Impact of tRNA-induced proline-to-serine mistranslation on the transcriptome of Drosophila melanogaster.

Mistranslation is the misincorporation of an amino acid into a polypeptide. Mistranslation has diverse effects on multicellular eukaryotes and is implicated in several human diseases. In Drosophila melanogaster, a serine transfer RNA (tRNA) that misincorporates serine at proline codons (P→S) affects male and female flies differently. The mechanisms behind this discrepancy are currently unknown. Here, we compare the transcriptional response of male and female flies to P→S mistranslation to identify genes and cellular processes that underlie sex-specific differences. Both males and females downregulate genes associated with various metabolic processes in response to P→S mistranslation. Males downregulate genes associated with extracellular matrix organization and response to negative stimuli such as wounding, whereas females downregulate aerobic respiration and ATP synthesis genes. Both sexes upregulate genes associated with gametogenesis, but females also upregulate cell cycle and DNA repair genes. These observed differences in the transcriptional response of male and female flies to P→S mistranslation have important implications for the sex-specific impact of mistranslation on disease and tRNA therapeutics.

Mistranslating tRNA variants have anticodon- and sex-specific impacts on Drosophila melanogaster.

Transfer RNAs (tRNAs) are vital in determining the specificity of translation. Mutations in tRNA genes can result in the misincorporation of amino acids into nascent polypeptides in a process known as mistranslation. Since mistranslation has different impacts, depending on the type of amino acid substitution, our goal here was to compare the impact of different mistranslating tRNASer variants on fly development, lifespan, and behaviour. We established two mistranslating fly lines, one with a tRNASer variant that misincorporates serine at valine codons (V→S) and the other that misincorporates serine at threonine codons (T→S). While both mistranslating tRNAs increased development time and developmental lethality, the severity of the impacts differed depending on amino acid substitution and sex. The V→S variant extended embryonic, larval, and pupal development whereas the T→S only extended larval and pupal development. Females, but not males, containing either mistranslating tRNA presented with significantly more anatomical deformities than controls. Mistranslating females also experienced extended lifespan whereas mistranslating male lifespan was unaffected. In addition, mistranslating flies from both sexes showed improved locomotion as they aged, suggesting delayed neurodegeneration. Therefore, although mistranslation causes detrimental effects, we demonstrate that mistranslation also has positive effects on complex traits such as lifespan and locomotion. This has important implications for human health given the prevalence of tRNA variants in humans.

Identification and genetic analysis of a pervasive 'needle-eye' sperm phenotype in Drosophila sterile hybrid males.

Interspecies hybrid sterility has been extensively studied, especially in the genus Drosophila. Hybrid sterility is more often found in the heterogametic (XY or ZW) sex, a trend called Haldane's rule. Although this phenomenon is pervasive, identification of a common genetic mechanism remains elusive, with modest support found for a range of potential theories. Here, we identify a single precise morphological phenotype, which we call 'needle-eye sperm', that is associated with hybrid sterility in three separate species pairs that span the Drosophila genus. The nature of the phenotype indicates a common point of meiotic failure in sterile hybrid males. We used 10 generations of backcross selection paired with whole-genome pooled sequencing to genetically map the regions underlying the needle-eye (NE) sperm phenotype. Surprisingly, the sterility phenotype was present in ~50% of males even after 10 generations of backcrossing, and only a single region of the X chromosome was associated with sterility in one direction of backcross. Owing to the common phenotype among sterile male hybrids, and the strong effect of individual loci, further exploration of these findings may identify a universal mechanism for the evolution of hybrid sterility.

Impact of tRNA-induced proline-to-serine mistranslation on the transcriptome of Drosophila melanogaster.

