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This article comes from the official account of Wechat: back to Park (ID:fanpu2019), author: Zhang Wei

The withered leaf butterfly, commonly known as the "dead leaf butterfly", has beautiful blue velvet-like wings with orange markings. When it stops to close its wings, its ornate wings disappear and become like dead leaves, and it can even simulate the shape of leaf midvein, secondary vein, petiole and even leaf decay very realistically. What kind of genetic mechanism is this magical evolution regulated? The withered leaf butterfly has received considerable attention in taxonomy, morphology and other disciplines, but the genetic basic research on their origin and mimicry evolution is still lacking. A recent study by the School of Life Sciences of Peking University and Zhang Wei of the Peking University-Tsinghua Joint Center for Life Sciences responded to the above questions.

In most people's impression of childhood, there may be a butterfly shadow fluttering scene, the gorgeous markings of butterfly wings are free and smart, opening a beautiful and diverse window for the first time to know the world.

As a fully metamorphosed insect, butterfly is closely related to its host plants during the larval stage, and is also sensitive to environmental and climate changes. it can be used as an indicator species to study biodiversity from multiple scales. therefore, it has a wealth of ecological data accumulation. Looking at more than 18000 butterfly species, nothing is more impressive than the small butterfly wings. The structure of this organ seems flat and simple, but it undertakes complex biological functions, such as movement, courtship, enemy defense, thermal regulation and so on. Butterfly wings with simple structure but complex functions are shaped by nature and driven by pressures such as natural selection and sexual selection, so they are concerned by biologists and become a model of ecological and evolutionary research.

How do we understand the diversity of butterfly wings? In the midst of butterflies, what kind of system should be chosen to carry out what kind of research? It has always been a problem. For example, in the Amazon jungle, the sleeve butterfly, which is toxic, different species show similar bright wing patterns and radiate evolution, is an ideal system for studying speciation, interspecific hybridization and adaptive radiation. In the rainforest of Southeast Asia, the non-toxic jade belt butterfly presents a bright wing pattern that mimics the poisonous butterfly, but this mimicry phenomenon is limited to the female butterfly and shows polymorphism, which brings inspiration for the study of phenotypic diversity, sexual dimorphism (differences in structure and function between female and male individuals in sexually reproductive organisms) and adaptive evolution.

However, this paper focuses on a kind of secret and low-key butterflies, whose camouflage and imitation of leaves are easy to be ignored by people and their natural enemies. Leaves are everywhere in nature, and the phenomenon of this kind of animals imitating plants is of great benefit and widely exists in the animal kingdom, such as Amazon leaf fish, Malaysia leaf frog, leaf katydids, etc., but it does not affect the protagonist of this article to become one of the most famous and eye-catching leaf mimicry.

Fig. 1 wing phenotype of Chinese subspecies of withered leaf butterfly. The ventral side of the wing shows a vivid leaf-shaped phenotype, and the back shows bright patches. Tengdequn is a butterfly species of the genus Butterfly, which is widely found in East and Southeast Asia and is famous for its unique wing patterns. When their wings fold backward, they cleverly simulate brown withered leaves, and the leaf-shaped patterns are vivid, including elements such as leaf midrib, secondary vein, petiole, and even with patterns similar to mildew spots (figure 1). This trait is probably driven by the pressure of natural selection. When it spreads its wings and shows a bright pattern on the back of its wings, some species of the genus Linnaeus have bright patchy patterns, which may be related to courtship and driven by the pressure of sexual selection. but it may also be to warn natural enemies, it is the result of another kind of natural selection, which needs to be further studied and confirmed.

Therefore, the author believes that the asymmetrical pattern on the dorsal ventral side of the wing well shows the evolution of phenotypic diversity under conservative developmental constraints, and can be used to study the shaping of phenotype and function under different selection pressures.

The attention of evolutionary biologists to withered leaf butterflies was recorded as early as in Wallace's 1889 book Darwinism: a discussion of the Theory of Natural selection and its partial Application. They are considered to be "the most wonderful and undoubtedly protective examples of butterflies" [1]. In the more than 100 years since then, scientists have studied it from the perspectives of classification, pedigree geography, physiology and morphology, and think that its leaf shape phenotype is the product of gradual evolution. it lays a foundation for exploring the evolution and genetic mechanism of various wing elements that make up leaf mimicry.

