Role of visual and olfactory cues in sex recognition in butterfly Cethosia cyane cyane

Tympanal ears mediate male-male competition, courtship and mating success in Bicyclus anynana butterflies

The presence of intra-specific acoustic communication in diurnal butterflies is not well established. Here, we examined the function of the tympanal ear (Vogel's organ, VO) in the seasonally polyphenic butterfly Bicyclus anynana in the context of sexual signalling. We investigated how the VO and the flanking enlarged veins, which are suggested sound resonance chambers, scale with wing size across sexes and seasonal forms, and how disruptions to the VO alter courtship behaviour and mating outcomes. We found that males have VOs similar in size to females despite having smaller wings, and dry season (DS) male cubital and anal veins do not scale with the wing size. This suggests that the VO plays an important role in males and that cubital and anal veins in DS males may be tuned to amplify specific sound frequencies. Behavioural assays performed with deafened and hearing males of different seasonal forms, in pair and triad settings, showed that deafened DS males, but not wet season males, experienced lower mating success relative to their hearing counterparts. Our study documents a novel function for the wing tympanal membrane in mediating courtship and mating outcomes in diurnal butterflies.

Colour Selection and Olfactory Responses of Papilio demoleus during Foraging and Courtship

Colours and odours are the most important cues for butterflies to localise food and mating partners. We studied the visual and olfactory responses of the widely distributed butterfly Papilio demoleus Linnaeus during foraging and courtship. P. demoleus visited odourless flowers with six colours except green and black, with red as the favourite colour (650-780 nm). Males and females differed in behaviour while visiting flowers. Males were more active than females during foraging. The application of honey water resulted in a significant increase in flower visits by both females and males, and they scarcely visited the apetalous branches with odours. Under natural conditions, four patterns were observed: males chasing males (42.28%), males chasing females (30.56%), females chasing females (13.73%), and females chasing males (13.43%). Males chasing males was the most frequent, probably because males drive away competing con-specific males. When butterflies visited odourless mimics, males chasing females (70.73%) and males chasing males (29.27%) also occurred, indicating that males could accurately distinguish mates using colours only without any chemical cues, and females need chemical cues. The behavioural responses of P. demoleus to floral visits and courtship suggest that colour is the dominant factor during foraging and courtship. We verified the presence of P. demoleus rhodopsin genes, including Rh2, Rh3, Rh4, and Rh5, for long wavelength, blue, and ultraviolet (UV) spectrum recognition, which is consistent with the colour recognition of flowers and wings during visiting flowers and courtship.

Exploitation of an ancestral pheromone biosynthetic pathway contributes to diversification in Heliconius butterflies

During courtship, male butterflies of many species produce androconial secretions containing male sex pheromones (MSPs) that communicate species identity and affect female choice. MSPs are thus likely candidates as reproductive barriers, yet their role in speciation remains poorly studied. Although Heliconius butterflies are a model system in speciation, their MSPs have not been investigated from a macroevolutionary perspective. We use GC/MS to characterize male androconial secretions in 33 of the 69 species in the Heliconiini tribe. We found these blends to be species-specific, consistent with a role in reproductive isolation. We detected a burst in blend diversification rate at the most speciose genus, Heliconius; a consequence of Heliconius and Eueides species using a fatty acid (FA) metabolic pathway to unlock more complex blends than basal Heliconiini species, whose secretions are dominated by plant-like metabolites. A comparison of 10 sister species pairs demonstrates a striking positive correlation between blend dissimilarity and range overlap, consistent with character displacement or reinforcement in sympatry. These results demonstrate for the first time that MSP diversification can promote reproductive isolation across this group of butterflies, showcasing how implementation of an ancestral trait, the co-option of the FA metabolic pathway for pheromone production, can facilitate rapid speciation.

