High level of novelty under the hood of convergent evolution

Evolution of gene regulatory networks in insects

Changes in gene regulatory networks (GRNs) underlying the evolution of traits have been intensively studied, with insects providing excellent model cases. In studies using Drosophila, butterflies, and other insects, several well-known cases have shown that changes in the cis-regulatory region of a gene controlling a trait can result in the co-option of the gene for a role different from that in its original developmental context. When the expression of a regulatory gene that controls the expression of multiple downstream genes is altered, the expression of these downstream genes changes accordingly, representing the simplest form of GRN co-option. Many studies have explored the applicability of this model to the acquisition of new traits, yielding substantial insights. However, no study has yet comprehensively elucidated the co-option of a GRN or the evolution of a network architecture, including associated genes and their regulatory relationships. In the near future, the use of single-cell multiomics and machine learning will allow for larger-scale data analysis, leading to a better understanding of the evolution of traits through the evolution of GRNs.

Multiple transitions to high l-DOPA 4,5-dioxygenase activity reveal molecular pathways to convergent betalain pigmentation in Caryophyllales

Many specialized metabolic pathways have evolved convergently in plants, but distinguishing multiple origins from alternative evolutionary scenarios can be difficult. Here, we explore the evolution of l-3,4-dihydroxyphenylalanine (l-DOPA) 4,5-dioxygenase (DODA) enzymes to better resolve the convergent evolution of the betalain biosynthetic pathway within the flowering plant order Caryophyllales. We use yeast-based heterologous assays to quantify enzymatic activity of extant proteins and then employ ancestral sequence reconstruction to resurrect and assay ancestral DODA enzymes. We use a combination of ancestral sequence reconstruction, model-based methods, and structural modelling to describe patterns of molecular convergence. We confirm that high l-DOPA 4,5-dioxygenase activity is polyphyletic and show that high activity DODAs evolved at least three times from ancestral proteins with low activity. We show that molecular convergence is concentrated proximally to the binding pockets but also appears distally to active sites. Moreover, our analysis also suggests that many unique and divergent substitutions contribute to the evolution of DODA. Given the key role of DODA in betalain biosynthesis, our analysis further supports the convergent origins of betalains and illustrates how the iterative evolution of betalain biosynthesis has drawn on a complex mixture of convergent, divergent, and unique variation.

Genetic basis of an adaptive polymorphism controlling butterfly silver iridescence

Identifying the genes and mutations that drive phenotypic variation and which are subject to selection is crucial for understanding evolutionary processes. Mormon Fritillary butterflies (Speyeria mormonia) exhibit a striking wing color polymorphism throughout their range: typical morphs bear silver spots on their ventral surfaces and can co-occur with unsilvered morphs displaying a dull coloration.1 Through genome-wide association studies in two polymorphic populations, we fine-map this difference in silvering to the 3' region of the transcription factor gene optix. The expression of optix is confined to the unsilvered regions that surround the spots, and these patterns are transformed to a silver identity upon optix RNA interference (RNAi) knockdown, implicating optix as a repressor of silver scales in this butterfly. We show that the unsilvered optix haplotype shows signatures of recent selective sweeps and that this allele is shared with an unsilvered population of Speyeria hydaspe, suggesting that introgressions may facilitate the exchange of variants of adaptive potential across species. Remarkably, these findings parallel the role of allelic sharing and cis-regulatory modulation of optix in shaping the aposematic red patterns of Heliconius butterflies,2,3,4,5,6,7 a lineage that separated from Speyeria 45 million years ago.8 The genetic basis of adaptive variation can thus be more predictable than often presumed, even for traits that appear divergent across large evolutionary distances.

