The molecular evolution of cancer associated genes in mammals.

Cancer is a disease that many multicellular organisms have faced for millions of years, and species have evolved various tumour suppression mechanisms to control oncogenesis. Although cancer occurs across the tree of life, cancer related mortality risks vary across mammalian orders, with Carnivorans particularly affected. Evolutionary theory predicts different selection pressures on genes associated with cancer progression and suppression, including oncogenes, tumour suppressor genes and immune genes. Therefore, we investigated the evolutionary history of cancer associated gene sequences across 384 mammalian taxa, to detect signatures of selection across categories of oncogenes (GRB2, FGL2 and CDC42), tumour suppressors (LITAF, Casp8 and BRCA2) and immune genes (IL2, CD274 and B2M). This approach allowed us to conduct a fine scale analysis of gene wide and site-specific signatures of selection across mammalian lineages under the lens of cancer susceptibility. Phylogenetic analyses revealed that for most species the evolution of cancer associated genes follows the species' evolution. The gene wide selection analyses revealed oncogenes being the most conserved, tumour suppressor and immune genes having similar amounts of episodic diversifying selection. Despite BRCA2's status as a key caretaker gene, episodic diversifying selection was detected across mammals. The site-specific selection analyses revealed that the two apoptosis associated domains of the Casp8 gene of bats (Chiroptera) are under opposing forces of selection (positive and negative respectively), highlighting the importance of site-specific selection analyses to understand the evolution of highly complex gene families. Our results highlighted the need to critically assess different types of selection pressure on cancer associated genes when investigating evolutionary adaptations to cancer across the tree of life. This study provides an extensive assessment of cancer associated genes in mammals with highly representative, and substantially large sample size for a comparative genomic analysis in the field and identifies various avenues for future research into the mechanisms of cancer resistance and susceptibility in mammals.

Organisation and evolution of the major histocompatibility complex class I genes in cetaceans.

A quarter of marine mammals are at risk of extinction, with disease and poor habitat quality contributing to population decline. Investigation of the Major Histocompatibility Complex (MHC) provides insight into species' capacity to respond to immune and environmental challenges. The eighteen available cetacean chromosome level genomes were used to annotate MHC Class I loci, and to reconstruct the phylogenetic relationship of the described loci. The highest number of loci was observed in the striped dolphin (Stenella coeruleoalba), while the least was observed in the pygmy sperm whale (Kogia breviceps) and rough toothed dolphin (Steno bredanensis). Of the species studied, Mysticetes had the most pseudogenes. Evolutionarily, MHC Class I diverged before the speciation of cetaceans. Yet, locus one was genomically and phylogenetically similar in many species, persisting over evolutionary time. This characterisation of MHC Class I in cetaceans lays the groundwork for future population genetics and MHC expression studies.

Temporal specific coevolution of Hsp70 and co-chaperone stv expression in Drosophila melanogaster under selection for heat tolerance.

Heat shock proteins (Hsps) have long been candidates for ecological adaptation given their unequivocal role in mitigating cell damage from heat stress, but linking Hsps to heat tolerance has proven difficult given the complexity of thermal adaptation. Experimental evolution has been utilized to examine direct and correlated responses to selection for increased heat tolerance in Drosophila, often focusing on the major Hsp family Hsp70 and/or the master regulator HSF as a selection response, but rarely on other aspects of the heat shock complex. We examined Hsp70 and co-chaperone stv isoform transcript expression in Australian D. melanogaster lines selected for static heat tolerance, and observed a temporal and stv isoform specific, coordinated transcriptional selection response with Hsp70, suggesting that increased chaperone output accompanied increased heat tolerance. We hypothesize that the coordinated evolutionary response of Hsp70 and stv may have arisen as a correlated response resulting from a shared regulatory hierarchy. Our work highlights the complexity and specificity of the heat shock response in D. melanogaster. The selected lines examined also showed correlated responses for other measures of heat tolerance, and the coevolution of Hsp70 and stv provide new avenues to examine the common mechanisms underpinning direct and correlated phenotypic responses to selection for heat tolerance.

Enhancing Ebony? Common Associations With a cis-Regulatory Haplotype for Drosophila melanogaster Thoracic Pigmentation in a Japanese Population and Australian Populations.

