conformation change and, thus, proceeds via a different mechanism (Stower and Logan 2008).Polymorphisms in Obp genes can serve as a substrate for natural selection and contribute to speciation. A polymorphism in Obp57e is responsible for differences in host plant preference between D. sechellia and D. melanogaster. D. melanogaster flies lacking the Obp57e and Obp57d genes were no longer repelled by hexanoic and octanoic acid, toxins produced by Morinda citrifolia, the host plant for D. sechellia. Here, inactivation of an Obp gene has enabled D. sechellia to occupy a specialist evolutionary niche (Matsuo et al. 2007). Differences in expression levels between Ors and Obps between D. sechellia and D. simulans have also been reported and postulated to contribute to the evolution of host plant preferences (Kopp et al. 2008).Despite the demonstrated importance of OBPs in pheromone and host plant recognition, little is known about how naturally occurring allelic variation in Obp genes affects individual variation in olfactory behavior. Previously, we identified polymorphisms associated with natural variation in olfactory behavior in response to benzaldehyde in Obp99a, Obp99c, and Obp99d in a population of wild-derived inbred lines of D. melanogaster (Wang et al. 2007). These studies indicated that these OBPs are likely to recognize benzaldehyde in a combinatorial manner, similar to odorant recognition by mammalian odorant receptors (Malnic et al. 1999). This observation enables us to begin to explore OBP odorant response profiles using a population genetics approach that capitalizes on naturally occurring mutations that affect behavior. As a first step, we asked whether variation in responses to odorants that are chemically similar would be associated with the same or overlapping sets of OBPs and, if so, whether the same or different polymorphisms in these OBPs would contribute to individual variation for olfactory behavior in response to these odorants. We focused on genes of the Obp99 group, previously associated with phenotypic variation in response to benzaldehyde. We obtained complete sequences of these genes from 297 inbred lines from the same wild-derived inbred population of D. melanogaster and measured variation in olfactory behavior in response to acetophenone, which is structurally similar to benzaldehyde. These odorants occur in fruits from host plants on which flies from the Raleigh population feed (e.g., apples and peaches). We find that overlapping sets of OBPs contribute to recognition of these two odorants, but that different SNPs are associated with odorant-specific individual variation.     

A Comprehensive Study of Genic Variation in Natural Populations of Drosophila melanogaster. III. Variations in Genetic Structure and Their Causes between Drosophila melanogaster and Its Sibling Species Drosophila simulans

The natural populations of Drosophila melanogaster and Drosophila simulans were compared for their genetic structure. A total of 114 gene-protein loci were studied in four mainland (from Europe and Africa) and an island (Seychelle) populations of D. simulans and the results were compared with those obtained on the same set of homologous loci in fifteen worldwide populations of D. melanogaster. The main results are as follows: (1) D. melanogaster shows a significantly higher proportion of loci polymorphic than D. simulans (52% vs. 39%, P<0.05), (2) both species have similar mean heterozygosity and mean number of alleles per locus, (3) the two species share some highly polymorphic loci but they do not share loci that show high geographic differentiation, and (4) D. simulans shows significantly less geographic differentiation than D. melanogaster. The differences in genetic differentiation between the two species are limited to loci located on the X and second chromosomes only; loci on the third chromosome show similar level of geographic differentiation in both species. These two species have previously been shown to differ in their pattern of variation for chromosomal polymorphisms, quantitative and physiological characters, two-dimensional electrophoretic (2DE) proteins, middle repetitive DNA and mitochondrial DNA. Variation in niche-widths and/or genetic "strategies" of adaptation appear to be the main causes of differences in the genetic structure of these two species.     