The Developmental Origin of Novel Complex Morphological Traits in Lepidoptera

Novel traits in the order Lepidoptera include prolegs in the abdomen of larvae, scales, and eyespot and band color patterns in the wings of adults. We review recent work that investigates the developmental origin and diversification of these four traits from a gene-regulatory network (GRN) perspective. While prolegs and eyespots appear to derive from distinct ancestral GRNs co-opted to novel body regions, scales derive from in situ modifications of a sensory bristle GRN. The origin of the basal and central symmetry systems of bands on the wing is associated with the expression of the WntA gene in those regions, whereas the more marginal bands depend on two other genes, Distal-less and spalt. Finally, several genes have been discovered that play important roles in regulating background wing color, via the regulation of pigmentation GRNs. The identification of shared and novel cis-regulatory elements of genes belonging to these distinct GRNs helps trace the developmental and evolutionary history of these traits. Future work should examine the extent to which ancestral GRNs are co-opted/modified to produce the novel traits and how these GRNs map to specific cell types in ancestral and derived traits.

Genomic convergence in terrestrial root plants through tandem duplication in response to soil microbial pressures

Despite increasing reports of convergent adaptation, evidence for genomic convergence across diverse species worldwide is lacking. Here, our study of 205 Archaeplastida genomes reveals evidence of genomic convergence through tandem duplication (TD) across different lineages of root plants despite their genomic diversity. TD-derived genes, notably prevalent in trees with developed root systems embedded in soil, are enriched in enzymatic catalysis and biotic stress responses, suggesting adaptations to environmental pressures. Correlation analyses suggest that many factors, particularly those related to soil microbial pressures, are significantly associated with TD dynamics. Conversely, flora transitioned to aquatic, parasitic, halophytic, or carnivorous lifestyles-reducing their interaction with soil microbes-exhibit a consistent decline in TD frequency. This trend is further corroborated in mangroves that independently adapted to hypersaline intertidal soils, characterized by diminished microbial activity. Our findings propose TD-driven genomic convergence as a widespread adaptation to soil microbial pressures among terrestrial root plants.

Understanding developmental system drift

Developmental system drift (DSD) occurs when the genetic basis for homologous traits diverges over time despite conservation of the phenotype. In this Review, we examine the key ideas, evidence and open problems arising from studies of DSD. Recent work suggests that DSD may be pervasive, having been detected across a range of different organisms and developmental processes. Although developmental research remains heavily reliant on model organisms, extrapolation of findings to non-model organisms can be error-prone if the lineages have undergone DSD. We suggest how existing data and modelling approaches may be used to detect DSD and estimate its frequency. More direct study of DSD, we propose, can inform null hypotheses for how much genetic divergence to expect on the basis of phylogenetic distance, while also contributing to principles of gene regulatory evolution.

Multi-omics analyzes of Rosa gigantea illuminate tea scent biosynthesis and release mechanisms

Rose is an important ornamental crop cultivated globally for perfume production. However, our understanding of the mechanisms underlying scent production and molecular breeding for fragrance is hindered by the lack of a reference genome for tea roses. We present the first complete telomere-to-telomere (T2T) genome of Rosa gigantea, with high quality (QV > 60), including detailed characterization of the structural features of repetitive regions. The expansion of genes associated with phenylpropanoid biosynthesis may account for the unique tea scent. We uncover the release rhythm of aromatic volatile organic compounds and their gene regulatory networks through comparative genomics and time-ordered gene co-expression networks. Analyzes of eugenol homologs demonstrate how plants attract pollinators using specialized phenylpropanoids in specific tissues. This study highlights the conservation and utilization of genetic diversity from wild endangered species through multi-omics approaches, providing a scientific foundation for enhancing rose fragrance via de novo domestication.