The molecular underpinnings of pigmentation diversity in Drosophila have recently emerged as a model for understanding how the evolution of different cis-regulatory variants results in common adaptive phenotypes within species. We compared sequence variation in a 5' regulatory region harboring a modular enhancer containing a ∼0.7-kb core element contributing to abdominal melanisation in African, and a ∼0.5-kb core element contributing to thoracic pigmentation in D. melanogaster from Japan, to tropical and temperate populations from eastern Australia previously shown to be divergent in thoracic pigmentation and ebony expression. The Australian populations exhibited strong association with the core enhancer polymorphism cluster in complete association with Dark and Light phenotypes from Iriomote, Japan. Moreover, the Iriomote Light and Dark core enhancer haplotypes are common to the Australian populations in the direction predicted by pigmentation phenotype. We also confirmed the Japanese patterns of linkage disequilibrium and association of the tropical inversion In(3R)Payne with the Light enhancer haplotype in the Australian tropical light population. A worldwide survey of the ∼0.5-kb ebony control region SNPs and haplotypes in a subset of the Drosophila Genome Nexus (DGN) populations suggest origins in the sub-Saharan ancestral region surrounding Zambia and subsequent invasion following colonization out of Africa. A previous study demonstrated complex within and between population genetic architecture for abdominal pigmentation which is also correlated with thoracic pigmentation in melanized DGN sub-Saharan populations; however, the ∼0.5-kb ebony control region was not associated and both haplotypes are common even in the most intensely pigmented D. melanogaster from high altitude Ethiopia. In the Australian populations, the strong phenotypic association with the enhancer SNPs and haplotypes that at least partly regulates ebony expression in the Iriomote population, our previous work demonstrating opposing clines for thoracic pigmentation and ebony expression, where the expression cline parallels the In(3R)Payne cline, and the concerted evolution of pigmentation intensity and ebony expression under rapid experimental evolution, all point to a common adaptive evolutionary pathway in distinct populations.

Transcriptional profiles of plasticity for desiccation stress in Drosophila.

We examined the transcriptional responses of desiccation resistance candidate genes in populations of Drosophila melanogaster divergent for desiccation resistance and in capacity to improve resistance via phenotypic plasticity. Adult females from temperate and tropical eastern Australian populations were exposed to a rapid desiccation hardening (RDH) treatment, and groups without RDH to acute desiccation stress, and the transcript expression of 12 candidate desiccation genes were temporally profiled during, and in recovery from stress. We found that desiccation exposure resulted in largely transitory, stress-specific transcriptional changes in all but one gene. However linking the expression profiles to the population-level phenotypic divergence was difficult given subtle, and time-point specific population expression variation. Nonetheless, rapid desiccation hardening had the largest effect on gene expression, resulting in distinct molecular profiles. We report a hitherto uncharacterised desiccation molecular hardening response where prior exposure essentially 'primes' genes to respond to subsequent stress without discernible transcript changes prior to stress. This, taken together with some population gene expression variation of several bona fide desiccation candidates associated with different water balance strategies speaks of the complexity of natural desiccation resistance and plasticity and provides new avenues for understanding the molecular basis of a trait of ecological significance.

Signatures of polygenic adaptation associated with climate across the range of a threatened fish species with high genetic connectivity.

Adaptive differences across species' ranges can have important implications for population persistence and conservation management decisions. Despite advances in genomic technologies, detecting adaptive variation in natural populations remains challenging. Key challenges in gene-environment association studies involve distinguishing the effects of drift from those of selection and identifying subtle signatures of polygenic adaptation. We used paired-end restriction site-associated DNA sequencing data (6,605 biallelic single nucleotide polymorphisms; SNPs) to examine population structure and test for signatures of adaptation across the geographic range of an iconic Australian endemic freshwater fish species, the Murray cod Maccullochella peelii. Two univariate gene-association methods identified 61 genomic regions associated with climate variation. We also tested for subtle signatures of polygenic adaptation using a multivariate method (redundancy analysis; RDA). The RDA analysis suggested that climate (temperature- and precipitation-related variables) and geography had similar magnitudes of effect in shaping the distribution of SNP genotypes across the sampled range of Murray cod. Although there was poor agreement among the candidate SNPs identified by the univariate methods, the top 5% of SNPs contributing to significant RDA axes included 67% of the SNPs identified by univariate methods. We discuss the potential implications of our findings for the management of Murray cod and other species generally, particularly in relation to informing conservation actions such as translocations to improve evolutionary resilience of natural populations. Our results highlight the value of using a combination of different approaches, including polygenic methods, when testing for signatures of adaptation in landscape genomic studies.

