Development and evolution of color patterns in ladybird beetles: A case study in Harmonia axyridis

Micro-CT data of complete metamorphosis process in Harmonia axyridis

Insect metamorphosis involves significant changes in insect internal structure and is thus a critical focus of entomological research. Investigating the morphological transformation of internal structures is vital to understanding the origins of adult insect organs. Beetles are among the most species-rich groups in insects, but the development and transformation of their internal organs have yet to be systematically documented. In this study, we have acquired a comprehensive dataset that includes 27 detailed whole-body tomographic image sets of Harmonia axyridis, spanning from the prepupal to the pupal stages. Utilizing this data, we have created intricate 3D models of key internal organs, encompassing the brain, ventral nerve cord, digestive and excretion systems, as well as the body wall muscles. These data documented the transformation process of these critical organs and correlations between the origin of adult and larval organs and can be used to enhance the understanding of holometabolous adult organ genesis and offers a valuable reference model for investigating complete metamorphosis in insects.

The loci of insect phenotypic evolution

Insects are important elements of terrestrial ecosystems because they pollinate plants, destroy crops, transmit diseases to livestock and humans, and are important components of food chains. Here, I used Gephebase, a manually curated database of genetic variants associated with natural and domesticated trait variation, to explore current knowledge about the genes and the mutations known to contribute to natural phenotypic variation in insects. Analysis of over 600 mutations reveals that data are concentrated toward certain species and traits and that experimental approaches have changed over time. The distribution of coding and cis-regulatory changes varies with traits, experimental approaches, and identified gene loci. Recent studies highlight the important role of standing variation, repeated mutations in hotspot genes, recombination, inversions, and introgression.

The origins of colour patterns in fossil insects revealed by maturation experiments

Many fossil insects show monochromatic colour patterns that may provide valuable insights into ancient insect behaviour and ecology. Whether these patterns reflect original pigmentary coloration is, however, unknown, and their formation mechanism has not been investigated. Here, we performed thermal maturation experiments on extant beetles with melanin-based colour patterns. Scanning electron microscopy reveals that melanin-rich cuticle is more resistant to degradation than melanin-poor cuticle: with progressive maturation, melanin-poor cuticle regions experience preferential thinning and loss, yet melanin-rich cuticle remains. Comparative analysis of fossil insects with monotonal colour patterns confirms that the variations in tone correspond to variations in preserved cuticle thickness. These preserved colour patterns can thus be plausibly explained as melanin-based patterning. Recognition of melanin-based colour patterns in fossil insects opens new avenues for interpreting the evolution of insect coloration and behaviour through deep time.

Global patterns of genomic and phenotypic variation in the invasive harlequin ladybird

BACKGROUND

The harlequin ladybird Harmonia axyridis (Coleoptera: Coccinellidae), native to Asia, has been introduced to other major continents where it has caused serious negative impacts on local biodiversity. Though notable advances to understand its invasion success have been made during the past decade, especially with then newer molecular tools, the conclusions reached remain to be confirmed with more advanced genomic analyses and especially using more samples from larger geographical regions across the native range. Furthermore, although H. axyridis is one of the best studied invasive insect species with respect to life history traits (often comparing invasive and native populations), the traits responsible for its colonization success in non-native areas warrant more research.

RESULTS

Our analyses of genome-wide nuclear population structure indicated that an eastern Chinese population could be the source of all non-native populations and revealed several putatively adaptive candidate genomic loci involved in body color variation, visual perception, and hemolymph synthesis. Our estimates of evolutionary history indicate (1) asymmetric migration with varying population sizes across its native and non-native range, (2) a recent admixture between eastern Chinese and American populations in Europe, (3) signatures of a large progressive, historical bottleneck in the common ancestors of both populations and smaller effective sizes of the non-native population, and (4) the southwest origin and subsequent dispersal routes within its native range in China. In addition, we found that while two mitochondrial haplotypes-Hap1 and Hap2 were dominant in the native range, Hap1 was the only dominant haplotype in the non-native range. Our laboratory observations in both China and USA found statistical yet slight differences between Hap1 and Hap2 in some of life history traits.

