Wing Pattern Evolution And The Origins Of Mimicry Of North American Admirals

Admiral butterflies (genus Limenitis) are a particularly appealing system to address questions regarding wing pattern evolution and speciation. This genus is unusual among other butterflies in that mimicry has evolved multiple times and hybridization is frequent between wing pattern forms.

An interesting fact of wing pattern evolution within this genus is the apparent differences between the largely stereotypic Paleartic fauna and the highly variable Nearctic forms of butterflies.

Palearctic and Nearctic forms regard to different ecozones dividing the earth’s land surface. The Palearctic is the largest ecozone. It includes Europe, Asia north of the Himalaya, northern Africa, and the northern and central parts of the Arabian Peninsula. The Nearctic ecozone covers most of North America, including Greenland and the highlands of Mexico, Southern Mexico, Southern Florida, Central America, and the Caribbean islands together with South America.

The Palearctic forms have a uniformly dark brown or black background wing color with one major band of white pigment running across both the dorsal and ventral surfaces of the wings. Variations among these species is predominately subtle in variations of this white-band wing pattern.

In contrast, wing patterns in Nearctic forms are diverse. There are three lineages of mimicry in admiral butterflies that occur in North America which may have played a large role in the diversification of this group.

The most recognized example of mimicry among the North American Limenitis butterflies is the relationship between the Viceroy butterfly and the Monarch butterfly. In the Monarch butterflies they are said to be foul tasting and poisonous due to the presence of cardenolides in their body, which they ingest as they feed on milkweed. In the viceroy butterflies they feed on Salicaceae plants and ingest salicylic acid in their bodies which makes them bitter and upsets predators stomachs. This is said to be a classic example of Mullerian mimicry. Mullerian mimicry is where two or more harmful species that are not closely related share one or more common predators, and mimic each other’s warning signals.

Another example of mimicry in the genus Limenitis occurs in the western United States where Batesian mimicry has evolved between the Lorquin’s admiral butterfly and the California Sister butterfly. Batesian mimicry is where a harmless species has evolved to imitate the warning signals of a harmful species directed at a common predator.

Final example of mimicry among the Nearctic Limenitis butterflies involves the white-banded admiral butterfly found in southeastern and southwestern United States. This is also considered to be a Batesian mimic of the Pipevine swallowtail.

These are all examples of divergent evolution

divergent evolution is the naturally selected changes in related species that once shared a characteristic in common but have come to be different.

Definitions

Lineage вЂ" descendants of an individual

Speciation вЂ" is the formation of new species through natural selection

Monophyletic вЂ" is a group of taxa that contains an ancestor and all of its descendants

Paraphyletic вЂ" is a taxonomic group which does not include all the descendants of an ancestral taxon

Polyphyletic вЂ" pertains to a group of species that does not include the most recent common ancestral species of the entire group

HYPOTHESIS

In this study it will show that 1.) the Nearctic Limenitis represents a monophyletic lineage and that each wing pattern race represents a discrete mitochondrial lineage; 2.) that the Poplar admiral is the closest Palearctic relative of the North American forms, and argue that an ancestral host plant switch to Salicaceae has played a significant role in the colonization of North American forms, and 3.) the differences in wing pattern diversity between the Nearctic and Palearctic Limenitis may be due to an increased mating opportunity resulting in wing pattern mimicry between North American admiral and northern models of South American butterflies.

Materials and Methods

Table 1

This table here shows the different species that were collected and where they were collected from.

There were fifteen species of Limenitis that were sampled, including all of the North American and European species and most of east Asia. Also, three outgroups genera were included, two from the subtribe which Limenitis belongs and one from the subfamily. They were collected from different parts of the United States, Canda, etc.

(All from same order just different family or tribe. ) Order, Family, Tribe, Genus, Species вЂ" Outgroups are where they don’t belong to this group of genus, used as a reference point.

The first thing that the researchers did was DNA amplification and DNA sequencing. They isolated DNA from the flight muscles and sequenced two highly conserved mitochondrial genes (COI, and COII), which stands for cytochrome oxidase 1 and cytochrome oxidase 2 and one nuclear gene (EF1-О±), which stands for elongation factor 1. COI and COII are enzymes found in the electron transport chain, and EF1 are proteins that facilitate elongation. After they sequenced everything they imported the data into several computer programs and came up with a final consensus of 1911 bp for the mitochrondrial genes and 904 bp for the EF1-О± genes. Then phylogenetic analyses were done using more computer programs and came up with phylogenetic relationships.

Results

Figure 2 . This is a phylogenetic tree of Nearctic and Palearctic admiral species just taking into account the mitochondrial genes.

In the mitochondrial phylogeny the North American admirals are a monophyletic group. Palearctic Limenitis are paraphyletic with respect to the Nearactic forms and the Poplar admiral is the sister descendant to the Nearctic forms. The genus Limenitis in this tree appears to be polyphyletic given the position of the Palearctic species L.pratti and L.staudingeri, because they fall together with species from two of the outgroup genera. The remaining Eurasian species with the exception of L.reducta, form a strongly supported monophylentic clade. Here you can also see