Cross-Study Comparison Reveals Common Genomic, Network, and Functional Signatures of Desiccation Resistance in Drosophila melanogaster.

Repeated attempts to map the genomic basis of complex traits often yield different outcomes because of the influence of genetic background, gene-by-environment interactions, and/or statistical limitations. However, where repeatability is low at the level of individual genes, overlap often occurs in gene ontology categories, genetic pathways, and interaction networks. Here we report on the genomic overlap for natural desiccation resistance from a Pool-genome-wide association study experiment and a selection experiment in flies collected from the same region in southeastern Australia in different years. We identified over 600 single nucleotide polymorphisms associated with desiccation resistance in flies derived from almost 1,000 wild-caught genotypes, a similar number of loci to that observed in our previous genomic study of selected lines, demonstrating the genetic complexity of this ecologically important trait. By harnessing the power of cross-study comparison, we narrowed the candidates from almost 400 genes in each study to a core set of 45 genes, enriched for stimulus, stress, and defense responses. In addition to gene-level overlap, there was higher order congruence at the network and functional levels, suggesting genetic redundancy in key stress sensing, stress response, immunity, signaling, and gene expression pathways. We also identified variants linked to different molecular aspects of desiccation physiology previously verified from functional experiments. Our approach provides insight into the genomic basis of a complex and ecologically important trait and predicts candidate genetic pathways to explore in multiple genetic backgrounds and related species within a functional framework.

Spatial analysis of gene regulation reveals new insights into the molecular basis of upper thermal limits.

The cellular stress response has long been the primary model for studying the molecular basis of thermal adaptation, yet the link between gene expression, RNA metabolism and physiological responses to thermal stress remains largely unexplored. We address this by comparing the transcriptional and physiological responses of three geographically distinct populations of Drosophila melanogaster from eastern Australia in response to, and recovery from, a severe heat stress with and without a prestress hardening treatment. We focus on starvin (stv), recently identified as an important thermally responsive gene. Intriguingly, stv encodes seven transcripts from alternative transcription sites and alternative splicing, yet appears to be rapidly heat inducible. First, we show genetic differences in upper thermal limits of the populations tested. We then demonstrate that the stv locus does not ubiquitously respond to thermal stress but is expressed as three distinct thermal and temporal RNA phenotypes (isoforms). The shorter transcript isoforms are rapidly upregulated under stress in all populations and show similar molecular signatures to heat-shock proteins. Multiple stress exposures seem to generate a reserve of pre-mRNAs, effectively 'priming' the cells for subsequent stress. Remarkably, we demonstrate a bypass in the splicing blockade in these isoforms, suggesting an essential role for these transcripts under heat stress. Temporal profiles for the weakly heat responsive stv isoform subset show opposing patterns in the two most divergent populations. Innate and induced transcriptome responses to hyperthermia are complex, and warrant moving beyond gene-level analyses.

Using genomics to characterize evolutionary potential for conservation of wild populations.

Genomics promises exciting advances towards the important conservation goal of maximizing evolutionary potential, notwithstanding associated challenges. Here, we explore some of the complexity of adaptation genetics and discuss the strengths and limitations of genomics as a tool for characterizing evolutionary potential in the context of conservation management. Many traits are polygenic and can be strongly influenced by minor differences in regulatory networks and by epigenetic variation not visible in DNA sequence. Much of this critical complexity is difficult to detect using methods commonly used to identify adaptive variation, and this needs appropriate consideration when planning genomic screens, and when basing management decisions on genomic data. When the genomic basis of adaptation and future threats are well understood, it may be appropriate to focus management on particular adaptive traits. For more typical conservations scenarios, we argue that screening genome-wide variation should be a sensible approach that may provide a generalized measure of evolutionary potential that accounts for the contributions of small-effect loci and cryptic variation and is robust to uncertainty about future change and required adaptive response(s). The best conservation outcomes should be achieved when genomic estimates of evolutionary potential are used within an adaptive management framework.

New levels of transcriptome complexity at upper thermal limits in wild Drosophila revealed by exon expression analysis.

