Biology subjects

McGregor, A. P.

Publications and source records attributed to McGregor, A. P..

At least 19 recordsLinked to original sources

bioRxiv2026

Lost and found: beer belly is a novel, ancient regulator of animal development

Single-cell RNA sequencing (scRNA-seq) can provide new insights into the regulation of developmental processes by revealing unknown or overlooked genes. We identified a novel gene called beer belly (bebe) as a marker of a segment addition zone (SAZ) associated cell cluster in a single-cell atlas of the spider Parasteatoda tepidariorum. Here we show that bebe is an ancient toolkit gene encoding a previously uncharacterised protein predicted to be secreted, which likely plays an important role in the development of most animal phyla. However, we could not identify a bebe ortholog in any mammal or holometabolous insect, suggesting it has been lost in these lineages and explaining why it has been overlooked to date. Knock-down of bebe expression in P. tepidariorum demonstrates that it regulates SAZ size and consequently segment size by interacting with Wnt8. Our findings also suggest that bebe may regulate segmentation in other invertebrates and play additional developmental roles in bilaterian and non-bilaterian animals, likely ancestrally, through modulation of Wnt signalling. This study emphasises the promise of applying single-cell sequencing across a diverse range of species to better understand the regulation of development beyond candidate genes from traditional models, allowing us to better characterise the animal developmental toolkit and its evolution.

developmental biology↗

bioRxiv2026

A developmental chimera: co-option of appendage, secretory and myogenic programs underlies spider venom gland evolution

The evolutionary emergence of complex organs requires the integration of distinct developmental modules, yet the mechanisms bridging divergent tissue programs into unified functional units remain poorly understood. Animal venom systems, combining high-output secretory epithelia with muscular delivery systems, a provide an exceptional model for studying this modular integration. Here, we show that the venom gland of the spider Parasteatoda tepidariorum is a developmental chimera built by the hierarchical co-option of three ancestral programs. The appendage-patterning gene Distal-less establishes the primary spatial coordinate system for gland development. Downstream, the transcription factor sage maintains secretory epithelium identity; its loss causes downregulation of key venom-processing enzymes and triggers a dramatic phenotypic reversion to a neuronal state. This neural upregulation, accompanied by the downregulation of Notch ligands (delta and jagged), reveals that sage actively represses a latent neural program to maintain secretory fate. Crucially, the myogenic factor sum-1 governs the surrounding muscle layer to drive paracrine crosstalk (Wnt) and sustain lipid metabolism. Knockdown of sum-1 downregulates wntless and fatty acid anabolism genes, alongside the abnormal accumulation of lipid droplets in the secretory epithelium. Ultimately, our findings demonstrate how a complex organ is assembled by fusing an appendicular address, a repressed neuroglandular program, and a myogenic metabolic driver. This modular architecture illustrates a fundamental principle in evolutionary developmental biology: novelty can arise by harnessing the physiological and metabolic capacities of adjacent tissues to fuel the physiological demands of a newly emerged organ. Significance statementHow novel organs evolve by integrating distinct developmental modules remains a fundamental question in biology. Using the spider venom gland as a model, we show that this complex organ is a developmental chimera built by recruiting three ancestral tissue programs. The appendage-selector gene Distal-less (Dll) establishes the glands primary proximal-distal structural axis. Downstream, the transcription factor sage maintains secretory epithelial identity by actively repressing a default neural program. Concurrently, the myogenic regulator sum-1 sustains epithelial lipid metabolism through paracrine Wnt signaling. These findings establish a mosaic model of organogenesis, demonstrating that radical evolutionary innovation can arise by forging morphogenetic and metabolic partnerships that harness adjacent tissues to fuel newly emerged structures.

evolutionary biology↗

bioRxiv2026

Evolutionary landscapes of zygotic genome activation across animals

During early animal embryogenesis, gene expression control shifts from maternally deposited products to newly transcribed RNA through zygotic genome activation (ZGA). Although essential and universal, ZGA remains poorly understood outside a few model species. Here, we generated a transcriptomic atlas of early embryogenesis across 61 animal species from 13 phyla and used a unified computational framework to infer ZGA timing systematically. We find extensive variation in ZGA onset across animals, but show that the ratio between genome size and egg cytoplasmic volume, a proxy for the nuclear-to-cytoplasmic ratio, robustly predicts when genome activation begins. We also reveal that zygotic genes also differ from maternal transcripts in genomic architecture, function and evolutionary conservation, suggesting that ZGA follows a conserved molecular logic despite flexible transcriptomic outputs.