CONCLUSIONS

Our study on H. axyridis provides new insights into its invasion processes into other major continents from its native Asian range, reconstructs a geographic range evolution across its native region China, and tentatively suggests that its invasiveness may differ between mitochondrial haplotypes.

Melanin Synthesis Pathway Interruption: CRISPR/Cas9-mediated Knockout of dopa decarboxylase (DDC) in Harmonia axyridis (Coleoptera: Coccinellidae)

CRISPR/Cas9 technology is a very powerful genome editing tool and has been used in many insect species for functional genomics studies through targeted gene mutagenesis. Here, we successfully established CRISPR/Cas9 research platform in Asian multi-colored ladybird beetle, Harmonia axyridis, an important natural enemy in biological control. In this study, one pivotal gene dopa decarboxylase (DDC) in melanin synthesis was targeted by CRISPR/Cas9 to generate mutants in H. axyridis by CRISPR/Cas9 technology. Our results showed that injection of single guide RNA of the DDC and Cas9 protein into preblastoderm eggs induced one insertion and four deletion (indels) mutant H. axyridis. Mutations of HaDDC gene generated 25% mutant rate with melanin missing phenotype in larva, pupa,l and adult stage. The predation ability of the fourth instar larvae has no significant difference between wild (control) and mutant H. axyridis (G0), while these mutant fourth instar larvae had longer developmental period than that of the wild type. Consequently, the total predation of the fourth instar larvae was significantly increased in H. axyridis mutants comparing with the wild type. These results indicated that the success of CRISPR/Cas9 gene editing in H. axyridis. The gene editing platform in H. axyridis would facilitate the gene function research and promote special strain of predatory ladybird beetle generation.

Hiding in plain sight: Cryptic enemies are found on cochineal (Hemiptera: Dactylopiidae), a scale insect of economic and cultural significance

Cochineal is the common name for cactus‐feeding scale insects in the Dactylopiidae. These ruby‐red insects include the domesticated dye insect Dactylopius coccus. Dactylopius coccus and congeners have been introduced around the world, some accidentally, to become pests of prickly pear cactus species (Opuntia), and some intentionally, for dye production or biological control of pest Opuntia. In the northern Sonoran Desert (Tucson, AZ, USA), we studied the enemy complex of D. opuntiae and D. confusus on Opuntia and characterized two cryptic enemies, a coccinellid beetle predator and a parasitoid wasp. (1) Hyperaspis sp. The coccinellid predator Hyperaspis trifurcata shares a niche with a similar, typically all‐black beetle. Morphological data, crossing tests, and phylogenetic results showed the black beetle to be a distinct, undescribed species in the genus Hyperaspis, with a rare spotted phenotype that is similar in appearance to H. trifurcata. Crossing tests among black and spotted forms showed the spotted morph is inherited as a single‐locus dominant allele. (2) Formicencyrtus thoreauini. Rearing of this ant‐like parasitoid wasp (Encyrtidae) in pure culture of D. opuntiae showed it to be a semi‐gregarious primary parasitoid of cochineal. To our knowledge, this is the first confirmed instance of a cochineal parasitoid. Observations of development show early instar larvae keep their posterior end within the egg chorion, attached to an aeroscopic plate with a connection to the cochineal body wall. Late instar larvae are eventually surrounded by a membrane, likely of larval origin. Wasps then pupate in a dry air‐filled chamber within the desiccated scale remains before chewing out as an adult. Both Hyperaspis sp. and F. thoreauini may have restricted distributions. Hyperaspis sp. does not appear to be a member of the cochineal community in Mexico or Texas, and scant records suggest F. thoreauini may also be restricted to the Southwestern USA.

The Molecular Properties and Roles of Pannier in Harmonia axyridis's Metamorphosis and Melanin Synthesis