While the cellular heat-shock response has been a paradigm for studying the impact of thermal stress on RNA metabolism and gene expression, the genome-wide response to thermal stress and its connection to physiological stress resistance remain largely unexplored. Here, we address this issue using an array-based exon expression analysis to interrogate the transcriptome in recently established Drosophila melanogaster stocks during severe thermal stress and recovery. We first demonstrated the efficacy of exon-level analyses to reveal a level of thermally induced transcriptome complexity extending well beyond gene-level analyses. Next, we showed that the upper range of both the cellular and physiological thermal stress response profoundly affected message expression and processing in D. melanogaster, limiting expression to a small subset of transcripts, many that share features of known rapidly responding stress genes. As predicted from cellular heat-shock research, constitutive splicing was blocked in a set of novel genes; we did not detect changes to alternative splicing during heat stress, but rather induction of intronless isoforms of known heat-responsive genes. We observed transcriptome plasticity in the form of differential isoform expression during recovery from heat shock, mediated by multiple mechanisms including alternative transcription and alternative splicing. This affected genes involved in DNA regulation, immune response, and thermotolerance. These patterns highlight the complex nature of innate transcriptome responses under stress and potential for adaptive shifts through plasticity and evolved genetic responses at different hierarchical levels.

High resolution mapping of candidate alleles for desiccation resistance in Drosophila melanogaster under selection.

The ability to counter periods of low humidity is an important determinant of distribution range in Drosophila. Climate specialists with low physiological tolerance to desiccation stress are restricted to the tropics and may lack the ability to further increase resistance through evolution. Although the physiological adaptations to desiccation stress are well studied in Drosophila and other ectotherms, factors underlying evolutionary responses remain unknown because of a paucity of genetic data. We address this issue by mapping evolutionary shifts in D. melanogaster under selection for desiccation resistance. Genomic DNA from five independent replicate selected, and control lines were hybridized to high density Affymetrix Drosophila tiling arrays resulting in the detection of 691 single feature polymorphisms (SFPs) differing between the treatments. While randomly distributed throughout the genome, the SFPs formed specific clusters according to gene ontology. These included genes involved in ion transport and respiratory system development that provide candidates for evolutionary changes involving excretory and respiratory water balance. Changes to genes related to neuronal control of cell signaling, development, and gene regulation provide candidates to explore novel biological processes in stress resistance. Sequencing revealed the nucleotide shifts in a subset of the SFPs and highlighted larger regions of genomic diversity surrounding SFPs. The association between natural desiccation resistance and a 463-bp region of the 5' promoter region of the Dys gene undergoing allele frequency changes in response to selection in the experimental evolution lines was tested in an independent population from Coffs Harbour, Australia. The allele frequencies of 23 SNPs common to the two populations were inferred from the parents of the 10% most and 10% least resistant Coffs Harbour flies. The frequencies of the selected alleles were higher at all sites, with three sites significantly associated with the resistant Coffs Harbour flies. This study illustrates how rapid mapping can be used for discovering natural molecular variants associated with survival to low humidity and provides a wealth of candidate alleles to explore the genetic basis of physiological differences among resistant and susceptible Drosophila populations and species.

The molecular genetics of clinal variation: a case study of ebony and thoracic trident pigmentation in Drosophila melanogaster from eastern Australia.

Widespread pigmentation diversity coupled with a well-defined genetic system of melanin synthesis and patterning in Drosophila provides an excellent opportunity to study phenotypes undergoing evolutionary change. Pigmentation variation is highly correlated with different ecological variables and is thought to reflect adaptations to different environments. Several studies have linked candidate genes from Drosophila melanogaster to intra-population variation and interspecific morphological divergence, but less clearly to variation among populations forming pigmentation clines. We characterized a new thoracic trident pigmentation cline in D. melanogaster populations from eastern Australia, and applied a candidate gene approach to explain the majority of the geographically structured phenotypic variation. More melanized populations from higher latitudes tended to express less ebony than their tropical counterparts, and an independent artificial selection experiment confirmed this association. By partitioning temperature dependent effects, we showed that the genetic differences underlying clinal patterns for trident variation at 25 °C do not explain the patterns observed at 16 °C. Changes in thoracic trident pigmentation could be a common evolutionary response to climatically mediated environmental pressures. On the Australian east coast most of the changes appear to be associated with regulatory divergence of the ebony gene but this depends on temperature.

Expression of defense genes in Drosophila evolves under a different selective regime from expression of other genes.

