Essay: Müllerian mimicry

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The South American rainforest contains a big range of different butterflies, even more so then initially suspected. In 1862 when these butterflies were first studied by Henry Bates (1) he quickly realized there were loads of species with similar appearances but they still were distinctly different species. Two of these species mimicking each other are H. erato and H. melpomene they live in the same parts of the rainforest and share more than one predator. These two species are an example of Müllerian mimicry, a biological phenomenon in which two or more unpalatable ( 2 : Chai 1986) species mimic each others warning signals. This phenomenon was named after the German zoologist and evolutionist Fritz Muller. He used Bates’s his initial discoveries to write his theory. Mullein mimicry is based on the use of warning signals to warn predators about their taste or poisonousness, they have these signals because being poisonous alone isn’t enough. In order for an animal to survive and use their unpalatablesness as a weapon their predators need to know about this. By using clear warning signals they try to scare of the predator or at least make them remember, warning signals are most commonly learned by animals instead of instinctively picked up (3 : Benson 1972, Kapan 2001). After an animal has eaten a poisonous or otherwise bad tasting animal they will learn fairly quick no to do this again especially if the animal has clear warning signs that are easy to remember. So what happens is that one ore more species mimic another species warning signals because their is strength in numbers. The more individuals with the same warning signs the faster a predator learns which means less individual casualties, they hide amongst their own. According to Muller this phenomenon is a mutualistic arrangement between multiple organisms.
Evolutionary hotspots
An interesting discovery has been that is these two species of butterflies use the same genome for wing coloring an patterning. So two species that have merged from each other 15 million years ago are now growing back to each other. Not only do they look alike they also use the same genes. This indicates these are evolutionary hotspots. These butterflies are interesting because it displays a rather rare natural phenomenon, it is very uncommon in the animal kingdom to see two species cooperate this way. What does this look like genetically ? And how have these genes developed over the past 200.000 years ?
These butterflies have developed the same traits after 15 millions years of separation (4: Pohl et al. 2009). Research has shown that H.melpomene mimics H.erato and not the other way around. We know that H.melpomene mimics H.erato in order to gain protection. It took the H.melpomene species approximately 200,000 years to develop in to the current situation by natural selection. This is how it works on a big scale but recently the focus has been on gene research. In order to take a good look a the genotype its important to first understand the fenotype. H.erato and H.melpomene are very similar but there are some slight differences. U can distinguish H.erato from H.melpomene, H. erato has four red dots on the underside of the wings while H.melpomene has three and a yellowish stripe. A big similarity and an important warning sign is the red banner that lays vertically across both wings. Recent studies have shown that only a couple of traits in the DNA are responsible for this banner (5: Martin and Orgogozo 2013). Individuals of both H.erato and H.melpomene have this banner and individuals without it have a 50 % bigger change to be eaten. Because of the importance of this banner scientists are doing a lot of research concerning the genes responsible for the creation of the banner. They aim of this type of research is to understand they origin of biological diversity and its development. These butterflies are a prime study object to discover functional changes in complex adaptive traits in natural populations.
Image 1
Linnaeus from 1770 from the South-American rainforest.The one on the top is Heliconius melpomene and the one on the bottom is Heliconius erato. On the left are the males and on the right the females. These pictures show the extreme resemblance between the two species.
Aproximaley 15 years ago it was discovered that these traits are controlled by one region of the genome less than one megabase long, known as the B locus in H. melpomene and the D locus in H. erato. The focus of a recent study has been trying to identify the exact genes responsible. In 2010 there was a study in which the whole genome was sequenced using bacterial artificial chromosomes (BACs). This region contained around 20 genes, the same in both butterflies. The optix gene placed on the B/D region is responsible for all red patterning on the wings. We know this because, a variation within the optix gene results in differences in the expression of red as a pattern color. This is true for a lot of Heliconius species not just the H.erato and the H. melpomene .
Substantial progress has been made in understanding the genetic basis of red phenotypic variation in Heliconius. The region was initially de- scribed as a complex of three tightly linked loci (6 : Sheppard et al. 1985). However, the region modulating red variation was posi- tionally cloned in both H. erato and H. melpomene to a single shared 400-kb genomic interval referred to as the D interval in H. erato and the B/D in- terval in H. melpomene. After more research it was identified that an 150- kb region is different between the two species. Gene expression analyses across this interval supported the transcription factor optix as the only gene with expression patterns consistent with a role in red pattern formation (7: Reed et al. 2011). Gene expression differences and the highly conserved amino acid sequence variation in optix suggest that variable red patterns are driven by cis-regulatory variation. Phenotypic recombinants between red pattern elements have been occasionally observed, implicating the potential involvement of multiple, tightly linked, cis-regulatory variants in this region that generate differences in the spatial expression of optix and result in diverse red color pattern phenotypes.

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