On August 2, 2022, the author and team members published a research article in the journal Cell, revealing the evolution and genetic mechanism of the withered leaf butterfly and its leaf mimicry. By collecting and analyzing the butterfly samples of 20 genera of the family Nymphalidae, it is found that the leaf mimicry shows a convergent evolution in the family Nymphalidae, that is, many kinds of butterflies independently evolve the leaf-like phenotype under similar selection pressure. On the other hand, many species of the genus Linnaeus have formed monophyletic groups, which means that the leaf mimicry of the genus may be inherited from their common ancestor.

In the process of collecting samples, the author and the team also collected samples of 6 species of the genus Linnaeus in 11 geographical regions of East and Southeast Asia (figure 2). Although we know that no two leaves are exactly the same, it is still a great test to identify the commonness from individual differences and to identify the species to which they belong. However, the greater difficulty may come from finding them from those jungle-covered habitats, which is an important prerequisite for in-depth research.

Fig. 2 geographical distribution of the samples collected by the withered leaf butterfly genus. Samples of three species can be collected in the collection area in the upper left corner of the picture (Motuo County), and samples of a single species can be collected at other collection sites. Reference [2] interdisciplinary team members have obtained a very important clue in their scientific investigations year after year, that is, at least three species of withered leaf butterflies have been found in the rainforest of Xizang Motuo County, which is very different from the situation in which only a single species can be collected in any other region. Even so, the Motuo rainforest in the author's eyes still contains too many unknown biodiversity resources, and it is still possible that there are unknown species of the withered leaf butterfly flying in it.

Fig. 3 big bend in Guotang of Yarlung Zangbo River in Motuo County. Zhang Wei / photo of the eastern Himalayas is the starting point of differentiation Motuo County is located at the eastern end of the Himalayas, where the Brahmaputra River, the highest river bed in the world, flows through, forming the deepest and longest river canyon, the Brahmaputra Grand Canyon, which opens a channel for the water vapor barrier between the Qinghai-Tibet Plateau and the Indian Ocean, and its elevation drop of several thousand meters also provides a variety of habitats for the local rich biological species. Motuo in the rainy season, the Brahmaputra River and its tributaries roar along the valley, as if playing a hymn of life, sweeping away the weariness of the distant visitors who have just crossed Mount Mira, Mount Sejila and Mount Galongla. In this secret land of lotus flowers, the origin of the withered leaf butterfly genus is about to be revealed.

Figure 4 is about to go through the Galongla tunnel to Moto. Based on the discovery in Motuo, the author and the team hypothesized that the eastern Himalayas have formed a variety of microenvironments due to its huge elevation gradient, which may be the differentiation center of the genus Linnaeus, but another possibility is not ruled out, that is, the area is a refuge for the genus Nymphae, allowing it to survive the ice age.

The actual research results confirm the first hypothesis. The results of phylogeny, population history and habitat model analysis show that the populations of many species of withered leaf butterflies tend to emigrate from the eastern Himalayas and spread to Southeast Asian islands, while the eastern Himalayas may provide a relatively suitable habitat for withered butterflies in many historical periods. The time of species differentiation within the genus can also correspond to the uplift period of the Qinghai-Xizang Plateau.

At this point, the withered leaf butterfly originated and differentiated in this magnificent environment of mountains and rivers, with light butterfly wings flying for hundreds of thousands of years, and the starting point of differentiation began in the southeast of the Qinghai-Tibet Plateau. At that time, the islands of Hainan and Taiwan were connected to the mainland by land bridges, and Java, Sumatra, Borneo and Thailand-Malay Peninsula formed the Sunda ancient land. With the help of the ancient land bridge during the ice age, the withered leaf butterfly further migrated and differentiated, and the team speculated that some island species endemic to the present are likely to be part of the previous large population. it was isolated and seeded due to subsequent climate change and sea level rise.

In this torrent of geological upheaval and climate change, the evolution of the genus Linnaeus reveals the relationship between mountain biota and other lowland biodiversity hotspots, and also provides an important starting point for understanding the formation of species diversity.

Figure 5 above the Pentium Brahmaputra River (collected near the Ani Bridge). Zhang Wei wants to make an in-depth study of a species, not only its habitat, but also its life history. The research team selected the withered leaf butterfly Chinese subspecies of the genus, which, like fruit flies, mice, nematodes and Arabidopsis thaliana, became a model (a biological species selected for scientific research to reveal the general laws of life science). However, there is no experience to unlock this model, only many attempts and explorations.

The feeding room is located on the second floor underground, which is hot and suffocating, but it happens to provide a constant temperature and humidity environment for the withered leaf butterfly and its host plants, and once again the author's team made an unexpected discovery. it was found that the withered leaf butterfly had at least 10 discrete leaf phenotypes (figure 6). These 10 phenotypes were further confirmed by pedigree experiments, and found that they may be controlled by Mendelian single locus. It is presumed that there may be five alleles.