Low Temperature Reveals Genetic Variability Against Male-Killing Spiroplasma in Drosophila melanogaster Natural Populations

Spiroplasma endosymbionts are maternally inherited microorganisms which infect many arthropod species. In some Drosophila species, it acts as a reproductive manipulator, spreading in populations by killing the sons of infected mothers. Distinct Drosophila melanogaster populations from Brazil exhibit variable male-killing Spiroplasma prevalences. In this study, we investigated the presence of variability for the male-killing phenotype among Drosophila and/or Spiroplasma strains and verified if it correlates with the endosymbiont prevalence in natural populations. For that, we analyzed the male-killing expression when Spiroplasma strains from different populations were transferred to a standard D. melanogaster line (Canton-S) and when a common Spiroplasma strain was transferred to different wild-caught D. melanogaster lines, both at optimal and challenging temperatures for the bacteria. No variation was observed in the male-killing phenotype induced by different Spiroplasma strains. No phenotypic variability among fly lines was detected at optimal temperature (23 °C), as well. Conversely, significant variation in the male-killing expression was revealed among D. melanogaster lines at 18.5 °C, probably caused by imperfect transmission of the endosymbiont. Distinct lines differed in their average sex ratios as well as in the pattern of male-killing expression as the infected females aged. Greater variation occurred among lines from one locality, although there was no clear correlation between the male-killing intensity and the endosymbiont prevalence in each population. Imperfect transmission or male killing may also occur in the field, thus helping to explain the low or intermediate prevalences reported in nature. We discuss the implications of our results for the dynamics of male-killing Spiroplasma in natural populations.     

Dynamics of Cytoplasmic Incompatibility and Mtdna Variation in Natural Drosophila Simulans Populations

In Drosophila simulans a cytoplasmically transmitted microorganism causes reduced egg hatch when infected males mate with uninfected females. The infection is rapidly spreading northward in California. Data on a specific mtDNA restriction site length polymorphism show that changes in the frequency of mtDNA variants are associated with this spread. All infected flies possess the same mtDNA allele, whereas the uninfected flies are polymorphic. Given that both paternal inheritance of the infection and imperfect maternal transmission have been demonstrated, one might expect instead that both infected and uninfected flies would possess both mtDNA variants. Our data suggest that imperfect female transmission of the infection (and/or the loss of the infection among progeny) is more common in nature than paternal transmission. A simple model of intrapopulation dynamics, with empirically supported parameter values, adequately describes the joint frequencies of the mtDNA variants and incompatibility types.     

Odorant Receptor Polymorphisms and Natural Variation in Olfactory Behavior in Drosophila melanogaster