A long noncoding RNA at the cortex locus controls adaptive coloration in butterflies

Evolutionary variation in the wing pigmentation of butterflies and moths offers striking examples of adaptation by crypsis and mimicry. The cortex locus has been independently mapped as the locus controlling color polymorphisms in 15 lepidopteran species, suggesting that it acts as a genomic hotspot for the diversification of wing patterns, but functional validation through protein-coding knockouts has proven difficult to obtain. Our study unveils the role of a long noncoding RNA (lncRNA) which we name ivory, transcribed from the cortex locus, in modulating color patterning in butterflies. Strikingly, ivory expression prefigures most melanic patterns during pupal development, suggesting an early developmental role in specifying scale identity. To test this, we generated CRISPR mosaic knock-outs in five nymphalid butterfly species and show that ivory mutagenesis yields transformations of dark pigmented scales into white or light-colored scales. Genotyping of Vanessa cardui germline mutants associates these phenotypes to small on-target deletions at the conserved first exon of ivory. In contrast, cortex germline mutant butterflies with confirmed null alleles lack any wing phenotype and exclude a color patterning role for this adjacent gene. Overall, these results show that a lncRNA gene acts as a master switch of color pattern specification and played key roles in the adaptive diversification of wing patterns in butterflies.

A distinct foliar pigmentation pattern formed by activator-repressor gradients upstream of an anthocyanin-activating R2R3-MYB

The emergence of novel traits is often preceded by a potentiation phase, when all the genetic components necessary for producing the trait are assembled. However, elucidating these potentiating factors is challenging. We have previously shown that an anthocyanin-activating R2R3-MYB, STRIPY, triggers the emergence of a distinct foliar pigmentation pattern in the monkeyflower Mimulus verbenaceus. Here, using forward and reverse genetics approaches, we identify three potentiating factors that pattern STRIPY expression: MvHY5, a master regulator of light signaling that activates STRIPY and is expressed throughout the leaf, and two leaf developmental regulators, MvALOG1 and MvTCP5, that are expressed in opposing gradients along the leaf proximodistal axis and negatively regulate STRIPY. These results provide strong empirical evidence that phenotypic novelties can be potentiated through incorporation into preexisting genetic regulatory networks and highlight the importance of positional information in patterning the novel foliar stripe.

Genome-wide analysis of histone modifications can contribute to the identification of candidate cis-regulatory regions in the threespine stickleback fish

BACKGROUND

Cis-regulatory mutations often underlie phenotypic evolution. However, because identifying the locations of promoters and enhancers in non-coding regions is challenging, we have fewer examples of identified causative cis-regulatory mutations that underlie naturally occurring phenotypic variations than of causative amino acid-altering mutations. Because cis-regulatory elements have epigenetic marks of specific histone modifications, we can detect cis-regulatory elements by mapping and analyzing them. Here, we investigated histone modifications and chromatin accessibility with cleavage under targets and tagmentation (CUT&Tag) and assay for transposase-accessible chromatin-sequencing (ATAC-seq).

RESULTS

Using the threespine stickleback (Gasterosteus aculeatus) as a model, we confirmed that the genes for which nearby regions showed active marks, such as H3K4me1, H3K4me3, and high chromatin accessibility, were highly expressed. In contrast, the expression levels of genes for which nearby regions showed repressive marks, such as H3K27me3, were reduced, suggesting that our chromatin analysis protocols overall worked well. Genomic regions with peaks of histone modifications showed higher nucleotide diversity within and between populations. By comparing gene expression in the gills of the marine and stream ecotypes, we identified several insertions and deletions (indels) with transposable element fragments in the candidate cis-regulatory regions.

CONCLUSIONS

Thus, mapping and analyzing histone modifications can help identify cis-regulatory elements and accelerate the identification of causative mutations in the non-coding regions underlying naturally occurring phenotypic variations.

Does coevolution in refugia drive mimicry in bumble bees? Insights from a South Asian mimicry group

Müllerian mimicry was proposed to be an example of a coevolved mutualism promoted by population isolation in glacial refugia. This, however, has not been well supported in butterfly models. Here, we use genomic data to test this theory while examining the population genetics behind mimetic diversification in a pair of co-mimetic bumble bees, Bombus breviceps Smith and Bombus trifasciatus Smith. In both lineages, populations were structured by geography but not as much by color pattern, suggesting sharing of color alleles across regions of restricted gene flow and formation of mimicry complexes in the absence of genetic differentiation. Demographic analyses showed mismatches between historical effective population size changes and glacial cycles, and niche modeling revealed only mild habitat retraction during glaciation. Moreover, mimetic subpopulations of the same color form in the two lineages only in some cases exhibit similar population history and genetic divergence. Therefore, the current study supports a more complex history in this comimicry than a simple refugium-coevolution model.

