Sunday, June 5, 2016


The genetic control of wing pattern variation in moths and butterflies


"The gene cortex controls mimicry and crypsis in butterflies and moths"

Nature 534, 106–110


"The industrial melanism mutation in British peppered moths is a transposable element"


Electron microscope image of
butterfly wing scales
Moths and butterflies belong to an order of insects called Lepidoptera. One of the main distinguishing features of this order is that they have tiny scales on their wings and other parts of their bodies. Other groups of insects do not have these scales.

These tiny scales can serve a number of functions in moths and butterflies, but one of the most important functions is their role in creating the vivid and unique patterns that these insects have on their wings.

We are all familiar with the beautiful patterns of color and shape that moths and butterflies carry on their wings. These diverse wing patterns are a stunning example of biological variation.

Examples of wing pattern variation
in moths and butterflies

What is variation? Simply put, it refers to groups of items that are 'similar but different'
The wing patterns of moths and butterflies are an example of variation:
  • they are similar: all the wing patterns share many basic features, e.g. on the microscopic level they are constructed from similar tiny scales;
  • but they are different: individual moths and butterflies have wing patterns that are different in shape and colour from other moths and butterflies

Biological variation is the main object of study in the science of genetics. Geneticists are interested in discovering the "genetic component of biological variation".

What does this mean? Biological variation arises from two sources:
  • Environmental variation: differences in organisms caused by differences in environment, e.g. Sam is shorter than Xi because Sam did not have enough food growing up
  • Genetic variation: differences in organisms caused by differences in their genes, e.g. Sam is shorter than Xi because Sam's parents are shorter than Xi's parents.
This is commonly referred to as the "nature versus nurture" debate, but there is not so much debate about this in biology. All organisms have a genetic code that determines what it looks like and how it behaves - there will always be a genetic component to all biological phenomena.

Most examples of biological variation involve both nature and nurture - the more correct question is "how much nature versus how much nurture".

By analyzing the DNA sequence of subjects and measuring different traits, geneticists can try and find correlations between differences in DNA and differences in traits. These experiments allow them to answer two questions:
  1. How much variation is controlled by genes versus the environment (nature vs. nurture)?
  2. What are the DNA mutations causing the genetic variation?
It is only recently, with the invention of genome sequencing technologies and powerful computational methods to analyze genetic data, that scientists have been able to answer the second question. Being able to identify the individual mutations that drive biological variations has huge implications for science and human life. We are just beginning to identify these mutations, and much work remains to be done.

A living histogram of human height variation
An example of this kind of genetic science can be found in a recent large study on the genetic factors controlling variation in human height.

Human height is a classic example of complex intraspecific variation - across human society people can be short, or tall, or somewhere in between.

The authors of this study measured height and analyzed DNA mutations in 253,288 subjects. This kind of experiment is called a genome-wide association study (or GWAS), and it requires very large study populations for the statistical precision needed to draw reliable inferences. 

This study identified 697 different mutations that account for about 1/5th of the observed variation in height among the subjects. What is the significance of this finding? Traits that are controlled by such a large number of genes are known as complex or quantitative traits

Because height is a complex trait, we cannot look at a single mutation in a baby and use it to accurately predict someone's adult height. We would have to identify all the known mutations that influence human height, and even then we would obtain an imperfect prediction of adult height as there still so many unknown factors controlling human height.

The opposite of a complex trait is called a Mendelian trait. In Mendelian traits, nearly all of the observed biological variation is controlled by mutations in a single gene. A classic example of this is sickle cell anemia, a genetic blood disease caused by mutations in a single gene: hemoglobin. By identifying whether a baby has the sickle cell mutant form of hemoglobin, we can perfectly predict whether they will have sickle cell anemia.

Scientists have recently begun to study the genetic mutations controlling wing pattern variation in different species of moths and butterflies. These studies have revealed that multiple mutations in a single gene called cortex can control wing pattern variations across many different species of Lepidoptera. 