Animals perceive and discriminate among a vast array of sensory cues in their environment. Both genetic and environmental factors contribute to individual variation in behavioral responses to these cues. Here, we asked to what extent sequence variants in six Drosophila melanogaster odorant receptor (Or) genes are associated with variation in behavioral responses to benzaldehyde by sequencing alleles from a natural population. Sequence analyses showed signatures of deviations from neutrality for Or42b and Or85f, and linkage disequilibrium analyses showed a history of extensive recombination between polymorphic markers for all six Or genes. We identified polymorphisms in Or10a, Or43a, and Or67b that were significantly associated with variation in response to benzaldehyde. To verify these associations, we repeated the analyses with an independent set of behavioral measurements of responses to a structurally similar odorant, acetophenone. Association profiles for both odorants were similar with many polymorphisms and haplotypes associated with variation in responsiveness to both odorants. Some polymorphisms, however, were associated with one, but not the other odorant. We also observed a correspondence between behavioral response to benzaldehyde and differences in Or10a and Or43a expression. These results illustrate that sequence variants that arise during the evolution of odorant receptor genes can contribute to individual variation in olfactory behavior and give rise to subtle shifts in olfactory perception.RESEARCHERS in many scientific fields have long appreciated that different animal species perceive the world differently. In fact, these differences are so striking that new disciplines have arisen to study the adaptations of sense organs to the environment (e.g., Ali 1978; Lythgoe 1979; Dusenbery 1992). Differences in sensory perception exist not only between species, but also between populations of a single species and between individuals within a population. What is the underlying genetic architecture for individual variation in sensory perception?Olfaction provides an excellent model for examining the underlying genetic mechanisms that result in variation in behavior. In both vertebrates and invertebrates, odorants are detected by families of odorant receptors expressed in populations of olfactory receptor neurons (ORNs), whose activation elicits a distinct spatial pattern of glomerular activity in the brain (Buck and Axel 1991; Vassar et al. 1994; Mombaerts et al. 1996; Laissue et al. 1999; Gao et al. 2000; Vosshall et al. 2000; Bhalerao et al. 2003; Wang et al. 2003). This combinatorial code allows for discrimination of a diverse repertoire of odorants.Drosophila melanogaster has a relatively simple olfactory system with only 60 odorant receptor (Or) genes (Vosshall and Stocker 2007) compared to ∼1000 in the mouse (Zhang and Firestein 2002; Zhang et al. 2004). The 60 genes are located throughout the genome, and 2 of these genes are alternatively spliced for a total of 62 identified proteins (Clyne et al. 1999; Gao and Chess 1999; Vosshall et al. 1999; Robertson et al. 2003). Furthermore, clusters of Ors throughout the genome suggest several recent gene duplication events (Robertson et al. 2003).The response spectra of individual ORNs have been extensively characterized using extracellular electrophysiological recordings from single sensilla on the antennae and maxillary palps. Recordings from basiconic sensilla on the antenna identified classes of neurons with distinct olfactory response profiles organized as two to four neurons in each sensillum with specific neuronal combinations occurring in distinct spatial regions of the antenna (de Bruyne et al. 1999, 2001).The majority of ORNs express a unique odorant receptor in addition to the highly conserved coreceptor, Or83b (Jones et al. 2005). Studies of a null mutant of Or83b implicated this receptor in positioning odorant receptor proteins in the sensory dendrites (Larsson et al. 2004; Benton et al. 2006). Odorant receptors in Drosophila have an atypical membrane topology with a cytoplasmic N terminus and an extracellular C terminus (Benton et al. 2006). Specific domains in the third cytoplasmic loops of two odorant receptors, Or22a and Or43a, have been implicated to interact with the third loop of Or83b (Benton et al. 2006). Drosophila odorant receptors act as ligand-gated nonselective cation channels formed by a dimeric complex between a unique Or and the Or83b coreceptor (Sato et al. 2008; Wicher et al. 2008).Several studies have examined ligand specificities of individual odorant receptor proteins and demonstrated that they respond to diverse and overlapping suites of ligands. Response profiles for many receptors have been characterized using the Gal4/UAS system to drive expression of individual odorant receptors in a mutant ORN lacking expression of its endogeneous receptor, followed by electrophysiological recording (Dobritsa et al. 2003; Hallem et al. 2004; Hallem and Carlson 2006). In addition, misexpression studies of Or43a resulted in a reduction of behavioral avoidance responses to benzaldehyde (Stortkuhl et al. 2005). This result combined with electrophysiological recordings from ORNs and heterologous expression in Xenopus oocytes further functionally characterized the odorant response profiles of this receptor (Wetzel et al. 2001) and identified several Or43a ligands, such as fruit- derived odorants benzaldehyde, cyclohexanone, cyclohexanol, and benzyl alcohol (Stortkuhl and Kettler 2001; Hallem et al. 2004).Despite advances in our understanding of odor coding, the molecular mechanisms responsible for variation in olfactory perception remain poorly understood. D. melanogaster is especially amenable to conducting such studies given its quantitatively simple olfactory system and since large numbers of genetically identical individuals can be reared in a common environment and these individuals can be subjected to simple, rapid, and highly reproducible quantitative behavioral assays Anholt and Mackay 2004). Here, we examine how molecular variation in odorant receptors contributes to variation in olfactory behavior in inbred lines derived from a natural population of D. melanogaster. We focused our analyses on six odorant receptors, Or7a, Or10a, Or42b, Or43a, Or67b, and Or85f, which have been shown by electrophysiology (Stortkuhl and Kettler 2001; Hallem et al. 2004; Stortkuhl et al. 2005; Hallem and Carlson 2006), through heterologous expression systems (Wetzel et al. 2001), or by calcium imaging studies (Wang et al. 2003) to respond to benzaldehyde. Significant variation in behavioral responses to benzaldehyde has been observed previously in this population and was normally distributed as is typical for a quantitative trait influenced by multiple genes (Wang et al. 2007). Here, we report associations between olfactory behavior and sequence variants in three Or genes. To validate the reliability of these associations we measured responses to a structurally similar odorant, acetophenone, in the same population, and showed that the associations with variation in responses to both odorants are largely similar with occasional molecular polymorphisms associated with variation in response to only one, but not the other odorant. These observations illustrate how sequence variants that arise during the evolution of Or genes can contribute to individual variation in olfactory behavior, how polymorphisms can give rise to subtle shifts in olfactory perception, and how naturally arising mutations within a population can combine to generate broad individual variation in sensory perception.     