Signatures of Adaptation and Purifying Selection in Highland Populations of Dasiphora fruticosa

High mountains harbor a considerable proportion of biodiversity, but we know little about how diverse plants adapt to the harsh environment. Here we finished a high-quality genome assembly for Dasiphora fruticosa, an ecologically important plant distributed in the Qinghai-Tibetan Plateau and lowland of the Northern Hemisphere, and resequenced 592 natural individuals to address how this horticulture plant adapts to highland. Demographic analysis revealed D. fruticosa underwent a bottleneck after Naynayxungla Glaciation. Selective sweep analysis of two pairs of lowland and highland populations identified 63 shared genes related to cell wall organization or biogenesis, cellular component organization, and dwarfism, suggesting parallel adaptation to highland habitats. Most importantly, we found that stronger purging of estimated genetic load due to inbreeding in highland populations apparently contributed to their adaptation to the highest mountain. Our results revealed how plants could tolerate the extreme plateau, which could provide potential insights for species conservation and crop breeding.

Spatiotemporal dynamics in butterfly hybrid zones

Evaluating whether hybrid zones are stable or mobile can provide novel insights for evolution and conservation biology. Butterflies exhibit high sensitivity to environmental changes and represent an important model system for the study of hybrid zone origins and maintenance. Here, we review the literature exploring butterfly hybrid zones, with a special focus on their spatiotemporal dynamics and the potential mechanisms that could lead to their movement or stability. We then compare different lines of evidence used to investigate hybrid zone dynamics and discuss the strengths and weaknesses of each approach. Our goal with this review is to reveal general conditions associated with the stability or mobility of butterfly hybrid zones by synthesizing evidence obtained using different types of data sampled across multiple regions and spatial scales. Finally, we discuss spatiotemporal dynamics in the context of a speciation/divergence continuum, the relevance of hybrid zones for conservation biology, and recommend key topics for future investigation.

Adaptive introgression of a visual preference gene

Visual preferences are important drivers of mate choice and sexual selection, but little is known of how they evolve at the genetic level. In this study, we took advantage of the diversity of bright warning patterns displayed by Heliconius butterflies, which are also used during mate choice. Combining behavioral, population genomic, and expression analyses, we show that two Heliconius species have evolved the same preferences for red patterns by exchanging genetic material through hybridization. Neural expression of regucalcin1 correlates with visual preference across populations, and disruption of regucalcin1 with CRISPR-Cas9 impairs courtship toward conspecific females, providing a direct link between gene and behavior. Our results support a role for hybridization during behavioral evolution and show how visually guided behaviors contributing to adaptation and speciation are encoded within the genome.

The diversification of butterfly wing patterns: progress and prospects

Butterfly wings display rich phenotypic diversity and are associated with complex biological functions, thus serving as an important evolutionary system to address the genetic basis and evolution of phenotypic diversification. We review recent butterfly studies that revealed complex functions underlying diversified wing patterns and describe the genetic and environmental factors involved in wing pattern determinations. These factors lead to inter-specific divergence, genetic polymorphism, and phenotypic plasticity, which in many cases are decided by several key genes. We also summarize the research advances on gene co-option as an important origin of functional complexity and evolutionary novelty. These findings reveal a pattern of evolutionary innovation within a constrained developmental framework during butterfly wing morphogenesis, but further research is required to gain a systematic and comprehensive understanding.