Mistranslation is the misincorporation of an amino acid into a polypeptide. Mistranslation has diverse effects on multicellular eukaryotes and is implicated in several human diseases. In Drosophila melanogaster, a serine transfer RNA (tRNA) that misincorporates serine at proline codons (P→S) affects male and female flies differently. The mechanisms behind this discrepancy are currently unknown. Here, we compare the transcriptional response of male and female flies to P→S mistranslation to identify genes and cellular processes that underlie sex-specific differences. Both males and females downregulate genes associated with various metabolic processes in response to P→S mistranslation. Males downregulate genes associated with extracellular matrix organization and response to negative stimuli such as wounding, whereas females downregulate aerobic respiration and ATP synthesis genes. Both sexes upregulate genes associated with gametogenesis, but females also upregulate cell cycle and DNA repair genes. These observed differences in the transcriptional response of male and female flies to P→S mistranslation have important implications for the sex-specific impact of mistranslation on disease and tRNA therapeutics.

A Test of Haldane's Rule in Neodiprion Sawflies and Implications for the Evolution of Postzygotic Isolation in Haplodiploids.

AbstractHaldane's rule-a pattern in which hybrid sterility or inviability is observed in the heterogametic sex of an interspecific cross-is one of the most widely obeyed rules in nature. Because inheritance patterns are similar for sex chromosomes and haplodiploid genomes, Haldane's rule may apply to haplodiploid taxa, predicting that haploid male hybrids will evolve sterility or inviability before diploid female hybrids. However, there are several genetic and evolutionary mechanisms that may reduce the tendency of haplodiploids to obey Haldane's rule. Currently, there are insufficient data from haplodiploids to determine how frequently they adhere to Haldane's rule. To help fill this gap, we crossed a pair of haplodiploid hymenopteran species (Neodiprion lecontei and Neodiprion pinetum) and evaluated the viability and fertility of female and male hybrids. Despite considerable divergence, we found no evidence of reduced fertility in hybrids of either sex, consistent with the hypothesis that hybrid sterility evolves slowly in haplodiploids. For viability, we found a pattern opposite to that of Haldane's rule: hybrid females, but not males, had reduced viability. This reduction was most pronounced in one direction of the cross, possibly due to a cytoplasmic-nuclear incompatibility. We also found evidence of extrinsic postzygotic isolation in hybrids of both sexes, raising the possibility that this form or reproductive isolation tends to emerge early in speciation in host-specialized insects. Our work emphasizes the need for more studies on reproductive isolation in haplodiploids, which are abundant in nature but underrepresented in the speciation literature.

A novel mistranslating tRNA model in Drosophila melanogaster has diverse, sexually dimorphic effects.

Transfer RNAs (tRNAs) are the adaptor molecules required for reading the genetic code and producing proteins. Transfer RNA variants can lead to genome-wide mistranslation, the misincorporation of amino acids not specified by the standard genetic code into nascent proteins. While genome sequencing has identified putative mistranslating transfer RNA variants in human populations, little is known regarding how mistranslation affects multicellular organisms. Here, we create a multicellular model of mistranslation by integrating a serine transfer RNA variant that mistranslates serine for proline (tRNAUGG,G26ASer) into the Drosophila melanogaster genome. We confirm mistranslation via mass spectrometry and find that tRNAUGG,G26ASer misincorporates serine for proline at a frequency of ∼0.6% per codon. tRNAUGG,G26ASer extends development time and decreases the number of flies that reach adulthood. While both sexes of adult flies containing tRNAUGG,G26ASer present with morphological deformities and poor climbing performance, these effects are more pronounced in female flies and the impact on climbing performance is exacerbated by age. This model will enable studies into the synergistic effects of mistranslating transfer RNA variants and disease-causing alleles.

Inheritance of pheromone profiles from aged D. melanogaster.

