An official website of the United States government
Abstract Nutrition-dependent growth of sexual traits is a major contributor to phenotypic diversity, and a large body of research documents insulin signalling as a major regulator of nutritional plasticity. However, findings across studies raise the possibility that the role of individual components within the insulin signalling pathway diverges in function among traits and taxa. Here, we use RNAi-mediated transcript depletion in the gazelle dung beetle to investigate the functions of forkhead box O (Foxo) and two paralogs of the insulin receptor (InR1 and InR2) in shaping nutritional plasticity in polyphenic male head horns, exaggerated fore legs, and weakly nutrition-responsive genitalia. Our functional genetic manipulations led to three main findings: FoxoRNAi reduced the length of exaggerated head horns in large males, while neither InR1 nor InR2 knock-downs resulted in measurable horn phenotypes. These results are similar to those documented previously for another dung beetle (Onthophagus taurus), but in stark contrast to findings in rhinoceros beetles. Secondly, knockdown of Foxo, InR1, and InR2 led to an increase in the intercept or slope of the scaling relationship of genitalia size. These findings are in contrast even to results documented previously for O. taurus. Lastly, while FoxoRNAi reduces male forelegs in D. gazella and O. taurus, the effects of InR1 and InR2 knockdowns diverged across dung beetle species. Our results add to the growing body of literature indicating that despite insulin signalling's conserved role as a regulator of nutritional plasticity, the functions of its components may diversify among traits and species, potentially fuelling the evolution of scaling relationships.
more »
« less
Genome evolution and divergence in cis-regulatory architecture is associated with condition-responsive development in horned dung beetles
Phenotypic plasticity is thought to be an important driver of diversification and adaptation to environmental variation, yet the genomic mechanisms mediating plastic trait development and evolution remain poorly understood. The Scarabaeinae, or true dung beetles, are a species-rich clade of insects recognized for their highly diversified nutrition-responsive development including that of cephalic horns—evolutionarily novel, secondary sexual weapons that exhibit remarkable intra- and interspecific variation. Here, we investigate the evolutionary basis for horns as well as other key dung beetle traits via comparative genomic and developmental assays. We begin by presenting chromosome-level genome assemblies of three dung beetle species in the tribe Onthophagini (> 2500 extant species) includingOnthophagus taurus,O.sagittarius, andDigitonthophagus gazella. Comparing these assemblies to those of seven other species across the order Coleoptera identifies evolutionary changes in coding sequence associated with metabolic regulation of plasticity and metamorphosis. We then contrast chromatin accessibility in developing head horn tissues of high- and low-nutritionO.taurusmales and females and identify distinctcis-regulatory architectures underlying nutrition- compared to sex-responsive development, including a large proportion of recently evolved regulatory elements sensitive to horn morph determination. Binding motifs of known and new candidate transcription factors are enriched in these nutrition-responsive open chromatin regions. Our work highlights the importance of chromatin state regulation in mediating the development and evolution of plastic traits, demonstrates gene networks are highly evolvable transducers of environmental and genetic signals, and provides new reference-quality genomes for three species that will bolster future developmental, ecological, and evolutionary studies of this insect group.