Cis-regulatory modes of Ultrabithorax inactivation in butterfly forewings

Hox gene clusters encode transcription factors that drive regional specialization during animal development: for example the Hox factor Ubx is expressed in the insect metathoracic (T3) wing appendages and differentiates them from T2 mesothoracic identities. Hox transcriptional regulation requires silencing activities that prevent spurious activation and regulatory crosstalks in the wrong tissues, but this has seldom been studied in insects other than Drosophila, which shows a derived Hox dislocation into two genomic clusters that disjoined Antennapedia (Antp) and Ultrabithorax (Ubx). Here, we investigated how Ubx is restricted to the hindwing in butterflies, amidst a contiguous Hox cluster. By analysing Hi-C and ATAC-seq data in the butterfly Junonia coenia, we show that a Topologically Associated Domain (TAD) maintains a hindwing-enriched profile of chromatin opening around Ubx. This TAD is bordered by a Boundary Element (BE) that separates it from a region of joined wing activity around the Antp locus. CRISPR mutational perturbation of this BE releases ectopic Ubx expression in forewings, inducing homeotic clones with hindwing identities. Further mutational interrogation of two non-coding RNA encoding regions and one putative cis-regulatory module within the Ubx TAD cause rare homeotic transformations in both directions, indicating the presence of both activating and repressing chromatin features. We also describe a series of spontaneous forewing homeotic phenotypes obtained in Heliconius butterflies, and discuss their possible mutational basis. By leveraging the extensive wing specialization found in butterflies, our initial exploration of Ubx regulation demonstrates the existence of silencing and insulating sequences that prevent its spurious expression in forewings.

Genetic Modification of a Hox Locus Drives Mimetic Color Pattern Variation in a Highly Polymorphic Bumble Bee

Müllerian mimicry provides natural replicates ideal for exploring mechanisms underlying adaptive phenotypic divergence and convergence, yet the genetic mechanisms underlying mimetic variation remain largely unknown. The current study investigates the genetic basis of mimetic color pattern variation in a highly polymorphic bumble bee, Bombus breviceps (Hymenoptera, Apidae). In South Asia, this species and multiple comimetic species converge onto local Müllerian mimicry patterns by shifting the abdominal setal color from orange to black. Genetic crossing between the orange and black phenotypes suggested the color dimorphism being controlled by a single Mendelian locus, with the orange allele being dominant over black. Genome-wide association suggests that a locus at the intergenic region between 2 abdominal fate-determining Hox genes, abd-A and Abd-B, is associated with the color change. This locus is therefore in the same intergenic region but not the same exact locus as found to drive red black midabdominal variation in a distantly related bumble bee species, Bombus melanopygus. Gene expression analysis and RNA interferences suggest that differential expression of an intergenic long noncoding RNA between abd-A and Abd-B at the onset setal color differentiation may drive the orange black color variation by causing a homeotic shift late in development. Analysis of this same color locus in comimetic species reveals no sequence association with the same color shift, suggesting that mimetic convergence is achieved through distinct genetic routes. Our study establishes Hox regions as genomic hotspots for color pattern evolution in bumble bees and demonstrates how pleiotropic developmental loci can drive adaptive radiations in nature.

Hybrid Epigenomes Reveal Extensive Local Genetic Changes to Chromatin Accessibility Contribute to Divergence in Embryonic Gene Expression Between Species

Chromatin accessibility plays an important role in shaping gene expression, yet little is known about the genetic and molecular mechanisms that influence the evolution of chromatin configuration. Both local (cis) and distant (trans) genetic influences can in principle influence chromatin accessibility and are based on distinct molecular mechanisms. We, therefore, sought to characterize the role that each of these plays in altering chromatin accessibility in 2 closely related sea urchin species. Using hybrids of Heliocidaris erythrogramma and Heliocidaris tuberculata, and adapting a statistical framework previously developed for the analysis of cis and trans influences on the transcriptome, we examined how these mechanisms shape the regulatory landscape at 3 important developmental stages, and compared our results to similar analyses of the transcriptome. We found extensive cis- and trans-based influences on evolutionary changes in chromatin, with cis effects generally larger in effect. Evolutionary changes in accessibility and gene expression are correlated, especially when expression has a local genetic basis. Maternal influences appear to have more of an effect on chromatin accessibility than on gene expression, persisting well past the maternal-to-zygotic transition. Chromatin accessibility near gene regulatory network genes appears to be distinctly regulated, with trans factors appearing to play an outsized role in the configuration of chromatin near these genes. Together, our results represent the first attempt to quantify cis and trans influences on evolutionary divergence in chromatin configuration in an outbred natural study system and suggest that chromatin regulation is more genetically complex than was previously appreciated.

