Spermatogenesis Drives Rapid Gene Creation and Masculinization of the X Chromosome in Stalk-Eyed Flies (Diopsidae)

X chromosome drive is constrained by sexual selection and influences ornament evolution

Experimental evolution provides an integrative method for revealing complex interactions among evolutionary processes. One such interaction involves sex-linked selfish genetic elements and sexual selection. X-linked segregation distorters, a type of selfish genetic element, influence sperm transmission to increase in frequency and consequently alter the population sex ratio and the opportunity for sexual selection, while sexual selection may impact the spread of X-linked distorters. Here we manipulated sexual selection by controlling female mating opportunities and the presence of a distorting X chromosome in experimental lines of the stalk-eyed fly, Teleopsis dalmanni, over 11 generations. We find that removal of sexual selection leads to an increase in the frequency of the X-linked distorter and sex ratio across generations and that post-copulatory sexual selection alone is sufficient to limit the frequency of distorters. In addition, we find that male eyestalk length, a trait under pre-copulatory sexual selection, evolves in response to changes in the strength of sexual selection with the magnitude of the response dependent on X chromosome type and the frequency of distorting X chromosomes. These results reveal how a selfish X can interact with sexual selection to influence the evolution of sexually selected traits in multiple ways.

Impacts of Sex Ratio Meiotic Drive on Genome Structure and Function in a Stalk-Eyed Fly

Stalk-eyed flies in the genus Teleopsis carry selfish genetic elements that induce sex ratio (SR) meiotic drive and impact the fitness of male and female carriers. Here, we assemble and describe a chromosome-level genome assembly of the stalk-eyed fly, Teleopsis dalmanni, to elucidate patterns of divergence associated with SR. The genome contains tens of thousands of transposable element (TE) insertions and hundreds of transcriptionally and insertionally active TE families. By resequencing pools of SR and ST males using short and long reads, we find widespread differentiation and divergence between XSR and XST associated with multiple nested inversions involving most of the SR haplotype. Examination of genomic coverage and gene expression data revealed seven X-linked genes with elevated expression and coverage in SR males. The most extreme and likely drive candidate involves an XSR-specific expansion of an array of partial copies of JASPer, a gene necessary for maintenance of euchromatin and associated with regulation of TE expression. In addition, we find evidence for rapid protein evolution between XSR and XST for testis expressed and novel genes, that is, either recent duplicates or lacking a Dipteran ortholog, including an X-linked duplicate of maelstrom, which is also involved in TE silencing. Overall, the evidence suggests that this ancient XSR polymorphism has had a variety of impacts on repetitive DNA and its regulation in this species.

Selfish genetic elements and male fertility

Selfish genetic elements (SGEs) are diverse and near ubiquitous in Eukaryotes and can be potent drivers of evolution. Here, we discuss SGEs that specifically act on sperm to gain a transmission advantage to the next generation. The diverse SGEs that affect sperm often impose costs on carrier males, including damaging ejaculates, skewing offspring sex ratios and in particular reducing sperm-competitive success of SGE-carrying males. How males and females tolerate and mitigate against these costs is a dynamic and expanding area of research. The intense intra-genomic conflict that these selfish elements generate could also have implications for male fertility and spermatogenesis more widely. This article is part of the theme issue 'Fifty years of sperm competition'.

Genomic signatures of globally enhanced gene duplicate accumulation in the megadiverse higher Diptera fueling intralocus sexual conflict resolution

Gene duplication is an important source of evolutionary innovation. To explore the relative impact of gene duplication during the diversification of major insect model system lineages, we performed a comparative analysis of lineage-specific gene duplications in the fruit fly Drosophila melanogaster (Diptera: Brachycera), the mosquito Anopheles gambiae (Diptera: Culicomorpha), the red flour beetle Tribolium castaneum (Coleoptera), and the honeybee Apis mellifera (Hymenoptera). Focusing on close to 6,000 insect core gene families containing maximally six paralogs, we detected a conspicuously higher number of lineage-specific duplications in Drosophila (689) compared to Anopheles (315), Tribolium (386), and Apis (223). Based on analyses of sequence divergence, phylogenetic distribution, and gene ontology information, we present evidence that an increased background rate of gene duplicate accumulation played an exceptional role during the diversification of the higher Diptera (Brachycera), in part by providing enriched opportunities for intralocus sexual conflict resolution, which may have boosted speciation rates during the early radiation of the megadiverse brachyceran subclade Schizophora.

A gene's-eye view of sexual antagonism

Females and males may face different selection pressures. Accordingly, alleles that confer a benefit for one sex often incur a cost for the other. Classic evolutionary theory holds that the X chromosome, whose sex-biased transmission sees it spending more time in females, should value females more than males, whereas autosomes, whose transmission is unbiased, should value both sexes equally. However, recent mathematical and empirical studies indicate that male-beneficial alleles may be more favoured by the X chromosome than by autosomes. Here we develop a gene's-eye-view approach that reconciles the classic view with these recent discordant results, by separating a gene's valuation of female versus male fitness from its ability to induce fitness effects in either sex. We use this framework to generate new comparative predictions for sexually antagonistic evolution in relation to dosage compensation, sex-specific mortality and assortative mating, revealing how molecular mechanisms, ecology and demography drive variation in masculinization versus feminization across the genome.

Comparative analysis of behavioural traits in insects

Comparative studies of insect behaviour based on evolutionary trees are currently blossoming, because of the increasing ease of phylogeny estimation, the availability of new trait data to analyze, and a vast and growing array of statistical techniques for exploring data and testing hypotheses. These studies address not only the selective forces and constraints on insect behaviour, which are the realm of traditional behavioural ecology, but also their ecological and evolutionary consequences. Recent studies have significantly increased our understanding of foraging behaviour, interspecific interactions, locomotion and dispersal, communication and signalling, mate choice and sexual selection, parental care and the evolution of sociality. The curating of trait data remains a significant challenge to maximize the future potential of insect comparative studies.

The Z chromosome is enriched for sperm proteins in two divergent species of Lepidoptera

Genes that promote sexual conflict, such as those with a sex-limited fitness benefit, are expected to accumulate differentially on sex chromosomes relative to autosomes. Few tests of this hypothesis exist for male homogametic (ZZ) taxa, however, and most use RNA expression data to identify such genes. Here, we employ a different identification method by using proteomic analysis of sperm cells to identify genes with a sex-limited benefit. We tested for a bias in genomic location of sperm protein genes in two species of Lepidoptera. An excess of sperm protein genes was identified on the Z chromosomes of both the Carolina sphinx moth (Manduca sexta) and the monarch butterfly (Danaus plexippus). Taking into consideration a Z-autosome fusion in monarchs, we discover that the ancestrally sex-linked portion of the genome is the source of this enrichment, while the newly sex-linked portion still appears similar to autosomes in relative abundance of sperm protein genes. Together, these results point to an enrichment of male-beneficial genes on the Z chromosome and demonstrate the usefulness of proteomic datasets in sexual conflict research.