evolutionary biology↗

bioRxiv2025

Pre-cheliceral region patterning in a spider provides new insights into the development and evolution of arthropod neurosecretory centres

Comparing head development among arthropods has helped identify ancestral aspects of brain patterning and structure in these animals and more broadly. Most understanding of arthropod head patterning has been learned from insects and the myriapod Strigamia maritima. Chelicerates represent an outgroup to mandibulate arthropods and can provide a valuable perspective to arthropod evolution and development. We assayed the expression of key markers of head patterning and neurosecretory centres in mandibulates in the pre-cheliceral region of embryos of the spider Parasteatoda tepidariorum. We found that, like mandibulates, this spider likely has a pars intercerebralis, marked by six3.2 and visual system homeobox/chx. We also found some evidence for another neurosecretory centre, the pars lateralis, marked by six3.2 and fasciclin 2. Furthermore, we identified anterior-medial cells in the spider pre-cheliceral region that express six3.2, foxQ2, and collier1, suggesting they may be pioneer neurons. However, these spider cells do not appear to be equivalent to the central pioneer neuronal cells identified in S. maritima because they lack expression of other key markers. Taken together, our study of spider pre-cheliceral region patterning adds a new chelicerate perspective to understanding the development and evolution of the arthropod head.

developmental biology↗

bioRxiv2024

Assessing the impact of whole genome duplication on gene expression and regulation during arachnid development

Whole genome duplication (WGD) generates new genetic material that can contribute to the evolution of the regulation of developmental processes and phenotypic diversification. A WGD occurred in an ancestor of arachnopulmonates (spiders, scorpions, and their relatives), which provides an important independent comparison to WGDs in other animal lineages like those in vertebrates and horseshoe crabs. After WGD, arachnopulmonates retained many duplicated copies (ohnologues) of developmental genes including clusters of homeobox genes, many of which have been inferred to have undergone subfunctionalisation. However, there has been little systematic analysis of gene regulatory sequences and comparison of the expression of ohnologues versus their single-copy orthologues between arachnids. Here we compare the regions of accessible chromatin and gene expression of ohnologues and single-copy genes during three embryonic stages between an arachnopulmonate arachnid, the spider Parasteatoda tepidariorum, and a non-arachnopulmonate arachnid, the harvestman Phalangium opilio. We found that the expression of each spider ohnologue was lower than their single-copy orthologues in the harvestman providing evidence for subfunctionalisation. However, this was not reflected in a reduction in peaks of accessible chromatin because both spider ohnologues and single-copy genes had more peaks than the harvestman genes. We also found peaks of open chromatin that increased in the late stage associated with activation of genes expressed later during embryogenesis in both species. Taken together, our study provides the first genome-wide comparison of gene regulatory sequences and gene expression in arachnids and thus provides new insights into the impact of the arachnopulmonate WGD. Significance statementThe comparison of independent WGD events is essential to understanding their evolutionary outcomes. Here we examine gene expression and chromatin accessibility during the development of two arachnids, one of which underwent an ancient WGD. Our data provide the first direct comparison of the regulation of ohnologues and single-copy genes in arachnids.

genomics↗

bioRxiv2024

Rewired gene interactions during development of serially homologous appendages in male and female Drosophila

Drosophila ventral appendages are considered to be serially homologous derived from a ventral appendage ground state and shaped by different Hox inputs. In the legs and antennae a combination of the transcription factors C15, LIM1 homeobox 1 (Lim1), and Al (Aristaless) is required for the development of the tarsal claws and aristae, respectively. However, the roles of these factors in genital development have remained unexplored. Here, we investigated the expression and function of C15, Lim1, and Al in the development of male and female terminalia (genitalia and analia). We found that C15 plays distinct roles in males and females, repressing male clasper bristle formation while promoting bristle development in the female epiproct. Unlike in the antennal and leg discs, C15, Lim1, and Al are not all simultaneously co-expressed in any anal or genital structures in either sex, indicating that the interactions among these factors have diverged across these appendages. Nevertheless, we inferred regulatory interactions between C15 and other factors, reflecting similarities between leg and male clasper development. Finally, we identified a male-specific C15 enhancer that is active in male claspers but not in the female epiproct, legs or antennae. This C15 enhancer modularity may underpin tissue- and sex-specific regulatory logic.

