The Viceroy (Limenitis archippus) is a North American butterfly with a range from the Northwest Territories along the eastern edges of the Cascade Range and Sierra Nevada mountains, southwards into central Mexico.

Its wings feature an orange and black pattern, and over most of its range it is a Müllerian mimic^[1] with the Monarch butterfly (Danaus plexippus). The viceroy's wingspan is between 53 and 81 mm.^[2] It can be distinguished from the Monarch by its smaller size and the postmedian black line that runs across the veins on the hindwing.^[2] In Florida, Georgia, and the Southwest, Viceroys share the pattern of the Queen (Danaus gilippus) and in Mexico they share the pattern of the Soldier (Danaus eresimus). In all three areas, the local Danaus population mimic the coloration of the Viceroy species. It was originally believed that the Viceroy was a Batesian mimic of the three other species, and presumed edible or only mildly unpalatable to predators, but this has since proven not to be true.^[1]

The caterpillar feeds on trees in the willow family Salicaceae, including willows (Salix), and poplars and cottonwoods (Populus). The caterpillars sequester the salicylic acid in their bodies, which makes them bitter, and upsets predators' stomachs. As further protection, the caterpillars, as well as their chrysalis stage, resemble bird droppings. Adults are strictly diurnal, flying preferentially in the late morning and early afternoon.^[3]

The Viceroy was named the state butterfly of Kentucky in 1990.^[4]

Life stages

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  Adult

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  Caterpillar

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  Pupa

Evolution of admiral butterflies (Nymphalidae: Limenitis)

The world is divided into eight biogeographic areas called ecozones: Palearctic, Nearctic, Afrotropic, Neotropic, Australasia, Indo-malaya, Oceania, and Antarctica. Palearctic includes most of Eurasia and North Africa while Nearctic includes most of North America. Limenitis butterfly wing patterns are much more diverse in the Nearctic than the Palearctic. Three lineages of mimetic butterflies occur in North America and the evolution of mimicry may have played a large role in the diversification of this group.^[5] In order for butterflies to travel from the Palearctic region to the Nearctic region of the world, the migration must have occurred during a time period when Beringia, the land bridge between Eurasia and North America, was still above water.^[6] Based off crude divergence rate calculations,^[7] the colonization of the Nearctic Leminitis dates back approximately four million years.^[8] Whether the migration event was a single or multiple occurrence event has a significant effect on how we look at the evolution of mimicry. A history of multiple migrations would suggest that speciation occurred before the evolution of mimicry, meaning mimicry was the result of speciation instead of the driver of speciation.

However, much evidence supports that a single event colonization is the best explanation. One theory of Nearctic colonization states that the reason for the colonization was a larvae host plant shift. The position of the Poplar admiral (L. populi), a Palearctic species, in a phylogenetic tree confirms that the Poplar is the closest existing relative of the Nearctic taxa and is consistent with the theory that the host plant had a large effect on the evolution of North American admirals. Just like the wing-pattern of the Palearctic butterflies has little evidence of divergence, the host plant use of these species also shows no sign of divergence. These species only feed on different species of honeysuckle (Lonicera ssp.) The exception is the Poplar that feeds exclusively on aspen (Salicaceae: Populus tremulus).^[5] All North American Limenitis feed on Salicaceae as well, suggesting that an (ancestral host plant shift) expansion of a novel host plant across the Bering land bridge could have driven the colonization of the Nearctic. Species level phylogenies based on the mitochondrial gene COI and the gene EFI-α of Nearctic and Palearctic species also indicate a single colonization of the Nearctic species.^[5] The phylogenies produced indicate that a white-banded ancestor similar to the species L. arthemis.^[9] established itself in North America and resulted in several major lineages, three of which involved mimicry independently of each other. Given the present monophyly of the Nearctic species, it is likely that a single migration and subsequent expansion of the population was the foundation of the Nearctic butterflies.

Evolution of Viceroy mimicry

Based on phylogenic evidence we know that mimicry in the North American admirals was a driver of speciation. An essential condition for the evolution of mimicry was the presence and abundance of unpalatable models. Mimetic evolution also involved direct selection with the model acting as a “starting block” for the mimic to evolve against.^[10] The drive behind this type of evolution must be predation. Eventually, the mimetic population undergoes phenotypic fixation, usually at a point where the wing pattern and colors of the mimic have reached the closest superficial resemblance of its model.^[10] As these processes continued, the subspecies divergences began occurring as the mimetic species expanded their geographical range and began mimicking other species of butterfly.

Determining what part of the butterfly genome controls wing color and pattern is also a major component that must be taken into account when trying to understand the evolution of mimicry. Each individual stripe or spot on a wing has a distinct identity that can be traced from species to species within a family.^[11] A fascinating feature of pattern genetics is that the dramatic phenotypic changes are primarily due to small changes in the gene that determines the sizes positions of patter elements.^[11] This discovery is in accord with the principal theory for the evolution of mimicry. The theory proposes that initial mimicry is achieved by a single mutation that has a large effect on the phenotype, which immediately gives the organism some protection, and is then refined by so-called modifier genes with lesser phenotypic effects.^[11] Consequently, if the genes for wing pattern and color were normal functioning genes, a single mating would produce several phenyotypically different offspring, making the ability for mimicry to evolve very difficult.

This unique puzzle led to proposal of a possible supergene. A supergene is a tight cluster of loci that facilitate the co-segregation of adaptive variation, providing integrated control of complex adaptive phenotypes.^[12] Different genomic rearrangements have tightened the genetic linkage between different color and pattern loci with complete suppression of recombination in experimental crosses in a 400,000 base section containing at least 18 genes.^[12] This single supergene locus controls differences in a complex phenotype like wing coloration that can involve modifications of wing pattern, shape, and body color. Mimetic patterns have high fitness correlated to locally abundant wing patterns and low fitness when the offspring have recombinant, non-mimetic phenotypes.^[12] This tight-linked area of wing pattern genes explains how mimetic phenotypes are not broken up during recombination during sexual reproduction.

References