Through aging, D. melanogaster males and females change their social spacing. Flies are initially more social, but reduce sociability as they grow older. This preferred social space is inherited in their progeny. Here, we report that in females, the profiles of cuticular hydrocarbons (CHC), which are known to promote social interaction between individuals, similarly are affected by age. Importantly, for a subset of those CHC, the progeny's CHC levels are comparable to those of their parents, suggesting that parental age influences offspring CHC expression. Those data establish a foundation to identify the relationship between CHC levels and social spacing, and to understand the mechanisms of the inheritance of complex traits.

Intraspecific Genetic Variation for Behavioral Isolation Loci in Drosophila.

Behavioral isolation is considered to be the primary mode of species isolation, and the lack of identification of individual genes for behavioral isolation has hindered our ability to address fundamental questions about the process of speciation. One of the major questions that remains about behavioral isolation is whether the genetic basis of isolation between species also varies within a species. Indeed, the extent to which genes for isolation may vary across a population is rarely explored. Here, we bypass the problem of individual gene identification by addressing this question using a quantitative genetic comparison. Using strains from eight different populations of Drosophila simulans, we genetically mapped the genomic regions contributing to behavioral isolation from their closely related sibling species, Drosophila mauritiana. We found extensive variation in the size of contribution of different genomic regions to behavioral isolation among the different strains, in the location of regions contributing to isolation, and in the ability to redetect loci when retesting the same strain.

Design and characterization of a 3D-printed axon-mimetic phantom for diffusion MRI.

PURPOSE: To introduce and characterize inexpensive and easily produced 3D-printed axon-mimetic diffusion MRI phantoms in terms of pore geometry and diffusion kurtosis imaging metrics. METHODS: Phantoms were 3D-printed with a composite printing material that, after the dissolution of the polyvinyl alcohol, exhibits microscopic fibrous pores. Confocal microscopy and synchrotron phase-contrast micro-CT imaging were performed to visualize and assess the pore sizes. Diffusion MRI scans of four identical phantoms and phantoms with varying print parameters in water were performed at 9.4 T. Diffusion kurtosis imaging was fit to both data sets and used to assess the reproducibility between phantoms and effects of print parameters on diffusion kurtosis imaging metrics. Identical scans were performed 25 and 76 days later, to test their stability. RESULTS: Segmentation of pores in three microscopy images yielded a mean, median, and SD of equivalent pore diameters of 7.57 µm, 3.51 µm, and 12.13 µm, respectively. Phantoms had T1 /T2 = 2 seconds/180 ms, and those with identical parameters showed a low coefficient of variation (~10%) in mean diffusivity (1.38 × 10-3 mm2 /s) and kurtosis (0.52) metrics and radial diffusivity (1.01 × 10-3 mm2 /s) and kurtosis (1.13) metrics. Printing temperature and speed had a small effect on diffusion kurtosis imaging metrics (< 16%), whereas infill density had a larger and more variable effect (> 16%). The stability analysis showed small changes over 2.5 months (< 7%). CONCLUSION: Three-dimension-printed axon-mimetic phantoms can mimic the fibrous structure of axon bundles on a microscopic scale, serving as complex, anisotropic diffusion MRI phantoms.

Genes underlying species differences in cuticular hydrocarbon production between Drosophila melanogaster and D. simulans.

Surface chemical compounds are key components of survival and reproduction in many species. Cuticular hydrocarbons (CHCs) are chemical compounds produced by all insects that are used for both desiccation resistance and chemical communication, including communication related to mating. In the species pair of Drosophila melanogaster and D. simulans, female CHCs stimulate conspecific males to mate and repel heterospecific males. While CHCs are a critical contributor to both reproductive success within a species and isolation between species, few genes underlying species variation in CHC profiles are known. Here, we use genetic mapping of the 3rd chromosome to test a suite of candidate genes for interspecies variation in CHCs. Candidate gene CG5946 was found to be involved in species differences in the production of 7,11-heptacosadiene and 7-tricosene between D. melanogaster and D. simulans. This is therefore a new candidate locus contributing to species-specific variation in the CHC profile. In the process of mapping genes for CHCs, we also identified 29 candidate genes for the reduced survival or inviability of interspecies hybrids.