Comparative analysis on visual and olfactory signals of Papilio xuthus (Lepidoptera: Papilionidae) during foraging and courtship

This study examined the roles of visual and olfactory responses during foraging and courtship in butterfly Papilio xuthus. P. xuthus showed obvious orientation to color in the range of 350–500 nm. Visits of P. xuthus females and males to blue, purple, and red artificial cloth flowers were ♀ 54.90% and ♂ 39.22%, ♀ 19.61% and ♂ 35.29%, and ♀ 9.80% and ♂ 19.61%, respectively. Application of 10% honey on these artificial flowers resulted in an increase of 3.41 and 3.26 fold in flower visits by the butterfly compared to controls. When 10% honey water was sprayed on flower branches without colorful flowers, branch visiting was very low, only seven times for females and two times for males, indicating that colors might be more critical than odor for foraging even though visual and olfactory perceptions both play important roles during foraging. During courtship, four types of chasing were observed in a natural population of P. Xuthus; the four types are males chasing females (49%), males chasing males (25%), females chasing males (13%), and females chasing females (10%). However, when odorless artificial models of butterflies were used, no significant differences were observed among these types of chasing, indicating that olfactory perception was crucial for the butterfly during courtship. Profiling volatile organic chemicals (VOCs) and individual bioassays revealed that VOCs contents of butterflies were not related to recognizing sex partners; by contrast, some level of α-farnesene, increased the frequency of male chasing female. This could be due to that α-farnesene is easy to be detected by butterflies because of its volatility and higher content in female.

The scent chemistry of butterflies

Butterflies use structurally highly diverse volatile compounds for communication, in addition to visual signals. These compounds originate from plants or a formed de novo especially by male butterflies that possess specific scent organs.

Re-emergence and diversification of a specialized antennal lobe morphology in ithomiine butterflies

How an organism's sensory system functions is central to how it navigates its environment. The insect olfactory system is a prominent model for investigating how ecological factors impact sensory reception and processing. Notably, work in Lepidoptera led to the discovery of vastly expanded structures, termed macroglomerular complexes (MGCs), within the primary olfactory processing centre. MGCs typically process pheromonal cues, are usually larger in males, and provide classic examples of how variation in the size of neural structures reflects the importance of sensory cues. Though prevalent across moths, MGCs were lost during the origin of butterflies, consistent with evidence that courtship initiation in butterflies is primarily reliant on visual cues, rather than long distance chemical signals. However, an MGC was recently described in a species of ithomiine butterfly, suggesting that this once lost neural adaptation has re-emerged in this tribe. Here, we show that MGC-like morphologies are widely distributed across ithomiines, but vary in both their structure and prevalence of sexual dimorphism. Based on this interspecific variation we suggest that the ithomiine MGC is involved in processing both plant and pheromonal cues, which have similarities in their chemical constitution, and co-evolved with an increased importance of plant derived chemical compounds.

Evidence for UV-green dichromacy in the basal hymenopteran Sirex noctilio (Siricidae)

A precondition for colour vision is the presence of at least two spectral types of photoreceptors in the eye. The order Hymenoptera is traditionally divided into the Apocrita (ants, bees, wasps) and the Symphyta (sawflies, woodwasps, horntails). Most apocritan species possess three different photoreceptor types. In contrast, physiological studies in the Symphyta have reported one to four photoreceptor types. To better understand the evolution of photoreceptor diversity in the Hymenoptera, we studied the Symphyta Sirex noctilio, which belongs to the superfamily Siricoidea, a closely related group of the Apocrita suborder. Our aim was to (i) identify the photoreceptor types of the compound eye by electroretinography (ERG), (ii) characterise the visual opsin genes of S. noctilio by genomic comparisons and phylogenetic analyses and (iii) analyse opsin mRNA expression. ERG measurements revealed two photoreceptor types in the compound eye, maximally sensitive to 527 and 364 nm. In addition, we identified three opsins in the genome, homologous to the hymenopteran green or long-wavelength sensitive (LW) LW1, LW2 and ultra-violet sensitive (UV) opsin genes. The LW1 and UV opsins were found to be expressed in the compound eyes, and LW2 and UV opsins in the ocelli. The lack of a blue or short-wavelength sensitive (SW) homologous opsin gene and a corresponding receptor suggests that S. noctilio is a UV-green dichromate.

