A mating pair of peppered moths, Biston betularia, showing the melanistic and pale forms.
A regularly-cited example of observed Darwinian evolution is that of the peppered moth which occurs in two forms, the white, speckled form and a melanistic, almost black form. During the industrial revolution, as English northern towns grew and became polluted by smoke from coal-burning factories, so the melanistic form became more common.
Experiments showed that the lighter form became easier for predators to see when the moths were roosting on tree trunks that had become coated in soot, while the melanistic form became harder for predators to see.
Following the decline of the northern towns, the light form again increased back to the former ratio, showing the importance of environmental change in evolution.
This tendency to have melanistic forms is common in the lepidoptera (moths and butterflies) and this tendency was believed to be under th control of as single genomic region surrounding the protein-coding gene “cortex“, common across many species, showing their descent from a common ancestor.
However, new research by international researchers from Singapore, Japan, and the United States of America, led by Professor Antónia MONTEIRO and Dr Shen TIAN from the Department of Biological Sciences at the National University of Singapore (NUS), has shown that 'cortex' is not directly involved in producing melanism, instead, this is controlled by a microRNA from within the 'cortex' genomic region, as another example of how microRNA's control many functions within cells, particular gene expression.
Tell me about how melanism contributed to the mimetic radiation of Heliconius butterflies, please. Melanism played a crucial role in the mimetic radiation of Heliconius butterflies by providing a basis for color pattern variation, which underpins mimicry and adaptive diversification in these species. Mimicry in Heliconius butterflies is driven by Müllerian mimicry, where toxic or unpalatable species evolve similar warning color patterns to reinforce predator learning and avoidance. Here's how melanism contributed:The team recently published their findings in the journal Science and explained it in a National University of Singapore blog post:
Example: Heliconius erato and Heliconius melpomene
- Genetic Basis of Melanism
- Melanism is primarily caused by genetic variation affecting pigmentation. In Heliconius, specific genetic loci, such as optix and WntA, control the color and patterning of their wings.
- These genes are highly polymorphic, enabling the expression of diverse melanic and brightly coloured patterns.
- Mimetic Convergence
- Melanic forms often serve as intermediate steps in the evolution of mimicry. For example, a butterfly with a predominantly black wing pattern can incrementally evolve coloured markings that match those of a local toxic model species.
- This convergence enhances survival as predators associate the shared pattern with toxicity.
- Geographic Variation
- The distribution of melanic and non-melanic forms correlates with the presence of different local mimicry complexes. For example, in certain regions, Heliconius species exhibit dark, melanic morphs that mimic co-occurring toxic species.
- This geographic variation facilitates speciation as populations adapt to distinct mimicry rings.
- Reinforcement of Mimicry
- In some cases, melanic forms stabilize mimicry by creating strong visual contrasts with brightly coloured bands or spots, making the warning signals more conspicuous to predators.
- Such patterns also reduce predation pressure on newly evolving mimetic phenotypes within the population.
- Role in Adaptive Radiation
- Melanism supports the rapid diversification of Heliconius butterflies by allowing them to occupy new ecological niches. For example:
- Different mimicry patterns enable species to specialize in different habitats where specific mimicry complexes predominate.
- Melanic and brightly coloured forms may exploit different thermal environments, as darker wings absorb more heat.
These two species exhibit parallel evolution in their wing patterns, including melanic traits, across their range in Central and South America. Their mimicry has contributed to a shared mimicry system involving many other co-mimics, highlighting the central role of melanism in their evolutionary success.
Melanism, therefore, is not just a pigmentary trait but a pivotal factor driving mimicry, ecological adaptation, and speciation in Heliconius butterflies.
A microRNA solves an evolutionary mystery of butterfly and moth wing colouration
Lepidopterans (butterflies and moths) exhibit a splendid diversity of wing colour patterns, and many species display black and white, or dark and bright, wing colour pattern variants associated with the presence and absence of melanin. Many of these wing colour pattern variants are textbook examples of natural selection and evolution. Iconic examples include the rapid increase in frequency of the melanic form of the British peppered moth Biston betularia, driven by the sootier and darker environment caused by carbon burning and industrialisation in the late 1800s in the United Kingdom, and the mimetic radiation of Heliconius butterflies, among others.
Despite the often well-understood ecological drivers that favour the presence or absence of melanin in the wings of these lepidopterans, the genetic and developmental basis of changes in colouration has remained unclear.
How do butterflies and moths paint their wings either black or white?