developmental biology↗

bioRxiv2024

Venom gland organogenesis in the common house spider

Venom is a remarkable innovation found across the animal kingdom, yet the evolutionary origins of venom systems in various groups, including spiders, remain enigmatic. Here, we investigated the organogenesis of the venom apparatus in the common house spider, Parasteatoda tepidariorum. The venom apparatus consists of a pair of secretory glands, each connected to an opening at the fang tip by a duct that runs through the chelicerae. We performed bulk RNA-seq to identify venom gland-specific markers and assayed their expression using RNA in situ hybridisation experiments on whole-mount time-series. These revealed that the gland primordium emerges during embryonic stage 13 at the chelicera tip, progresses proximally by the end of embryonic development and extends into the prosoma post-eclosion. The initiation of expression of an important toxin component in late postembryos marks the activation of venom-secreting cells. Our selected markers also exhibited distinct expression patterns in adult venom glands: sage and the toxin marker were expressed in the secretory epithelium, forkhead and sum-1 in the surrounding muscle layer, while Distal-less was predominantly expressed at the gland extremities. Our study provides the first comprehensive analysis of venom gland morphogenesis in spiders, offering key insights into their evolution and development.

developmental biology↗

bioRxiv2023

Development and patterning of a highly versatile visual system in spiders

Visual systems provide a key interface between organisms and their surroundings, and have evolved in many forms to perform diverse functions across the animal kingdom. Spiders exhibit a range of visual abilities and ecologies, the diversity of which is underpinned by a highly versatile, modular visual system architecture. This typically includes eight eyes of two developmentally distinct types, but the number, size, location, and function of the eyes can vary dramatically between lineages. Previous studies of visual system development in spiders have confirmed that many components of the retinal determination gene (RDG) network are conserved with other arthropods, but so far, comparative studies among spiders are lacking. We characterised visual system development in eight species of spiders representing a range of morphologies, visual ecologies, and phylogenetic positions, to determine how these diverse configurations are formed, and how they might evolve. Combining synchrotron radiation tomography, transcriptomics, in situ hybridisation, and selection analyses, we characterise the repertoires and expression of key RDGs in relation to adult morphology. We identify key molecular players, timepoints, and developmental events that may contribute to adult diversity, in particular the molecular and developmental underpinnings of eye size, number, position, and identity across spiders.

developmental biology↗

bioRxiv2023

Sox21b underlies the rapid diversification of a novel male genital structure between Drosophila species

The emergence and subsequent diversification of morphological novelties is a major feature of animal evolution1-9. However, in most cases little is known about the molecular basis of the evolution of novel structures and the genetic mechanisms underlying their diversification. The epandrial posterior lobes of the male genital arch is a novelty of some species of the Drosophila melanogaster subgroup10-13. The posterior lobes grasp the ovipositor of the female and then integrate between her abdominal tergites, and therefore these structures are important for copulation and species-recognition10-12,14-17. The posterior lobes evolved from co-option of a Hox regulated gene network from the posterior spiracles10 and have since diversified in shape and size in the D. simulans clade in particular over the last 240,000 years driven by sexual selection18-21. The genetic basis of this diversification is highly polygenic but to the best of our knowledge none of the causative genes have yet been identified despite extensive mapping22-30. Identifying the genes underlying the diversification of these secondary sexual structures is essential to understanding the basis of changes in their morphology and the evolutionary impact on copulation and species recognition. Here, we show that the transcription factor encoded by Sox21b negatively regulates posterior lobe size during development. This is consistent with higher and expanded expression of Sox21b in D. mauritiana, which develops smaller posterior lobes compared to D. simulans. We tested this by generating reciprocal hemizygotes and confirmed that changes in Sox21b underlie posterior lobe evolution between these two species. Furthermore, we found that differences in posterior lobe size caused by the species-specific allele of Sox21b significantly affect the duration of copulation. Taken together, our study reveals the genetic basis for the sexual selection driven diversification of a novel morphological structure and its functional impact on copulatory behaviour. HighlightsO_LISox21b regulates the development of the epandrial posterior lobes, a recently evolved novel structure of some species of the Drosophila melanogaster subgroup, which has subsequently rapidly diversified in size and shape. C_LIO_LID. mauritiana has smaller posterior lobes than D. simulans and more expansive expression of Sox21b in the developing genitalia. Using a reciprocal hemizygosity test, we show that variation in Sox21b underlies the diversification of epandrial posterior lobe size and shape between D. simulans and D. mauritiana. C_LIO_LIBehavioural tests show that the species allele of Sox21b causes differences in the duration of copulation in otherwise genetically identical backgrounds. C_LIO_LISox21b has evolved between D. simulans and D. mauritiana, and contributed to the divergence of a morphological novelty and copulatory behaviour between these two species. C_LI

evolutionary biology↗

bioRxiv2023

Evolution of compound eye morphology underlies differences in vision between closely related Drosophila species