The GATA transcription factor Pannier is identified as the major regulatory gene in color pattern formation in the Asian multi-colored ladybird beetle (Harmonia axyridis). however, the mechanisms of Pannier in regulating melanin synthesis and development in H. axyridis remain elusive. In this study, we identified and characterized Pannier in H. axyridis (HaPnr) and showed it to have two alternative spliced variants named HaPnr-α and HaPnr-β. Analyses of developmental stage expression patterns revealed that HaPnr, HaPnr-α and HaPnr-β were constitutively expressed throughout all developmental stages. To examine the role of HaPnr in H. axyridis development, RNA interference was performed in late larvae (the fourth instar) and early pupae (the first day of pupa stage). The transcript levels of HaPnr were effectively suppressed after the injection of double-stranded RNA of HaPnr (dsHaPnr). The fourth instar larvae injected with dsHaPnr reduced the pupation rates to only 61.50%, compared with 88.5% in the dsGFP-injected group. The un-pupated larvae gradually died after 1 week, and visually unaffected pupae emerged into abnormal adults with malformed hind wings and melanin absent from the cuticle. These abnormal adults gradually died 10 days after eclosion. However, when early pupae were injected with dsHaPnr, the normal eclosion rate was achieved at 88.41% on day 6 after the injection. In addition, these successful eclosion adults also showed an absence of melanin in the cuticle, but they could mate normally and have normal fecundity as compared with the control. We further demonstrated that the suppression of HaPnr-α or HaPnr-β individually did not affect the pupation and eclosion process. The suppression of HaPnr-α expression resulted in elytra melanin decreasing in both the conspicua and the succinea subgroup in H. axyridis. Even though the suppression of HaPnr-β expression only affected the melanin synthesis in the succinea subgroup, it significantly prolonged the time taken for melanin synthesis to occur in the conspicua subgroup in H. axyridis. These results indicate that HaPnr plays an essential role in insect development, especially during their metamorphosis, and also support our hypothesis that HaPnr could regulate melanin synthesis in H. axyridis under the combined action with its two splicing variants, HaPnr-α and HaPnr-β.

Erotylina Curran (Coleoptera, Erotylidae, Erotylini): redescription of type species, potential species groups and diversity of color patterns

The colorful genus Erotylina Curran was proposed to include a group of species originally described in Erotylus Fabricius but differing by the lack of three longitudinal carinae on the tibiae. The taxonomy of Erotylina is mostly based on coloration while information on mouthparts, thorax, male and female abdominal terminalia remain unknown. In addition, little is known about the factors underlying coloration in Erotylina and Erotylidae in general. Here we redescribe the type species of Erotylina, Erotylus leopardus Lacordaire providing the first descriptions of several morphological structures, and new data on its geographical distribution. Erotylus nicaraguae Crotch is proposed as new junior synonymy of E. leoparda. Lectotypes are designated for the following species: Erotylus leopardus Lacordaire, Erotylus nicaraguae Crotch and Erotylus confluens Crotch. Based on the geographical records, specimen labels, literature and online environmental databases, we show that E. leoparda includes two discrete morphs distributed across a latitudinal gradient, with intermediate and continuous variations distributed across altitudinal and temperature gradients. The present study sheds light on the taxonomy of Erotylina and provides the first clearest evidence of a relation between coloration, latitude, altitude, temperature variations and the distributional patterns of a taxon in Erotylidae.

Evo-devo of wing colour patterns in beetles

Insects have evolved tremendously diverse wing colour patterns that fulfil ecologically vital functions, including intraspecific sexual signalling, mimesis, mimicry, and detering predators. Beetles, which form the most species-rich order Coleoptera, have amazingly diverse wing colour patterns; however, the molecular mechanisms that give rise to these patterns remain poorly understood. Recently, the gene pannier (pnr), which encodes a transcription factor of the GATA family, was identified as an essential player in the wing patterning of the multi-coloured Asian ladybird beetle Harmonia axyridis. Here we review recent progress in understanding the molecular underpinnings of wing colour pattern formation in H. axyridis.

Molecular and Potential Regulatory Mechanisms of Melanin Synthesis in Harmonia axyridis