The Dryas iulia Genome Supports Multiple Gains of a W Chromosome from a B Chromosome in Butterflies

In butterflies and moths, which exhibit highly variable sex determination mechanisms, the homogametic Z chromosome is deeply conserved and is featured in many genome assemblies. The evolution and origin of the female W sex chromosome, however, remains mostly unknown. Previous studies have proposed that a ZZ/Z0 sex determination system is ancestral to Lepidoptera, and that W chromosomes may originate from sex-linked B chromosomes. Here, we sequence and assemble the female Dryas iulia genome into 32 highly contiguous ordered and oriented chromosomes, including the Z and W sex chromosomes. We then use sex-specific Hi-C, ATAC-seq, PRO-seq, and whole-genome DNA sequence data sets to test if features of the D. iulia W chromosome are consistent with a hypothesized B chromosome origin. We show that the putative W chromosome displays female-associated DNA sequence, gene expression, and chromatin accessibility to confirm the sex-linked function of the W sequence. In contrast with expectations from studies of homologous sex chromosomes, highly repetitive DNA content on the W chromosome, the sole presence of domesticated repetitive elements in functional DNA, and lack of sequence homology with the Z chromosome or autosomes is most consistent with a B chromosome origin for the W, although it remains challenging to rule out extensive sequence divergence. Synteny analysis of the D. iulia W chromosome with other female lepidopteran genome assemblies shows no homology between W chromosomes and suggests multiple, independent origins of the W chromosome from a B chromosome likely occurred in butterflies.

Linking ecological specialisation to adaptations in butterfly brains and sensory systems

Butterflies display incredible ecological and behavioural diversity. As such, they have been subject to intense study since the birth of evolutionary biology. However, with some possible exceptions, they are underused models in comparative and functional neurobiology. We highlight a series of areas, spanning sensory ecology to cognition, in which butterflies are particularly promising systems for investigating the neurobiological basis for behavioural or ecological variation. These fields benefit from a history of molecular and quantitative genetics, and basic comparative neuroanatomy, but these strands of research are yet to be widely integrated. We discuss areas for potential growth and argue that new experimental techniques, growing genomic resources, and tools for functional genetics will accelerate the use of butterflies in neurobiology.

Uncertainty about flying conspecifics causes territorial contests of the Old World swallowtail, Papilio machaon

Background

Male-male aerial contests of territorial butterflies are difficult to explain by major contest models based on game theory because of butterflies' apparent inability to inflict substantial costs on their opponent. As an alternative, the "erroneous courtship hypothesis" was presented. This hypothesis is based on the assumption that territorial butterflies cannot discriminate the sex of flying conspecifics. The hypothesis regards the aerial contest of male butterflies as an inevitable same-sex entanglement in the butterflies' behavioral sequence. To test the robustness of the hypothesis, we investigated the sex recognition abilities of the Old World swallowtail, Papilio machaon.

Results

We presented four types of flapping butterfly specimens (fresh male and female, chemicals-removed male and female) to territorial males. The males touched fresh female specimens and showed typical courtship flight. For the other types of specimens, they rarely showed courtship flight although they approached or touched them. In addition, territorial males reacted longer to fresh males than to fresh females.

Conclusions

The results indicated that although territorial males recognize flying females as sexual partners by sensing their semiochemicals, they cannot identify flying conspecific males, and continue to gather information on them. P. machaon is one of the species whose behavior is most incompatible with the erroneous courtship hypothesis, as its males perform a typical courtship flight to flying females, suggesting the ability of sexing flying conspecifics. Nevertheless, the erroneous courtship hypothesis was not disproved by our results.

Chemically mediated species recognition in two sympatric Grayling butterflies: Hipparchia fagi and Hipparchia hermione (Lepidoptera: Nymphalidae, Satyrinae)

Pheromones are known to play an important role in butterfly courtship and may influence both individual reproductive success and reproductive isolation between species. Recent studies have focused on courtship in Hipparchia butterflies (Nymphalidae: Satyrinae) emphasizing morphological and behavioural traits, as well as genetic differences. Behavioural observations suggested a role for chemical cues in mate and species recognition, where the androconial scales on the forewings of these species may be involved in chemical communication between individuals. Cchemical-mediated signals have received relatively little attention in this genus. Here, we report the results of a three-year investigation of the volatile organic compounds (VOCs) released by Hipparchia fagi and H. hermione in order to identify differences in VOCs between these species where they live in syntopy. Our study was carried out using an array of cross-selective sensors known as an "Electronic Nose" (EN) that operates by converting chemical patterns into patterns of sensor signals. While the identity of volatile compounds remained unknown, sensor signals can be compared to identify similar or dissimilar chemical patterns. Based on the EN signals, our results showed that: 1) the two sexes have a similar VOCs pattern in H. fagi, while they significantly diverge in H. hermione; 2) VOCs patterns were different between females of the two species, while those of males were not.