Insects have evolved complex visual systems and display an astonishing range of adaptations for diverse ecological niches. Differences in eye size within and between Drosophila species provide the opportunity to study the impact of eye structure on vision. Here we further explored differences in D. mauritiana and its sibling species D. simulans and confirmed that D. mauritiana have rapidly evolved larger eyes as a result of more and wider ommatidia than D. simulans since their recent common ancestor. The functional impact of eye size, and specifically ommatidia size, is often only estimated based on the rigid surface morphology of the compound eye. Therefore, we used 3D synchrotron radiation tomography to measure optical parameters in 3D, predict optical capacity, and compare the modelled vision to in vivo optomotor responses. Our optical models predicted higher contrast sensitivity for D. mauritiana, which we verified by presenting sinusoidal gratings to tethered flies in a flight arena. Similarly, we confirmed the higher spatial acuity predicted for Drosophila simulans with smaller ommatidia and found evidence for higher temporal resolution.

evolutionary biology↗

bioRxiv2023

Evolution of the spider homeobox gene repertoire by tandem and whole genome duplication

Gene duplication generates new genetic material that can contribute to the evolution of gene regulatory networks and phenotypes. Duplicated genes can undergo subfunctionalisation to partition ancestral functions and/or neofunctionalisation to assume a new function. We previously found there had been a whole genome duplication (WGD) in an ancestor of arachnopulmonates, the lineage including spiders and scorpions but excluding other arachnids like mites, ticks, and harvestmen. This WGD was evidenced by many duplicated homeobox genes, including two Hox clusters, in spiders. However, it was unclear which homeobox paralogues originated by WGD versus smaller-scale events such as tandem duplications. Understanding this is key to determining the contribution of the WGD to arachnopulmonate genome evolution. Here we characterised the distribution of duplicated homeobox genes across eight chromosome-level spider genomes. We found that most duplicated homeobox genes in spiders are consistent with an origin by WGD. We also found two copies of conserved homeobox gene clusters, including the Hox, NK, HRO, Irx, and SINE clusters, in all eight species. Consistently, we observed one copy of each cluster was degenerated in terms of gene content and organisation while the other remained more intact. Focussing on the NK cluster, we found evidence for regulatory subfunctionalisation between the duplicated NK genes in the spider Parasteatoda tepidariorum compared to their single-copy orthologues in the harvestman Phalangium opilio. Our study provides new insights into the relative contributions of multiple modes of duplication to the homeobox gene repertoire during the evolution of spiders and the function of NK genes.

evolutionary biology↗

bioRxiv2022

An atlas of spider development at single-cell resolution provides new insights into arthropod embryogenesis

Spiders are a diverse order of chelicerates that diverged from other arthropods over 500 million years ago. Research on spider embryogenesis, particular studies using the common house spider Parasteatoda tepidariorum, has made important contributions to understanding the evolution of animal development, including axis formation, segmentation, and patterning. However, we lack knowledge about the cells that build spider embryos, their gene expression profiles and fate. Single-cell transcriptomic analyses have been revolutionary in describing these complex landscapes of cellular genetics in a range of animals. Therefore, we carried out single-cell RNA sequencing of P. tepidariorum embryos at stages 7, 8 and 9, which encompass the establishment and patterning of the body plan, and initial differentiation of many tissues and organs. We identified 20 cell clusters, from 18.5k cells, which were marked by many developmental toolkit genes, as well as a plethora of genes not previously investigated. There were differences in the cell cycle transcriptional signatures, suggestive of different proliferation dynamics, which related to distinctions between endodermal and some mesodermal clusters, compared with ectodermal clusters. We found many Hox genes were markers of cell clusters, and Hox gene ohnologs often were present in different clusters. This provided additional evidence of sub- and/or neo-functionalisation of these important developmental genes after the whole genome duplication in the arachnopulmonate ancestor (spiders, scorpions, and allies). We also examined the spatial expression of marker genes for each cluster to generate a comprehensive cell atlas of these embryonic stages. This revealed new insights into the cellular basis and genetic regulation of head patterning, hematopoiesis, limb development, gut development, and posterior segmentation. This atlas will serve as a platform for future analysis of spider cell specification and fate, and studying the evolution of these processes among animals at cellular resolution.