In a 2009 paper, geneticists from Chris Jiggins' group at the University of Edinburgh studied the genetic basis of intraspecific wing pattern variation in three separate species of Heliconius butterflies.

Intraspecific wing pattern variation in 4 Heliconius species (source)

As you can see, each species contains individuals with different wing patterns. This is an example of intraspecific variation, similar to human height.

The authors determined the genetic component controlling wing pattern variation in three separate species, using a separate set experiments for each species. In order to do this, they used an experimental technique called linkage mapping.

The breeding scheme used to map
mutations causing wing pattern variation (source)
Similar to a GWAS, linkage mapping involves analyzing DNA mutations and measuring wing patterns in a large number of individual butterflies. But while a GWAS analyzes large natural populations, linkage mapping involves breeding animals in the lab and genetically analyzing their children.

In this study, male and female butterflies with different wing patterns were bred to each other, resulting in a number of children with different wing patterns.

Each butterfly child has its wing pattern measured and its DNA analyzed for mutations. By looking at correlations between different mutations and wing patterns, the scientists showed that in each species a single locus (or region of DNA) controlled the difference in wing pattern.

Further analysis showed that not only was a single locus responsible in each case, but that all three of the loci identified were homologous to each other - i.e. the same sequence of DNA appeared to control wing patterns in all 3 species!

However, there is a limitation to their study. Most linkage mapping experiments do not discover a single gene or mutation that is responsible, but rather a large sequence of DNA that contains multiple possible mutations.

In order to precisely narrow down the exact mutation responsible, scientists have to use a technique called fine-mapping. This traditionally means carrying out more breeding experiments and linkage analysis - the increased sample size gives greater statistical precision, allowing us to narrow down the sequence to a single gene.

A recent paper from the Jiggins lab used a newer, GWAS-based approach to fine-map the gene responsible for this pattern of variation in some Heliconius butterflies. By combining the results from the previous linkage analysis with a large GWAS of wing pattern variation in wild butterflies, the authors were able to pinpoint all the mutations to a single gene called cortex.


From the paperIn situ hybridization shows the
location of cortex in pupal wings (below in blue)
matches location of dark spots in adult wings (above in color)
It is currently not known how the cortex gene influences wing patterning. The authors hypothesize that it controls the formation of the scale cells that create the pattern. Using a technique called in situ hybridization, the authors were able to visualize the anatomical locations where the cortex gene is expressed.

They find that the patterns in which the cortex gene is expressed in the developing larval wing of the butterfly closely match the location and shape of colored spots and stripes in the adult wing. This kind of very specific, restricted gene expression suggests that the cortex gene is specifically turned on in groups of cells that give rise to adult wing patterns.


The same issue of Nature with this Heliconius paper contains a second study on a completely different example of Lepidopteran wing pattern - industrial melanism in moths. Industrial melanism occurs when insects like moths evolve a dark black color in response to increased air pollution from industrialization.

An example of industrial melanism in moths.
The moth on the right has evolved industrial melanism.
Although these moths are a very distant species from Heliconius butterflies, the authors find that a single mutation in the same gene - cortex - leads to the evolution of industrial melanism in British peppered moths.

So we find that many different mutations in a single gene - cortex - control intraspecific differences in a single trait - wing patterning - across multiple species in the order Lepidoptera. The figure below shows all the known Lepidoptera species for which mutations in cortex completely control the observed variations in wing pattern. It is likely that there are many more moths and butterflies for which this will also be true.

Known Lepidopteran species where cortex mutations control wing pattern variation (source)

Some Heliconius butterflies have an alternative
'multi-locus' control of wing pattern variation
But, despite these remarkable findings, cortex is not the only gene that controls wing pattern variation in moths and butterflies. Studies on other species of Heliconius butterflies have demonstrated that they use an alternative system where multiple genes interact to control wing pattern variation.






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