Reproductive consequences of an extra long-term sperm storage organ.

BACKGROUND: Sperm storage plays a key role in the reproductive success of many sexually-reproducing organisms, and the capacity of long-term sperm storage varies across species. While there are theoretical explanations for why such variation exists, to date there are no controlled empirical tests of the reproductive consequences of additional long-term sperm storage. While Dipterans ancestrally have three long-term sperm organs, known as the spermathecae, Drosophila contain only two. RESULTS: We identified a candidate gene, which we call spermathreecae (sp3), in which a disruption cause the development of three functional spermathecae rather than the usual two in Drosophila. We used this disruption to test the reproductive consequences of having an additional long-term sperm storage organ. Compared to females with two spermathecae, females with three spermathecae store a greater total number of sperm and can produce offspring a greater length of time. However, they did not produce a greater total number of offspring. CONCLUSIONS: Thus, additional long-term sperm storage in insects may increase female fitness through extending the range of conditions where she produces offspring, or through increasing the quality of offspring via enhanced local sperm competition at fertilization.

The fruitless gene affects female receptivity and species isolation.

Female mate rejection acts as a major selective force within species, and can serve as a reproductive barrier between species. In spite of its critical role in fitness and reproduction, surprisingly little is known about the genetic or neural basis of variation in female mate choice. Here, we identify fruitless as a gene affecting female receptivity within Drosophila melanogaster, as well as female Drosophila simulans rejection of male D. melanogaster. Of the multiple transcripts this gene produces, by far the most widely studied is the sex-specifically spliced transcript involved in the sex determination pathway. However, we find that female rejection behaviour is affected by a non-sex-specifically spliced fruitless transcript. This is the first implication of fruitless in female behaviour, and the first behavioural role identified for a fruitless non-sex-specifically spliced transcript. We found that this locus does not influence preferences via a single sensory modality, examining courtship song, antennal pheromone perception, or perception of substrate vibrations, and we conclude that fruitless influences mate choice via the integration of multiple signals or through another sensory modality.

A common suite of cellular abnormalities and spermatogenetic errors in sterile hybrid males in Drosophila.

When two species interbreed, the resulting hybrid offspring are often sterile, with the heterogametic (e.g. XY) hybrid usually being more severely affected. The prevailing theory for this pattern of sterility evokes divergent changes in separate lineages having maladaptive interactions when placed together in a hybrid individual, with recessive factors on the sex chromosome interacting with dominant factors on the autosomes. The effect of these interactions on gametogenesis should not be uniform across species pairs unless genetic divergence follows the same paths in different lineages or if a specific stage of gametogenesis is more susceptible to detrimental genetic interactions. Here, we perform a detailed cellular characterization of hybrid male sterility across three recently diverged species pairs of Drosophila. Across all three pairs, sterile hybrid sperm are alive but exhibit rapid nuclear de-condensation with age, with active, but non-differentiated, mitochondria. Surprisingly, all three sets of interspecies hybrids produce half of the number of sperm per round of spermatogenesis, with each sperm cell containing two tails. We identify non-disjunction failures during meiosis I as the likely cause. Thus, errors during meiosis I may be a general phenomenon underlying Drosophila male sterility, indicating either a heightened sensitivity of this spermatogenic stage to failure, or a basis to sterility other than the prevailing model.

Social behavior and aging: A fly model.

The field of behavioral genetics has recently begun to explore the effect of age on social behaviors. Such studies are particularly important, as certain neuropsychiatric disorders with abnormal social interactions, like autism and schizophrenia, have been linked to older parents. Appropriate social interaction can also have a positive impact on longevity, and is associated with successful aging in humans. Currently, there are few genetic models for understanding the effect of aging on social behavior and its potential transgenerational inheritance. The fly is emerging as a powerful model for identifying the basic molecular mechanisms underlying neurological and neuropsychiatric disorders. In this review, we discuss these recent advancements, with a focus on how studies in Drosophila melanogaster have provided insight into the effect of aging on aspects of social behavior, including across generations.