Melanization is a common phenomenon in insects, and melanin synthesis is a conserved physiological process that occurs in epidermal cells. Moreover, a comprehensive understanding of the mechanisms of melanin synthesis influencing insect pigmentation are well-suited for investigating phenotype variation. The Asian multi-colored (Harlequin) ladybird beetle, Harmonia axyridis, exhibits intraspecific polymorphism based on relative levels of melanization. However, the specific characteristics of melanin synthesis in H. axyridis remains elusive. In this study, we performed gene-silencing analysis of the pivotal inverting enzyme, tyrosine hydroxylase (TH), and DOPA decarboxylase (DDC) in the tyrosine metabolism pathway to investigate the molecular and regulatory mechanism of melanin synthesis in H. axyridis. Using RNAi of TH and DDC genes in fourth instar larvae, we demonstrated that dopamine melanin was the primary contributor to the overall body melanization of H. axyridis. Furthermore, our study provides the first conclusive evidence that dopamine serves as a melanin precursor for synthesis in the early pupal stage. According to transcription factor Pannier, which is essential for the formation of melanic color on the elytra in H. axyridis, we further demonstrated that suppression of HaPnr can significantly decrease expression levels of HaTH and HaDDC. These results in their entirety lead to the conclusion that transcription factor Pannier can regulate dopamine melanin synthesis in the dorsal elytral epidermis of H. axyridis.

Fifty Shades of the Harlequin Ladybird and a Sexually Transmitted Fungus

The ectoparasitic fungus Hesperomyces virescens was studied on its invasive host, the harlequin ladybird Harmonia axyridis, in the Czech Republic. A primary aim was to examine the relationship between fungal infection and elytral coloration of the ladybird. Furthermore, the role of host sex and mating status of females were analyzed. Beetles (n = 1,102) were sampled during autumn migration, and then sexed, weighed, and screened for infection. Females were dissected for detection of sperm in their spermathecae. Ladybirds were sorted according to color form and absorbance spectrophotometry was used to quantify carotenoid contents in their elytra. In individuals of the nonmelanic succinea form, the degree of melanization was measured using digital photographs and putative age groups were estimated based on background color of elytra. Sexual differences in infection patterns indicated transmission during copulation: males were infected mostly on elytra and venter, and females had infection almost exclusively on elytra. Mated females had higher infection rate than virgins. There was no influence of genetic color form on the fungal infection. Putative age groups (visual sorting to yellow, orange, and red) correlated with fungal infection. Infected individuals had elevated elytral carotenoid levels in comparison to uninfected individuals, which could be explained by host age. Infection-free succinea beetles were extensively melanized because they emerged later in the season at lower temperatures which induced melanization. Overall, we highlight that H. axyridis is a multivoltine species whose age, if not taken into account in ecophysiological studies, might present a considerable confounding factor.

The Role of the Dopamine Melanin Pathway in the Ontogeny of Elytral Melanization in Harmonia axyridis

Polymorphic melanism in insects is a conspicuous phenotype which is derived from specific genotypes, and might be central to speciation processes via assortative sexual selection. At the molecular level, melanism in insects is attributed to the melanin pathway. DOPA decarboxylase (DDC) protein encoded by the DDC gene plays a central role in dopamine-melanin synthesis, the main component of melanin in insects. Although the mechanism of melanism has been elucidated in holometabolous insects, other physiological processes coupled with melanin synthesis are unknown. Herein, we identified DDC from the Asian multi-colored ladybird (Harmonia axyridis), an ideal holometabolous insect for studies of melanization due to highly variable color on their elytra. Analyses revealed that HaDDC (the DDC gene of H. axyridis) was constitutively expressed throughout all developmental stages. We performed RNAi technique to examine the melanin synthesis pathway of elytra in H. axyridis. The transcript levels of HaDDC were significantly suppressed after the injection of double-strand RNA of HaDDC (dsHaDDC) at 300 ng/individual in third instar larvae. Silencing HaDDC in third instar larvae did not result in mortality nor significantly affect pupation and eclosion. We further demonstrated that all adults of H. axyridis (forms succinea, spectabilis, and conspicua) with HaDDC silenced in third larvae showed abnormal phenotype which emerged as decreased elytra melanin. However, melanin was still observed in other parts of the adults such as head or pronotum. These results demonstrate for the first time that dopamine-derived melanin is the main contributor in elytra melanization in H. axyridis. Additionally, we provide evidence for DDC in regulating fecundity by showing that silencing of HaDDC in third instar larvae significantly reduced female egg-laying and egg hatching. As such, DDC is likely pleiotropic in respect of its role in melanin production and fecundity processes. These findings bring novel insights into melanin production in holometabolous insects, and contribute to the framework on which further studies may be conducted on the mechanism of pigment production and patterning in various types of insect coloration.