Müllerian Mimicry
The South American rainforest contains a big range of different butterflies, even more so then initially suspected. In 1862 when the butterflies were studied for the first time by Henry Bates (1), he quickly realized there were many species with similar appearances. These species of butterflies looked very much alike but were distinctly different. The Heliconius genus contains over forty different species. Two of these species that are mimicking each other are H.erato and H.melpomene, they live in the same parts of the rainforest and share multiple predators. These two species are an example of Müllerian mimicry(2), a biological phenomenon in which two or more unpalatable species mimic each others warning signals. This phenomenon was named after the German zoologist and evolutionist Fritz Müller. He used Bates’s his initial discoveries to write his theory. Müllerian mimicry is based on the use of warning signals to warn predators about their taste or poisonousness. Organisms 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 it. By using clear warning signals they try to scare of the predator or at least make them remember. A warning signal is most commonly learned by an animal instead of instinctively picked up (3). 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 mimics another species warning signals because there 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 Müller this phenomenon is a mutualistic arrangement between multiple organisms. First I will explain the importance of the subject then I will focus on the origin, fenotype and its relation to the genotype and last I will explain some of the shared genetics.
Evolutionary hotspots
H.erato and H.melpomene are interesting because they display a very rare natural phenomenon, in the animal kingdom it is uncommon to see two species collaborate this way. Not too long ago it was discovered that is these two species of butterflies use the same genes for wing colouring patterning. This is interesting because this might have an influence on their separation as species. So two species that have merged from each other 3 million years ago are now growing back to each other. Not only do they look alike they also use the same genes. The genes they share are interesting because apparently these genes are very adaptable indicating that, that region is an evolutionary hotspot. I will answer the following questions. How are these butterflies similar ? How did these species originate and what how is that in relation to their genetic similarities ? What do the genes whom are responsible for their similar appearances look like?
Origin and fenotype
The H.erato species is approximately 2.8 million years old. A study from 2011 used amplified fragment length polymorphism and mitochondrial DNA data sets to date the species. They did the same for H.melpomene which is a bit younger at 2.1 million years old.There has always been a lot of hassle surrounding H.erato and H.melpomene because both Linnaeus and Bates weren’t completely certain about the fact the H.erato and H.melpomen were two different speciesUsing a phylogenetic tree we can take a closer look at their connection and the most recent common ancestor. In image 1 u can see the most recent common ancestor is at the beginning of the genus Heliconius. Another conclusion I took from this image is that H.erato and H.melpomene are not as closely related as some of the other species in the genus which is strange because they have most in common. These butterflies have developed the same traits after 3 million years of separation (4). Which means that in the past 200.000 years or so a lot of development has occurred. Research has shown that H.melpomene mimics H.erato and not the other way around (5). H.melpomene mimics H.erato in order to gain protection. H.melpomene changed over the course of generations by simple natural selection, individuals who mimic H.erato have a bigger chance at survival.. This is how it works on a big scale but recently the focus has been on gene research.
Image 1
Phylogentical tree from the Heliconiiae family.
‘’A sample of the morphological diversity of wing patterns in Heliconius and related genera. Each row represents a phylogenetic clade in the tribe Heliconiini. © Chris Jiggins and Mathieu Joro. ‘’ (10)
This image shows the helicons genus with the different species. The H.erato and H.melpomene are very distant and there most recent common ancestor was at the beginning of the Heliconius line.
H.melpomene
H.erato
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 see image 2. Recent studies have shown that only a couple of traits in the DNA are responsible for this banner (5). 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 population.
Image 2
Linnaeus his personal collection 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. In this picture you can clearly see the similarities especially the red banner.
Red color optix
The gene expression for the color red is very important, I will focus greatly on this because it is a very distinguished trait in both the fenotype and genotype. In the genome there are genes that express color and genes that express shapes and form on the wings. Markers just for patterning are les conclusive than markers for coloring (8). Another fact to consider is that only the H.melpomene and H.erato share this genetic trait so none of the other species in the genus have this. The red banner is seen al across the Rainforest from Panama to Brazil. A great deal of progress has been made in understanding the genetic basis of red patterning in Heliconius.
In 2011 almost the entire genome of both H.erato and H.melpomene were transcribed which leads to new insights to their similarities. The expression off the red banner is 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 for the expression of the red banner. In 2012 there was a study in which this megabase 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 the only gene responsible for all red patterning on the wings (7). 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 which is strange because the other genes are different in all other Heliconius species. The optix contains three closely linked loci (6). Other scientist discovered that only a 150- kb region is different between the two butterflies. In the optix gene a variation in wel conserved acid sequences is responsible for different phenotypic expressions. This results from cis-regulatory variations within the optix. Cis-regulatory genes are regions of non-coding DNA which influences the coding of nearby DNA. So variation in the Phenotype is created when these cis-regulatory regions are a bigger or smaller influence on the nearby genes. A study in 2014 used neutral markers across different loci between the two species in order to investigate the origin of these genes in both species. They used separated genes to make sure the results could be conclusive. The loci used are genes linked to colour pattern and the optix. They found that even though the optix has the same gene expression the history and structuring lineages were different in both species. Suggesting that although the same genes (optix) are responsible for a certain phenotype the red coloring they are still different.
Discussion
I found this subject to be very interesting, over the past 150 years a lot of research has been done which is very impressive. But there isn’t a lot of research that towards the origin of both species which to me would be very interesting because it would be a great insight behind evolutionary hotspot development. Another critical note would be that a lot of research focusses solo on very small details without a specific context. A good follow up research would be to develop a practical use for the research. These butterflies are so identical it be interesting to see what will happen if they were to be exposed to DNA modification. Another question I had during my research was how hybrids have changed during evolutionary development did they become more viable ? because H.erato and H.melpomene became more closely related.
Conclusion
From these results found by different studies all over the word I was able to answer most of my questions. The butterflies are almost indistinguishable but there are some differences. The butterflies use the same genes but not in the exact same way. They share the complete optix gene which is responsible for red colouring. They haven’t become the same species (yet?) because a hybrid is not fertile. However they do resemble each other much more than they once did both physically and genetically. If natural selection were to continue this way the two species might become even more tightly linked but will probably remain separated because of their differences in other loci area. In conclusion both species mostly share genes that are responsible for their visible features in order tot mimic one and other.