The genomes of 101 monarch butterflies from migratory and resident populations have been sequenced, revealing genes and molecular pathways that underlie insect migration and colouration. See Article p.317
In 1902, Rudyard Kipling wrote the Just So Stories, which explained, in colourful terms, how the leopard got his spots and the camel got his hump. Half a century later, Francis Crick and James Watson discovered the molecular structure of DNA1, but despite the molecular revolution that followed, today we still struggle to explain many examples of natural selection at more than a 'just so' level. Now, owing to the application of modern DNA sequencing to systems other than mice and humans, butterflies are leading a renaissance in our understanding of the molecular basis of natural selection2,3. In this issue, Zhan et al.4 (page 317) sequence a remarkable 101 butterfly genomes, and tell a story of two parts — migration and colouration.
During the summer, the monarch butterfly (Danaus plexippus) searches for milkweed plants on which to lay its eggs. Monarch caterpillars acquire cardiac glycoside compounds, which are toxic to predators, from the plant. These compounds are stored in both the caterpillars and the butterflies, which display the warning colours orange, black and white (Fig. 1). In the autumn, North American monarchs migrate and congregate in trees in the Mexican mountains. Tropical monarchs are not strictly migratory, although populations do make short-range migrations in the dry season.
Which evolved first, the temperate migratory populations or the resident tropical groups? For birds, the 'southern home' theory suggests that migratory populations arose from non-migratory tropical populations5. Surprisingly, Zhan and colleagues' analysis of migratory and non-migratory monarchs shows that these butterflies originated in North America, from a migratory ancestor. Tropical groups had reduced genetic diversity compared with their North American relatives, because they have gone through step-wise genetic bottlenecks during their colonization of the tropics, each of which reduced the diversity of their genomes.
Even more unexpectedly, the authors' analysis of monarch DNA suggests that migratory ability is linked to a single gene, encoding the protein collagen IV subunit α-1. Collagen IV is essential for the formation and efficient function of muscles6. When Zhan et al. undertook a detailed analysis of evolutionary selection patterns within this gene, they found evidence to suggest that alteration of a single amino acid affects the ability of the collagen subunits to co-assemble or trimerize, perhaps conferring on migratory monarchs some undisclosed advantage for long-distance flight.
What about the butterflies' warning colouration? Historically, vertebrate and invertebrate pigmentation have been viewed as distinct. For example, although the tiny coloured scales on the wings of butterflies are known to be related to flies' bristles7, there has been little evidence to suggest that the processes of pigmenting a butterfly-wing scale and a mouse hair involve the same genetic players. Zhan and colleagues have shed the first light on this subject by examining the ghostly white 'nivosus' monarchs — a variant found on the island of Oahu in Hawaii.
Kipling might have said that the white monarchs got their colouring from roosting on the peaks of the ancient volcanoes Waianae and Koolau when they were capped with snow. But by sequencing nivosus and orange butterflies from Oahu, the authors show that a single gene is strongly associated with the change in colour. That gene encodes a myosin protein related to the mammalian myosin 5a — a two-headed motor protein that can 'walk' along filaments of another protein, actin, within the cell8. Myosin 5a acts as a transporter of the light-absorbing pigment melanin, dispersing melanin-containing structures called melanosomes along actin filaments9. Melanosomes are cellular subunits that both synthesize and display melanin, giving colour to human hair and to the coats of other mammals. In mice, mutations in myosin 5a render the protein unable to properly transport melanosomes, resulting in a diluted coat colour.
Although the presence of melanin-containing pigment granules in the cuticles of some butterfly larvae has been documented10, it is unclear exactly how these granules relate to the melanosomes found in most vertebrates11. Precisely how a myosin protein might shunt pigment-containing structures around the scale of a butterfly wing therefore remains a mystery. What is clear, however, is that other structural cellular components, such as helical actin filaments, are involved in generating the microribs and lamellae, structures on wing scales that interfere with the wavelength of light and thus cause iridescence through refraction12. This fact, taken together with the current study, suggests that structural proteins within the wing scale might play a part in both pigment- and refraction-based colouration.
Zhan et al. have taken us beyond the realms of Kipling's stories. The authors have shown how current sequencing technologies can allow us to look directly at the traits under strong selection pressure in the species in which selection is actually acting, rather than just mice and fruit flies in the laboratory. But, of course, any good study raises more questions than answers.
What are the genes that prompt the migratory behaviour in North American monarchs? For example, does the monarch begin its migration in response to shortening day length in North America? The authors suggest that the molecular mechanisms contributing to migration are probably complex, and that the pathways involved range from those regulating circadian rhythms to those that control navigation. However, the gene that their study highlights is central to efficient muscle function. Although this finding suggests that it is the monarch's muscles that allow them to perform their migration, we still do not understand the complex role of shortening day length and environmental sensing in instructing the butterflies to migrate at a certain time of year.
Zhan and colleagues do not describe the mutation in the myosin-5a-like gene that causes the white nivosus variant, even though the answer probably lies within the sequenced mutant genomes. It is noteworthy that mutations within the globular tail domain of myosin 5a (its putative melanosome-binding site) result in a lighter coat colour in mice13. Does the nivosus-associated mutation also lie in this globular tail and therefore somehow disrupt its binding of butterfly pigment granules? Finally, the dilute-coat mutant in mice arises owing to the integration of a virus into the mouse genome, and full coat colour can be restored by virus excision14. Bizarrely, this suggests that, if similar mutations occur in the butterfly myosin-5a-like gene, they might be unstable, and the butterflies may therefore seem to spontaneously appear and disappear like white ghosts.