Hierarchical morphogenesis of swallowtail butterfly wing scale nanostructures

The study of color patterns in the animal integument is a fundamental question in biology, with many lepidopteran species being exemplary models in this endeavor due to their relative simplicity and elegance. While significant advances have been made in unraveling the cellular and molecular basis of lepidopteran pigmentary coloration, the morphogenesis of wing scale nanostructures involved in structural color production is not well understood. Contemporary research on this topic largely focuses on a few nymphalid model taxa (e.g., Bicyclus, Heliconius), despite an overwhelming diversity in the hierarchical nanostructural organization of lepidopteran wing scales. Here, we present a time-resolved, comparative developmental study of hierarchical scale nanostructures in Parides eurimedes and five other papilionid species. Our results uphold the putative conserved role of F-actin bundles in acting as spacers between developing ridges, as previously documented in several nymphalid species. Interestingly, while ridges are developing in P. eurimedes, plasma membrane manifests irregular mesh-like crossribs characteristic of Papilionidae, which delineate the accretion of cuticle into rows of planar disks in between ridges. Once the ridges have grown, disintegrating F-actin bundles appear to reorganize into a network that supports the invagination of plasma membrane underlying the disks, subsequently forming an extruded honeycomb lattice. Our results uncover a previously undocumented role for F-actin in the morphogenesis of complex wing scale nanostructures, likely specific to Papilionidae.

Evolutionary origin of genomic structural variations in domestic yaks

Yak has been subject to natural selection, human domestication and interspecific introgression during its evolution. However, genetic variants favored by each of these processes have not been distinguished previously. We constructed a graph-genome for 47 genomes of 7 cross-fertile bovine species. This allowed detection of 57,432 high-resolution structural variants (SVs) within and across the species, which were genotyped in 386 individuals. We distinguished the evolutionary origins of diverse SVs in domestic yaks by phylogenetic analyses. We further identified 334 genes overlapping with SVs in domestic yaks that bore potential signals of selection from wild yaks, plus an additional 686 genes introgressed from cattle. Nearly 90% of the domestic yaks were introgressed by cattle. Introgression of an SV spanning the KIT gene triggered the breeding of white domestic yaks. We validated a significant association of the selected stratified SVs with gene expression, which contributes to phenotypic variations. Our results highlight that SVs of different origins contribute to the phenotypic diversity of domestic yaks.

Evolutionary dynamics of genome size and content during the adaptive radiation of Heliconiini butterflies

Heliconius butterflies, a speciose genus of Müllerian mimics, represent a classic example of an adaptive radiation that includes a range of derived dietary, life history, physiological and neural traits. However, key lineages within the genus, and across the broader Heliconiini tribe, lack genomic resources, limiting our understanding of how adaptive and neutral processes shaped genome evolution during their radiation. Here, we generate highly contiguous genome assemblies for nine Heliconiini, 29 additional reference-assembled genomes, and improve 10 existing assemblies. Altogether, we provide a dataset of annotated genomes for a total of 63 species, including 58 species within the Heliconiini tribe. We use this extensive dataset to generate a robust and dated heliconiine phylogeny, describe major patterns of introgression, explore the evolution of genome architecture, and the genomic basis of key innovations in this enigmatic group, including an assessment of the evolution of putative regulatory regions at the Heliconius stem. Our work illustrates how the increased resolution provided by such dense genomic sampling improves our power to generate and test gene-phenotype hypotheses, and precisely characterize how genomes evolve.