Geographical Distribution and Diversity of Bacteria Associated with Natural Populations of Drosophila melanogaster

Drosophila melanogaster is one of the most widely used model systems in biology. However, little is known about its associated bacterial community. As a first step towards understanding these communities, we compared bacterial 16S rRNA gene sequence libraries recovered from 11 natural populations of adult D. melanogaster. Bacteria from these sequence libraries were grouped into 74 distinct taxa, spanning the phyla Proteobacteria, Bacteroidetes, and Firmicutes, which were unevenly spread across host populations. Summed across populations, the distribution of abundance of genera was closely fit by a power law. We observed differences among host population locations both in bacterial community richness and in composition. Despite this significant spatial variation, no relationship was observed between species richness and a variety of abiotic factors, such as temperature and latitude. Overall, bacterial communities associated with adult D. melanogaster hosts are diverse and differ across host populations.     

Heritability of Two Morphological Characters within and among Natural Populations of Drosophila melanogaster

Heritabilities of wing length and abdominal bristle number, as well as genetic correlations between these characters, were determined within and among populations of Drosophila melanogaster in nature. Substantial "natural" heritabilities were found when wild-caught flies from one population were compared to their laboratory-reared offspring. Natural heritabilities of bristle number approximated those derived from laboratory-raised parents and offspring, but wing length heritability was significantly lower in nature than in the laboratory. Among-population heritabilities, estimated by regressing population means of wild-caught flies against those of their laboratory-reared descendants, were close to 0.5. The genetic differentiation of populations was clinal with latitude, and was accompanied by significant geographic differences in the norms of reaction to temperature. These clines are similar to those reported on other continents and in other Drosophila species, and are almost certainly caused by natural selection. Genetic regressions between the characters reveal that the cline in bristle number may be a correlated response to geographic selection on wing length, but not vice versa. Our results indicate that there is a sizable genetic component to phenotypic variation within and among populations of D. melanogaster in nature.     

A Comprehensive Study of Genic Variation in Natural Populations of Drosophila melanogaster. I. Estimates of Gene Flow from Rare Alleles

In order to assess the evolutionary significance of molecular variation in natural populations of Drosophila melanogaster, we have started a comprehensive genetic variation study program employing a relatively large number of gene-protein loci and an array of populations obtained from various geographic locations throughout the world. In this first report we provide estimates of gene flow based on the spatial distributions of rare alleles at 117 gene loci in 15 worldwide populations of D. melanogaster . Estimates of Nm (number of migrants exchanged per generation among populations) range from 1.09 in East-Asian populations (Taiwan, Vietnam and Australia) to 2.66 in West-Coast populations of North America. These estimates, among geographic populations separated by hundreds or even thousands of miles, suggest that gene flow among neighboring populations of D. melanogaster is quite extensive. This means that, for selectively neutral genes, we should expect little differentiation among neighboring populations. A survey of eight West-Coast populations of D. melanogaster (geographically comparable to Drosophila pseudoobscura) showed that in spite of extensive gene flow, populations of D. melanogaster show much more geographic differentiation than comparable populations of D. pseudoobscura. From this we conclude that migration in combination with natural selection rather than migration alone is responsible for the geographic uniformity of molecular polymorphisms in D. pseudoobscura.     

A Comprehensive Study of Genic Variation in Natural Populations of Drosophila melanogaster. II. Estimates of Heterozygosity and Patterns of Geographic Differentiation