Genes involved in host-pathogen interactions are expected to be evolving under complex coevolutionary dynamics, including positive directional and/or frequency-dependent selection. Empirical work has largely focused on the evolution of immune genes at the level of the protein sequence. We examine components of genetic variance for transcript abundance of defense genes in Drosophila melanogaster and D. simulans using a diallel and a round robin breeding design, respectively, and infer modes of evolution from patterns of segregating genetic variation. Defense genes in D. melanogaster are overrepresented relative to nondefense genes among genes with evidence of significant additive variance for expression. Directional selection is expected to deplete additive genetic variance, whereas frequency-dependent selection is expected to maintain additive variance. However, relaxed selection (reduced or no purifying selection) is an alternative interpretation of significant additive variation. Of the three classes of defense genes, the recognition and effector classes show an excess of genes with significant additive variance; whereas signaling genes, in contrast, are overrepresented for dominance variance. Analysis of protein-coding sequences revealed no evidence for an association between additive or dominance variation in expression and directional selection. Both balancing selection driven by host-pathogen coevolution and relaxed selection for expression of uninduced defense genes are viable interpretations of these data.

SOLiD sequencing of four Vibrio vulnificus genomes enables comparative genomic analysis and identification of candidate clade-specific virulence genes.

BACKGROUND: Vibrio vulnificus is the leading cause of reported death from consumption of seafood in the United States. Despite several decades of research on molecular pathogenesis, much remains to be learned about the mechanisms of virulence of this opportunistic bacterial pathogen. The two complete and annotated genomic DNA sequences of V. vulnificus belong to strains of clade 2, which is the predominant clade among clinical strains. Clade 2 strains generally possess higher virulence potential in animal models of disease compared with clade 1, which predominates among environmental strains. SOLiD sequencing of four V. vulnificus strains representing different clades (1 and 2) and biotypes (1 and 2) was used for comparative genomic analysis. RESULTS: Greater than 4,100,000 bases were sequenced of each strain, yielding approximately 100-fold coverage for each of the four genomes. Although the read lengths of SOLiD genomic sequencing were only 35 nt, we were able to make significant conclusions about the unique and shared sequences among the genomes, including identification of single nucleotide polymorphisms. Comparative analysis of the newly sequenced genomes to the existing reference genomes enabled the identification of 3,459 core V. vulnificus genes shared among all six strains and 80 clade 2-specific genes. We identified 523,161 SNPs among the six genomes. CONCLUSIONS: We were able to glean much information about the genomic content of each strain using next generation sequencing. Flp pili, GGDEF proteins, and genomic island XII were identified as possible virulence factors because of their presence in virulent sequenced strains. Genomic comparisons also point toward the involvement of sialic acid catabolism in pathogenesis.

Sex-specific splicing in Drosophila: widespread occurrence, tissue specificity and evolutionary conservation.

Many genes in eukaryotic genomes produce multiple transcripts through a variety of molecular mechanisms including alternative splicing. Alternatively spliced transcripts often encode functionally distinct proteins, indicating that gene regulation at this level makes an important contribution to organismal complexity. The multilevel splicing cascade that regulates sex determination and sex-specific development in Drosophila is a classical example of the role of alternative splicing in cell differentiation. Recent evidence suggests that a large proportion of genes in the Drosophila genome may be spliced in a sex-biased fashion, raising the possibility that alternative splicing may play a more general role in sexually dimorphic development and physiology. However, the prevalence of sex-specific splicing and the extent to which it is shared among genotypes are not fully understood. Genetic variation in the splicing of key components of the sex determination pathway is known to influence the expression of downstream target genes, suggesting that alternative splicing at other loci may also vary in functionally important ways. In this study, we used exon-specific microarrays to examine 417 multitranscript genes for evidence of sex-specific and genotype-specific splicing in 80 different genotypes of Drosophila melanogaster. Most of these loci showed sex-biased splicing, whereas genotype-specific splicing was rare. One hundred thirty-five genes showed different alternative transcript use in males vs. females. Real-time PCR analysis of 6 genes chosen to represent a broad range of biological functions showed that most sex-biased splicing occurs in the gonads. However, somatic tissues, particularly adult heads, also show evidence of sex-specific splicing. Comparison of splicing patterns at orthologous loci in seven Drosophila species shows that sexual biases in alternative exon representation are highly conserved, indicating that sex-specific splicing is an ancient feature of Drosophila biology. To investigate potential mechanisms of sex-biased splicing, we used real-time PCR to examine the expression of six known regulators of alternative splicing in males vs. females. We found that all six loci are themselves spliced sex specifically in gonads and heads, suggesting that regulatory hierarchies based on alternative splicing may be an important feature of sexual differentiation.

Simpler mode of inheritance of transcriptional variation in male Drosophila melanogaster.