Over the past two decades, scientists discovered that the majority of melanic wing colour variants are controlled by a single genomic region surrounding the protein-coding gene “cortex“. It was assumed, then, that cortex was the melanic colour switch. A team of international researchers from Singapore, Japan, and the United States of America, led by Professor Antónia MONTEIRO and Dr Shen TIAN from the Department of Biological Sciences at the National University of Singapore (NUS), discovered that cortex does not affect melanic colouration. Instead, a previously ignored microRNA (miRNA), is the actual colour switch.
The findings were published in the journal Science.
Piles of evidence from previous studies cast doubt on whether cortex was really the melanic colour switch, which inspired me to test the function of some other genomic features within this genomic region – miRNAs. MiRNAs are small RNA molecules that do not encode proteins like most genes do, yet they play essential roles in gene regulation by repressing the expression of target genes.
Dr Shen Tian, first author
Department of Biological Sciences
Faculty of Science
National University of Singapore, Singapore.
He conducted this research work as a PhD/postdoctoral researcher in Professor Monteiro’s laboratory at NUS, and is now a postdoctoral researcher at Duke University, USA.
In this study, Dr Tian and colleagues found a miRNA located next to cortex, mir-193. The team disrupted mir-193 using a gene editing tool CRISPR-Cas9 in three deeply diverged lineages of butterflies. The complete disruption of mir-193 eliminated black and dark wing colours in the African squinting bush brown butterfly, Bicyclus anynana, the Indian cabbage white butterfly, Pieris canidia, and the common mornon butterfly, Papilio polytes. In contrast, disrupting cortex and three other protein-coding genes from the same genomic region in B. anynana had no effect on wing colours. This indicated that mir-193, not cortex or any other nearby gene, is the key melanic colour regulator across these Lepidoptera.
The team further confirmed that mir-193 is processed from a long non-protein-coding RNA, ivory, and it functions by directly repressing multiple pigmentation genes. Since the sequence of mir-193 is deeply conserved not only in Lepidoptera but across the animal kingdom, the team also tested the role of mir-193 in Drosophila flies. Surprisingly, mir-193 was also found to control melanic colouration in these flies, suggesting a deeply conserved role for mir-193 beyond Lepidoptera.While previous studies exclusively focused on the role of cortex in generating melanic colour variations, this work brings a twist to this long-standing hypothesis and demonstrates that a small, non-protein coding RNA is the switch that, by being expressed or not expressed, brings about the diverse melanic wing colour variations in nature. This study shows that poorly annotated non-protein-coding RNAs, such as miRNAs, should never be ignored in genotype-phenotype association studies, which would otherwise lead to misleading conclusions.
Professor Antónia Monteiro, lead author. Department of Biological Sciences
Faculty of Science
National University of Singapore, Singapore.
The role of non-coding RNAs in phenotypic diversification is largely understudied. This study prompts further investigations on how non-coding RNAs such as miRNAs can contribute to phenotypic diversifications in organisms.
Dr Shen Tian.
AbstractThe variances that can be seen in butterflies and moths and upon which natural selection acts to push the species towards greater fitness in a changing environment, or, in the case of the Heliconius butterflies, towards better mimicry of the local toxic species, is nothing to do with magic tricks performed by an invisible magic god as creationists like to imagine, but has a perfectly natural explanation in terms of the action of a microRNA molecule that is able to act as a switch to switch malanism on and off in different parts of the wing.
In Lepidoptera (butterflies and moths), the genomic region around the gene cortex is a “hotspot” locus, repeatedly implicated in generating intraspecific melanic wing color polymorphisms across 100 million years of evolution. However, the identity of the effector gene regulating melanic wing color within this locus remains unknown. We show that none of the four candidate protein-coding genes within this locus, including cortex, serve as major effectors. Instead, a microRNA (miRNA), mir-193, serves as the major effector across three deeply diverged lineages of butterflies, and its role is conserved in Drosophila. In Lepidoptera, mir-193 is derived from a gigantic primary long noncoding RNA, ivory, and it functions by directly repressing multiple pigmentation genes. We show that a miRNA can drive repeated instances of adaptive evolution in animals.
Shen Tian et al.
A microRNA is the effector gene of a classic evolutionary hotspot locus. Science 386, 1135-1141 (2024). DOI:10.1126/science.adp7899
© 2024 American Association for the Advancement of Science.
Reprinted under the terms of s60 of the Copyright, Designs and Patents Act 1988.
And of course it shows the common ancestry of not only the lepidoptera, but, because they have the same mechanism, the common ancestry of fruit flies and the lepidoptera. Just one more tint scrap of evidence to be added to the massive pile refuting every aspect of the childish superstition - creationism - that makes its followers into the laughing stock of the civilised world.
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