developmental biology↗

bioRxiv2021

Differences in orthodenticle expression promote ommatidial size variation between Drosophila species

The compound eyes of insects exhibit extensive variation in ommatidia number and size, which affects how they see and underlies adaptations in their vision to different environments and lifestyles. However, very little is known about the genetic and developmental bases of differences in compound eye size. We previously showed that the larger eyes of Drosophila mauritiana compared to D. simulans is generally caused by differences in ommatidia size rather than number. Furthermore, we identified an X-linked chromosomal region in D. mauritiana that results in larger eyes when introgressed into D. simulans. Here, we used a combination of fine-scale mapping and gene expression analysis to further investigate positional candidate genes on the X chromosome. We found earlier expression of orthodenticle (otd) during ommatidial maturation in third instar larvae in D. mauritiana than in D. simulans, and we show that this gene is required for the correct organisation and size of ommatidia in D. melanogaster. We discovered that the activity of an otd eye enhancer is consistent with the difference in the expression of this gene between species, with the D. mauritiana enhancer sequence driving earlier expression than that of D. simulans. We also identified potential direct targets of Otd that are differentially expressed between D. mauritiana and D. simulans during ommatidial maturation. Taken together, our results suggest that differential timing of otd expression contributes to natural variation in ommatidia size between D. mauritiana and D. simulans, which provides new insights into the mechanisms underlying the regulation and evolution of compound eye size in insects.

evolutionary biology↗

bioRxiv2020

Unravelling the genetic basis for the rapid diversification of male genital structures between Drosophila species

In the last 240,000 years, males of the Drosophila simulans species clade have evolved striking differences in the morphology of their epandrial posterior lobes and claspers (surstyli). These changes have most likely been driven by sexual selection and mapping studies indicate a highly polygenic and generally additive genetic basis. However, we have limited understanding of the gene regulatory networks that control the development of genital structures and how they evolved to result in this rapid phenotypic diversification. Here, we used new D. simulans / D. mauritiana introgression lines on chromosome 3L to generate higher resolution maps of posterior lobe and clasper differences between these species. We then carried out RNA-seq on the developing genitalia of both species to identify the genes expressed during this process and those that are differentially expressed between the two species. This allowed us to test the function of expressed positional candidates during genital development in D. melanogaster. We identified several new genes involved in the development and possibly the evolution of these genital structures, including the transcription factors Hairy and Grunge. Furthermore, we discovered that during clasper development Hairy negatively regulates tartan, a gene known to contribute to divergence in clasper morphology. Taken together our results provide new insights into the regulation of genital development and how this evolves between species.

evolutionary biology↗

bioRxiv2020

Characterisation of the role and regulation of Ultrabithorax in sculpting fine-scale leg morphology

Hox genes are expressed during embryogenesis and determine the regional identity of animal bodies along the antero-posterior axis. However, they also function post-embryonically to sculpt fine-scale morphology. To better understand how Hox genes are integrated into post-embryonic gene regulatory networks, we further analysed the role and regulation of Ultrabithorax (Ubx) during leg development in Drosophila melanogaster. Ubx regulates several aspects of bristle and trichome patterning on the femurs of the second (T2) and third (T3) leg pairs. We found that repression of trichomes in the proximal posterior region of the T2 femur by Ubx is likely mediated by activation of the expression of microRNA-92a by this Hox protein. Furthermore, we identified a novel enhancer of Ubx that recapitulates the temporal and regional activity of this gene in T2 and T3 legs. We then used transcription factor binding motif analysis in regions of accessible chromatin in T2 leg cells to predict and functionally test transcription factors that may regulate the Ubx leg enhancer. We also tested the role of the Ubx co-factors Homothorax (Hth) and Extradenticle (Exd) in T2 and T3 femurs. We found several transcription factors that may act upstream or in concert with Ubx to modulate trichome patterning along the proximo-distal axis of developing femurs and that the repression of trichomes also requires Hth and Exd. Taken together our results provide insights into how Ubx is integrated into a postembryonic gene regulatory network to determine fine-scale leg morphology.

developmental biology↗

bioRxiv2020

The evolution of Sox gene repertoires and regulation of segmentation in arachnids