Sexual selection drives faster evolution in males. The X chromosome is potentially an important target for sexual selection, because hemizygosity in males permits accumulation of alleles, causing tradeoffs in fitness between sexes. Hemizygosity of the X could cause fundamentally different modes of inheritance between the sexes, with more additive variation in males and more nonadditive variation in females. Indeed, we find that genetic variation for the transcriptome is primarily additive in males but nonadditive in females. As expected, these differences are more pronounced on the X chromosome than the autosomes, but autosomal loci are also affected, possibly because of X-linked transcription factors. These differences may be of evolutionary significance because additive variation responds quickly to selection, whereas nonadditive genetic variation does not. Thus, hemizygosity of the X may underlie much of the faster male evolution of the transcriptome and potentially other phenotypes. Consistent with this prediction, genes that are additive in males and nonadditive in females are overrepresented among genes responding to selection for increased mating speed.

Sex-specific expression of alternative transcripts in Drosophila.

BACKGROUND: Many genes produce multiple transcripts due to alternative splicing or utilization of alternative transcription initiation/termination sites. This 'transcriptome expansion' is thought to increase phenotypic complexity by allowing a single locus to produce several functionally distinct proteins. However, sex, genetic and developmental variation in the representation of alternative transcripts has never been examined systematically. Here, we describe a genome-wide analysis of sex-specific expression of alternative transcripts in Drosophila melanogaster. RESULTS: We compared transcript profiles in males and females from eight Drosophila lines (OregonR and 2b, and 6 RIL) using a newly designed 60-mer oligonucleotide microarray that allows us to distinguish a large proportion of alternative transcripts. The new microarray incorporates 7,207 oligonucleotides, satisfying stringent binding and specificity criteria that target both the common and the unique regions of 2,768 multi-transcript genes, as well as 12,912 oligonucleotides that target genes with a single known transcript. We estimate that up to 22% of genes that produce multiple transcripts show a sex-specific bias in the representation of alternative transcripts. Sexual dimorphism in overall transcript abundance was evident for 53% of genes. The X chromosome contains a significantly higher proportion of genes with female-biased transcription than the autosomes. However, genes on the X chromosome are no more likely to have a sexual bias in alternative transcript representation than autosomal genes. CONCLUSION: Widespread sex-specific expression of alternative transcripts in Drosophila suggests that a new level of sexual dimorphism at the molecular level exists.

A new set of laboratory-selected Drosophila melanogaster lines for the analysis of desiccation resistance: response to selection, physiology and correlated responses.

Artificial selection experiments provide insights into the evolutionary factors that can shape adaptive responses and have previously been utilized to examine the physiological adaptations that can improve survival to desiccation in Drosophila melanogaster. While such studies demonstrate that multiple resistance mechanisms may arise via different base populations and selection regimes, water retention emerges as a key mechanism for desiccation survival. Here, we present the physiological, correlated response and life history data for a new set of selection lines designed for the genetic dissection of desiccation resistance. After 26 generations of selection for desiccation resistance, female survival increased twofold. In contrast to previous studies, the altered resistance was associated primarily with enhanced dehydration tolerance and increased mass and less consistently with decreased rates of water loss. Life history tradeoffs and correlated selection responses were examined and overlap with previously published data. We crossed the resistant selected lines to desiccation-sensitive lines from the same control background to examine how each heterozygous resistant chromosome (excluding four) may improve desiccation resistance and observed that most of the resistance was due to genes on the third and first chromosomes, although interaction effects with the second chromosome were also detected. Results are compared to other selection responses and highlight the multiple evolutionary solutions that can arise when organisms are faced with a common selection pressure, although water loss rate remains a common mechanism in all studies.

Isolation of a Drosophila melanogaster desiccation resistant mutant.

Mutagenesis provides a powerful way of isolating genetic and physiological processes underlying complex traits, but this approach has rarely been applied to investigating water balance in insects. Here, we describe the isolation of a desiccation-resistant mutant of Drosophila melanogaster. Mutagenesis of a desiccation sensitive line resulted in the isolation of a mutant with two-fold higher resistance. The mutant was partially dominant and mapped to the second chromosome. Mutant flies showed lower rates of water loss, and had a higher water content, but showed no change in body mass, glycogen content, hemolymph volume or water content tolerated at death from desiccation. These physiological differences are contrasted to changes in lines of D. melanogaster mass selected for altered stress resistance. Isolation of this mutant provides an opportunity to identify a gene involved in water balance in insects.