A study of genic variation in natural population of D. melanogaster was undertaken (1) to obtain a better estimate of heterozygosity by sampling a relatively large number of gene loci and (2) to identify different groups of polymorphic loci whose variation patterns might suggest different kinds of selection forces. A total of 117 gene loci (coding for 79 enzymes and 38 abundant proteins) were studied in 15 geographically distant populations originating from different continents. The findings of this study are as follows: (1) of the 117 gene loci studied, 61 are polymorphic and 56 are uniformly monomorphic everywhere. (2) An average population is polymorphic for 43% of its gene loci and an average individual is heterozygous for 10% of its gene loci. These estimates are remarkably similar among populations. (3) The average within-locality heterozygosity (H(S)) for polymorphic loci is uniformly distributed over the range of heterozygosity observed; i.e. , given that a locus has any local variation, it is nearly as likely to have a lot as a little. (4) The distribution of F(ST) (fixation index) is strongly skewed, with a prominent mode at 8-10% and a long tail of high values reaching a maximum of 58%. Two-thirds of all loci fall within the bell-shaped distribution centered on an F(ST) of 8-10%, a result compatible with the notion that they are experiencing a common tendency toward small interlocality differences owing to extensive gene flow among populations. (5) The distribution of total heterozygosity (H(T)) has a prominent bimodal distribution. The lower mode consists of loci with single prominent allele and a few uncommon ones and the upper mode consists of clinally varying loci with a high F(ST ) (e.g., Adh and G6-pd), loci with many alleles in high frequency (e.g., Ao and Xdh) and loci with two alleles in high frequency in all populations but, with little interpopulational differentiation (e.g., Est-6 and alpha-Fuc). The loci in the lower mode are probably under purifying selection; a large proportion of those in the latter mode may be under balancing selection. (6) Comparison of genic variation for loci located inside vs. outside inversions, comparison of F(ST) for inversions and their associated genes, and comparison of F(ST) and map position for pairs of loci all suggest that, while linkage has some influence, it does not seem to constrain the pattern of variation that a locus may develop. (7) Eighteen polymorphic loci show latitudinal variation in allele frequencies which are consistent in populations from different continents. (8) Estimates of Nei genetic distance between population pairs are generally low between populations on the same continent and high between populations on different continents. There are two important exceptions: population pairs for which both localities are in the temperate zone show no relationship to distance, and in cases where both populations are tropical or subtropical, the genetic distance is higher than for the temperate-tropical comparisons and seem even higher than one would expect from the geographic distance separating them. The latter observation suggests that either geographic separation outweighs differences in environment in determining the genetic composition of a population or that all tropical populations are not experiencing the same environment.-The results are discussed in relation to the neutralist-selectionist controversy of genic variation and two important conclusions are drawn: First, there is a negative correlation between the number of loci sampled and the resulting heterozygosity. This means that available estimates of heterozygosity, 85% of which are based on 30 or fewer loci, are high and hence not appropriate for making between-taxa comparisons. Secondly, there is a group of loci, comprising one-third of polymorphic loci (or about 15% of all loci studied), that is distinguishable by different patterns of variation within and among populations. Most of these loci have clinal variation which is consistent with the hypothesis that their genetic variation is maintained by balancing selection.     

Transmission of Herpetomonas in Laboratory Populations of Drosophila melanogaster

Methods of transmission and the effects of temperature and mites on ageledeme development of Herpetomonas were examined in populations of Drosophila melanogaster maintained in the laboratory. Herpetomonas was observed in feces of infected adults taken from population cages and in the vomitus of clean flies shortly after feeding on a saline suspension of flagellates. Free-swimming flagellates were found in the moist areas of food cups. Adult D. melanogaster became infected when they fed on flagellates taken from the endoperitrophic space, the ectoperitrophic space or the Malpighian tubules. At 25°C the flagellates infected approximately 90% of the host population within 20 days. The high transmission rate was prematurely disrupted if host populations were subjected to changes in temperature. Free-swimming flagellates did not appear to be affected at these temperature changes. Food mites (Tyrophagus) established in the growth media of the fly nearly eliminated the Herpetomonas from Drosophila populations.     

Latitudinal variation in wild populations of Drosophila melanogaster: heritabilities and reaction norms

Large amounts of genetic variation for wing length and wing area were demonstrated both within and between Drosophila melanogaster populations along a latitudinal gradient in South America. Wing length and wing area showed a strong positive correlation with latitude in both wild flies and laboratory-raised descendants. Large population differences were observed for heritability and coefficient of variation of these two traits, whereas relatively small population differences were found for development time, viability, pupal mortality, sex ratio and their norms of reaction to four developmental temperatures. No clear-cut latitudinal clines were established for these life-history characters. These results are discussed in the light of Bergmann's Rule and the relation between larval development and adult body size.