The Sox family of transcription factors regulate many processes during metazoan development, including stem cell maintenance and nervous system specification. Characterising the repertoires and roles of these genes can therefore provide important insights into animal evolution and development. We further characterised the Sox repertoires of several arachnid species with and without an ancestral whole genome duplication (WGD), and compared their expression between the spider Parasteatoda tepidariorum and the harvestman Phalangium opilio. We also found that most Sox families have been retained as ohnologs after WGD and evidence for potential subfunctionalisation and/or neofunctionalization events. Our results also suggest that Sox21b-1 likely regulated segmentation ancestrally in arachnids, playing a similar role to the closely related SoxB gene, Dichaete, in insects. We previously showed that Sox21b-1 is required for the simultaneous formation of prosomal segments and sequential addition of opisthosomal segments in P. tepidariorum. We studied the expression and function of Sox21b-1 further in this spider and found that while this gene regulates the generation of both prosomal and opisthosomal segments, it plays different roles in the formation of these tagmata reflecting their contrasting modes of segmentation and deployment of gene regulatory networks with different architectures.

developmental biology↗

bioRxiv2019

Characterisation of the genetic architecture underlying eye size variation within Drosophila melanogaster and Drosophila simulans

The compound eyes of insects exhibit striking variation in size, reflecting adaptation to different lifestyles and habitats. However, the genetic and developmental bases of variation in insect eye size is poorly understood, which limits our understanding of how these important morphological differences evolve. To address this, we further explored natural variation in eye size within and between four species of the Drosophila melanogaster species subgroup. We found extensive variation in eye size among these species, and flies with larger eyes generally had a shorter inter-ocular distance and vice versa. We then carried out quantitative trait loci (QTL) mapping of intra-specific variation in eye size and inter-ocular distance in both D. melanogaster and D. simulans. This revealed that different genomic regions underlie variation in eye size and inter-ocular distance in both species, which we corroborated by introgression mapping in D. simulans. This suggests that although there is a trade-off between eye size and inter-ocular distance, variation in these two traits is likely to be caused by different genes and so can be genetically decoupled. Finally, although we detected QTL for intra-specific variation in eye size at similar positions in D. melanogaster and D. simulans, we observed differences in eye fate commitment between strains of these two species. This indicates that different developmental mechanisms and therefore, most likely, different genes contribute to eye size variation in these species. Taken together with the results of previous studies, our findings suggest that the gene regulatory network that specifies eye size has evolved at multiple genetic nodes to give rise to natural variation in this trait within and among species.

evolutionary biology↗

bioRxiv2018

tartan underlies the evolution of male genital morphology

Male genital structures are among the most rapidly evolving morphological traits and are often the only features that can distinguish closely related species. This process is thought to be driven by sexual selection and may reinforce species separation. However, while the genetic basis of many phenotypic differences have been identified, we still lack knowledge about the genes underlying evolutionary differences in male genital organs and organ size more generally. The claspers (surstyli) are periphallic structures that play an important role in copulation in insects. Here we show that natural variation in clasper size and bristle number between Drosophila mauritiana and D. simulans is caused by evolutionary changes in tartan (trn), which encodes a transmembrane leucine-rich repeat domain protein that mediates cell-cell interactions and affinity differences. There are no fixed amino acid differences in trn between D. mauritiana and D. simulans but differences in the expression of this gene in developing genitalia suggest cis-regulatory changes in trn underlie the evolution of clasper morphology in these species. Finally, analysis of reciprocal hemizyotes that are genetically identical, except for which species the functional allele of trn is from, determined that the trn allele of D. mauritiana specifies larger claspers with more bristles than the allele of D. simulans. Therefore we have identified the first gene underlying evolutionary change in the size of a male genital organ, which will help to better understand the rapid diversification of these structures and the regulation and evolution of organ size more broadly.\n\nSignificance StatementThe morphology of male genital organs evolves rapidly driven by sexual selection. However, little is known about the genes underlying genitalia differences between species. Identifying these genes is key to understanding how sexual selection acts on development to produce rapid phenotypic change. We have found that the gene tartan underlies differences between male Drosophila mauritiana and D. simulans in the size and bristle number of the claspers - genital projections that grasp the female during copulation. Moreover, since tartan encodes a protein that is involved in cell affinity, this may represent a new developmental mechanism for morphological change. Therefore, our study provides new insights into genetic and developmental bases for the rapid evolution of male genitalia and organ size more generally.

evolutionary biology↗