Local thermal adaptation detected during multiple life stages across populations of Drosophila melanogaster.

Thermal adaptation is typically detected by examining the tolerance of a few populations to extreme temperatures within a single life stage. However, the extent to which adaptation occurs among many different populations might depend on the tolerance of multiple life stages and the average temperature range that the population experiences. Here, we examined local adaptation to native temperature conditions in eleven populations of the well-known cosmopolitan fruit fly, Drosophila melanogaster. These populations were sampled from across the global range of D. melanogaster. We measured traits related to fitness during each life stage to determine whether certain stages are more sensitive to changes in temperature than others. D. melanogaster appeared to show local adaptation to native temperatures during the egg, larval and adult life stages, but not the pupal stage. This suggests that across the entire distribution of D. melanogaster, certain life stages might be locally adapted to native temperatures, whereas other stages might use phenotypic plasticity or tolerance to a wide range of temperatures experienced in the native environment of this species.

Veiled preferences and cryptic female choice could underlie the origin of novel sexual traits.

Males in many species have elaborated sexual traits that females strongly prefer, and these traits often conspicuously differ among species. How novel preferences and traits originate, however, is a challenging evolutionary problem because the initial appearance of only the female preference or only the male trait should reduce the ability to find a suitable mate, which could reduce fitness for individuals possessing those novel alleles. Here, we present a hypothesis for how novel preferences, as well as the novel male traits that females prefer, can originate, be favoured and spread in polyandrous species. Novel preference mutations can arise as 'veiled preferences' that are not expressed when the corresponding male trait is not present in the population, allowing preferences to be hidden from selection, and thus persist. In those cases when a male trait is present, veiled preferences provide a selective advantage, and females disproportionately produce offspring from preferred males through either mate choice or cryptic female choice. This tips the fitness advantage for novel males, allowing both preference and trait to spread, and limiting selection against them in the absence of the corresponding trait or preference.

Males from populations with higher competitive mating success produce sons with lower fitness.

Female mate choice can result in direct benefits to the female or indirect benefits through her offspring. Females can increase their fitness by mating with males whose genes encode increased survivorship and reproductive output. Alternatively, male investment in enhanced mating success may come at the cost of reduced investment in offspring fitness. Here, we measure male mating success in a mating arena that allows for male-male, male-female and female-female interactions in Drosophila melanogaster. We then use isofemale line population measurements to correlate male mating success with sperm competitive ability, the number of offspring produced and the indirect benefits of the number of offspring produced by daughters and sons. We find that males from populations that gain more copulations do not increase female fitness through increased offspring production, nor do these males fare better in sperm competition. Instead, we find that these populations have a reduced reproductive output of sons, indicating a potential reproductive trade-off between male mating success and offspring quality.

Identification of a novel sperm class and its role in fertilization in Drosophila.

In many species, males have evolved to produce a sterile sperm (parasperm) in conjunction with fertilizing sperm (eusperm). Here, we document evidence of males depositing two morphologically distinct types of parasperm (1 and 2) into the female reproductive tract in Drosophila pseudoobscura. These parasperm differ in length, shape, amount produced, amount in long-term storage and may have separate roles in ensuring male fertilization success. Although both parasperm types protect eusperm from female spermicides, only parasperm 2, which has a corkscrew shape, is associated with sperm competition. Increased production of parasperm 2 is also negatively correlated with the eusperm and parasperm 1 production. Thus, selection may be acting on parasperm production in the presence of sperm competition. Our findings show how both sperm competition and cryptic female choice may be acting in conjunction to influence the evolution of ejaculate composition. Our identification and characterization of two distinct parasperm morphs will enhance the ability for further evaluation of parasperm's role in fertilization.