Parallel evolution of opsin visual pigments in hawkmoths by tuning of spectral sensitivities during transition from a nocturnal to a diurnal ecology

Genome assembly of a nocturnal butterfly (Macrosoma leucophasiata) reveals convergent adaptation of visual genes

Nearly all animals exhibit a preferred period of daily activity (diel-niche), strongly influenced by the light environment. Vision is a sensory system that is strongly adapted to light, and evolutionary transitions to novel light environments can impose strong constraints on eye evolution, color, and motion vision. While the genetic and neural basis of visual adaptation are well-studied in a few model systems, our understanding across the tree of life remains incomplete. Butterflies and moths are an ideal system to investigate the association between gene evolution and diel-niche transitions. While most butterflies are day-flying, hedylid butterflies are unique in being primarily nocturnal, representing an important evolutionary shift from diurnality to nocturnality. We sequenced the first Hedylidae genome and annotated it to understand genomic changes associated with diel niche shifts. Comparing Hedylidae visual genes to those of other diurnal and nocturnal Lepidoptera revealed that visual genes are highly conserved, with no major losses. However, hedylid opsins were more similar to nocturnal moths than their diurnal congeners, suggesting that these opsins convergently evovled to adapt to the nocturnal environment. Evolutionary rate tests (dN/dS) confirmed strong selection on color vision opsins, with some sites being mapped to the functional domain of the blue opsin. Our study provides new insight into the molecular evolutionary adaptations associated with species' changes to new light environments.

Chromosome-level genome assembly of the forest pest Achelura yunnanensis (Lepidoptera: Zygaenidae)

Achelura yunnanensis is a destructive pest of forests, causing substantial damage on tree growth and severe economic losses. Additionally, as a daytime-active moth, this species also holds important scientific value for investigating the genetic mechanisms governing day-night activity patterns of Lepidoptera. To facilitate effective pest control and deepen our understanding of the diurnal behavior's genetic basis of moths, genomic data for this species are crucial. In this study, we present a chromosome-level reference genome of A. yunnanensis (368.15 Mb in 32 chromosomes; scaffold N50 = 12.61 Mb; BUSCO completeness = 98.0%). Genome annotation shows that the new assembly comprises 37.10% (136.55 Mb) repetitive elements, 1,828 non-coding RNAs, and 15,523 protein-coding genes. Genes involved in lipid metabolism and xenobiotics biodegradation and metabolism, such as cytochrome P450 families, experienced significant expansion in the A. yunnanensis genome. The chromosome-level genome of A. yunnanensis provides a valuable genomic resource for devising novel pest control strategies, and will also help to study the genetic mechanism of the shift of diurnal behavior in Lepidoptera.

Day-night gene expression reveals circadian gene disco as a candidate for diel-niche evolution in moths

Temporal ecological niche partitioning is an underappreciated driver of speciation. While insects have long been models for circadian biology, the genes and circuits that allow adaptive changes in diel-niches remain poorly understood. We compared gene expression in closely related day- and night-active non-model wild silk moths, with otherwise similar ecologies. Using an ortholog-based pipeline to compare RNA-Seq patterns across two moth species, we find over 25 pairs of gene orthologs showing differential expression. Notably, the gene disco, involved in circadian control, optic lobe and clock neuron development in Drosophila, shows robust adult circadian mRNA cycling in moth heads. Disco is highly conserved in moths and has additional zinc-finger domains with specific nocturnal and diurnal mutations. We propose disco as a candidate gene for the diversification of temporal diel-niche in moths.

Brain size scaling through development in the whitelined sphinx moth (Hyles lineata) shows mass and cell number comparable to flies, bees, and wasps

Factors regulating larval growth and determinants of adult body size are described for several holometabolous insects, but less is known about brain size scaling through development. Here we use the isotropic fractionation ("brain soup") method to estimate the number of brain cells and cell density for the whitelined sphinx moth (Lepidoptera: Hyles lineata) from the first instar through the adult stage. We measure mass and brain cell number and find that, during the larval stages, body mass shows an exponential relationship with head width, while the total number of brain cells increases asymptotically. Larval brain cell number increases by a factor of ten from nearly 8000 in the first instar to over 80,000 in the fifth instar. Brain cell number increases by another factor of 10 during metamorphosis, with the adult brain containing more than 900,000 cells. This is similar to increases during development in the vinegar fly (Drosophila melanogaster) and the black soldier fly (Hermetia illucens). The adult brain falls slightly below the brain-to-body allometry for wasps and bees but is comparable in the number of cells per unit brain mass, indicating a general conservation of brain cell density across these divergent lineages.

Selection on Visual Opsin Genes in Diurnal Neotropical Frogs and Loss of the SWS2 Opsin in Poison Frogs

Amphibians are ideal for studying visual system evolution because their biphasic (aquatic and terrestrial) life history and ecological diversity expose them to a broad range of visual conditions. Here, we evaluate signatures of selection on visual opsin genes across Neotropical anurans and focus on three diurnal clades that are well-known for the concurrence of conspicuous colors and chemical defense (i.e., aposematism): poison frogs (Dendrobatidae), Harlequin toads (Bufonidae: Atelopus), and pumpkin toadlets (Brachycephalidae: Brachycephalus). We found evidence of positive selection on 44 amino acid sites in LWS, SWS1, SWS2, and RH1 opsin genes, of which one in LWS and two in RH1 have been previously identified as spectral tuning sites in other vertebrates. Given that anurans have mostly nocturnal habits, the patterns of selection revealed new sites that might be important in spectral tuning for frogs, potentially for adaptation to diurnal habits and for color-based intraspecific communication. Furthermore, we provide evidence that SWS2, normally expressed in rod cells in frogs and some salamanders, has likely been lost in the ancestor of Dendrobatidae, suggesting that under low-light levels, dendrobatids have inferior wavelength discrimination compared to other frogs. This loss might follow the origin of diurnal activity in dendrobatids and could have implications for their behavior. Our analyses show that assessments of opsin diversification in across taxa could expand our understanding of the role of sensory system evolution in ecological adaptation.