more »
« less
- PAR ID:
- 10511386
- Editor(s):
- Duncan, Elizabeth J
- Publisher / Repository:
- PLoS Genetics
- Date Published:
- Journal Name:
- PLOS Genetics
- Volume:
- 20
- Issue:
- 3
- ISSN:
- 1553-7404
- Page Range / eLocation ID:
- e1011165
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Understanding the origin of novel morphological traits is a long‐standing objective in evolutionary developmental biology. We explored the developmental genetic mechanisms that underpin the formation of a textbook example of evolutionary novelties, the cephalic horns of beetles. Previous work has implicated the gene regulatory networks associated with compound eye and ocellar development in horn formation and suggested that horns and compound eyes may influence each other's sizes. Therefore, we investigated the functional significance of genes central to visual system formation in the initiation, patterning, and size determination of head horns across three horned beetle species. We find that while the downregulation of canonical eye patterning genes reliably reduces or eliminates compound eye formation, it does not alter the position or shape of head horns yet does result in an increase in relative horn length. We discuss the implications of our results for our understanding of the genesis of cephalic horns in particular and evolutionary novelties in general.more » « less
-
Abstract Developmental and evolutionary processes underlying phenotypic variation frequently target several traits simultaneously, thereby causing covariation, or integration, among phenotypes. While phenotypic integration can be neutral, correlational selection can drive adaptive covariation. Especially, the evolution and development of exaggerated secondary sexual traits may require the adjustment of other traits that support, compensate for, or otherwise function in a concerted manner. Although phenotypic integration is ubiquitous, the interplay between genetic, developmental, and ecological conditions in shaping integration and its evolution remains poorly understood. Here, we study the evolution and plasticity of trait integration in the bull‐headed dung beetleOnthophagus tauruswhich is characterized by the polyphenic expression of horned (‘major’) and hornless (‘minor’) male morphs. By comparing populations subject to divergent intensities of mate competition, we tested whether mating system shifts affect integration of traits predicted to function in a morph‐specific manner. We focussed on fore and hind tibia morphology as these appendages are used to stabilize major males during fights, and on wings, as they are thought to contribute to morph‐based differences in dispersal behavior. We found phenotypic integration between fore and hind tibia length and horn length that was stronger in major males, suggesting phenotypic plasticity in integration and potentially secondary sexual trait compensation. Similarly, we observed that fore tibiashapewas also integrated with relative horn length. However, although we found population differentiation in wing and tibia shape and allometry, populations did not differ in integration. Lastly, we detected little evidence for morph differences in integration in either tibia or wing shape, although wing allometries differed between morphs. This contrasts with previous studies documenting intraspecific differentiation in morphology, behavior, and allometry as a response to varying levels of mate competition acrossO. tauruspopulations. We discuss how sexual selection may shape morph‐specific integration, compensation, and allometry across populations.more » « less
-
Abstract The Japanese rhinoceros beetleTrypoxylus dichotomusis a giant beetle with distinctive exaggerated horns present on the head and prothoracic regions of the male.T. dichotomushas been used as a research model in various fields such as evolutionary developmental biology, ecology, ethology, biomimetics, and drug discovery. In this study, de novo assembly of 615 Mb, representing 80% of the genome estimated by flow cytometry, was obtained using the 10 × Chromium platform. The scaffold N50 length of the genome assembly was 8.02 Mb, with repetitive elements predicted to comprise 49.5% of the assembly. In total, 23,987 protein-coding genes were predicted in the genome. In addition, de novo assembly of the mitochondrial genome yielded a contig of 20,217 bp. We also analyzed the transcriptome by generating 16 RNA-seq libraries from a variety of tissues of both sexes and developmental stages, which allowed us to identify 13 co-expressed gene modules. We focused on the genes related to horn formation and obtained new insights into the evolution of the gene repertoire and sexual dimorphism as exemplified by the sex-specific splicing pattern of thedoublesexgene. This genomic information will be an excellent resource for further functional and evolutionary analyses, including the evolutionary origin and genetic regulation of beetle horns and the molecular mechanisms underlying sexual dimorphism.more » « less
-
Abstract BackgroundNutrient availability is among the most widespread means by which environmental variability affects developmental outcomes. Because almost all cells within an individual organism share the same genome, structure-specific growth responses must result from changes in gene regulation. Earlier work suggested thathistone deacetylases(HDACs) may serve as epigenetic regulators linking nutritional conditions to trait-specific development. Here we expand on this work by assessing the function of diverseHDACsin the structure-specific growth of both sex-shared and sex-specific traits including evolutionarily novel structures in the horned dung beetleOnthophagus taurus. ResultsWe identified fiveHDACmembers whose downregulation yielded highly variable mortality depending on whichHDACmember was targeted. We then show thatHDAC1,3, and4operate in both a gene- and trait-specific manner in the regulation of nutrition-responsiveness of appendage size and shape. Specifically,HDAC 1, 3,or4knockdown diminished wing size similarly while leg development was differentially affected by RNAi targetingHDAC3andHDAC4. In addition, depletion ofHDAC3transcript resulted in a more rounded shape of genitalia at the pupal stage and decreased the length of adult aedeagus across all body sizes. Most importantly, we find thatHDAC3andHDAC4pattern the morphology and regulate the scaling of evolutionarily novel head and thoracic horns as a function of nutritional variation. ConclusionCollectively, our results suggest that both functional overlap and division of labor amongHDACmembers contribute to morphological diversification of both conventional and recently evolved appendages. More generally, our work raises the possibility thatHDAC-mediated scaling relationships and their evolution may underpin morphological diversification within and across insect species broadly.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1371/journal.pgen.1011165