Fig. 6 10 discrete leaf phenotypes have been found in the withered leaf butterfly Chinese subspecies, which may be controlled by a Mendelian locus. ? map source: reference [2] one gene controls various leaf-wing types by integrating a variety of genomics and gene editing analysis methods, the team identified a single gene cortex that controls this series of leaf phenotypes and has five haplotypes. This not only confirmed the previous hypothesis, but also found that the linkage disequilibrium among haplotypes was maintained by many mechanisms, such as chromosome inversion, topological association domain and so on. Corresponding to the regulation of wing phenotype, the gene showed expression characteristics related to the development of withered butterfly wings, and its chimeric deletion mutants also showed mottled wing patterns (Fig. 7).

Fig. 7 individual phenotype of Chinese subspecies of withered leaf butterfly based on CRISPR / Cas9 gene editing. The chimeric deletion mutant of cortex gene shows mottled wing phenotype, that is, knockout of the gene may affect the formation of leaf phenotype. Reference [2] in fact, this famous cortex gene is no ordinary person. As a tool box gene that controls the development of butterfly wings (genes involved in controlling the development of biological morphology and structure), it has also been found to be involved in controlling the industrial blackening phenotype of birch inchworm and the wing pattern of sleeve butterfly Mu's mimicry. This kind of toolbox genes play an important role in the process of development, and are easily favored by the pressure of natural selection, so as to obtain more regulatory ways and participate in the exercise of more functions. Taking this as an example, the seemingly contradictory concepts of developmental constraint and evolutionary innovation are reconciled, which also shows the evolution of the conservative development process, and further reveals the possible mechanism of genetic diversity and phenotypic diversity.

So far, the genetic mechanism of leaf mimicry has been explained in the Chinese subspecies of the withered leaf butterfly, and it is necessary to continue to explore the unknown in the origin and evolution of many species of the withered leaf butterfly. The team found that different leaf shape phenotypes existed in many species of withered leaf butterflies, while different phenotypes and genotypes had different frequencies in the wild populations of many species. To maintain this polymorphism, the team hypothesized that it might be under the pressure of natural selection, that is, it was subjected to long-term balanced selection so that the polymorphism could be retained in all species.

If this hypothesis is confirmed, it will be another exciting discovery. For a long time, polymorphisms driven by balanced selection pressure are mostly found among species below the level or for a short period of time, which is due to the fact that if individuals with different phenotypes have different fitness, the opportunity to pass on the genetic information corresponding to its phenotype to offspring is different, and this difference will eventually cause the number of offspring produced by individuals in the population to be lower than expected, which puts pressure on the continuation of the population. Also known as genetic load.

By typing the cortex genes of several species and constructing a population genetic model, the most suitable model obtained by the team shows that the leaf mimicry polymorphism of the genus Linnaeus has undergone balanced selection. The pressures of natural selection shape this diverse and exquisite leaf camouflage.

The revelation of the withered leaf butterfly genus and its leaf mimicry has come to an end, and the pace of exploration has never stopped, and brainstorming has just begun. The asymmetry of the dorsal ventral side of butterfly wings, structural colors and pigments dotted with colors, dry and rainy season wing pattern transformation, hidden species, unknown phenotype. And the adaptive evolution, phenotypic plasticity, ontogeny and evolution innovation, species formation, biodiversity generation implied below the surface. The evolution of life. Since 3.5 billion years ago, all reservations need to be re-recognized, and all unknowns need to be further explored. This is a new rainy season. Among the ten thousand ravines of Motuo, rainbows and birds sing in the empty mountains after the rain (figure 8).

Figure 8 is in the Motuo rainforest (collected near Dexing Township). Zhang Wei / people who chase butterflies will set out again for this butterfly shadow for thousands of times.

reference

Wallace, A.R. (1889). Darwinism: An Exploitation of the Theory of Natural Selection with Some of its Applications. (London: Macmillan)

[2] Wang, S., Teng, D., Li X., Yang, P., Da, W., Zhang, Y., Zhang, Y., Liu, G., Zhang, X., Wan, W., et al. (2022). The evolution and diversification of oakleaf butterflies. Cell 185,1-15.

[3] Nadeau, N.J., Pardo-Diaz, C., Whibley, A., Supple, M.A., Saenko, S.V., Wallbank, R.W., Wu, G.C., Maroja, L., Ferguson, L., Hanly, J.J., et al. (2016). The gene cortex controls mimicry and crypsis in butterflies and moths. Nature 534,106-110.

Product: popular science China

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