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About a month ago a question was raised in Dawn Chorus on a Sunday morning.  Why do ducks have such elegant and intricate color patterns?  Rashly I said I would do a diary on animal coloration.  The more I thought about it the more it seemed like a short series is in order.  So here is the first part of an exploration of the role of color in the natural world.

Color fits into the theme of biodiversity because it is one of the most obvious (to us) ways that organisms differ from one another.  Coloration can also be tied in to a vast array of interesting topics in ecology and evolution.  So let's get started.  This diary ended up being a bit longer than I expected and I ran out of time to insert all my references.  I'll come back and do that in the next couple of days so if you are interested check back.

This first diary is along the lines of crawl before you can walk, before you can run.  An introduction to the basics.  I'll throw in some interesting tidbits along the way so those who already know a bit don't get too bored.  Here's my plan for the diaries

1 (tonight) - Color and pigment basics.  Common color variations.  Genetic control of color.  Two case studies in humans and in mice.

  1. Development of animal color patterns.  Color as an anti-predator mechanism.
  1. Variation in the perception of color.  Color and plant-animal interactions.
  1. Color, pattern, and mate choice
  1. Color and pattern variation within populations

I. What is color?  I'm sure most of you know this but perceived color is the result of some wavelengths of light being absorbed by objects and other wavelengths being reflected.  For example a cardinal is red because its feathers reflect red light and absorb other visible wavelengths of light.

One important point to note here is that the perception of color is dependent on having photoreceptors (cells that detect light) that are sensitive to the wavelength in question.  For example our eyes cannot detect ultraviolet light and thus we cannot see differences in reflectance  of ultraviolet from different surfaces - there are colors we cannot see.  We'll discuss this in more detail in a future installment.

II. What causes different colors in plants and animals?  The green color of most above ground plant tissue is due to the presence of the photosynthetic pigment chlorophyll.  Some plants and other photosynthetic organisms have other photosynthetic pigments as well leading to somewhat different colors in groups such as the brown algae (kelp).

Of course plants are known for the brilliant colors of their flowers and their fruits and to a lesser extent their seeds.  Plants are capable of synthesizing a very large number of different types of pigments.  Pigments are chemicals that reflect particular colors.  Plants pignments have, of course, been widely used as dyes by humans.  We will also consider the reasons for the evolution of plant coloration in a later diary.

Animals are  less capable when it comes to making their own pigments.  Animal color is effected by many different classes of pigments but here are a few of the main ones.

Melanin - The main type of pigment influencing animal coloration are the different types of melanin.  Depending on the type and amount of melanin the color produced can by black, brown, tan, yellow or red (generally not bright red or yellow).

Carotenoids - Red and orange.  Carotenoids are synthesized by plants and fungi.  Animals get them through their diet. For example, it is well known that flamingos will lose their brilliant colors in captivity unless they are fed a diet containing carotenoids.  Interestingly animals are capable of synthesizing pigments known as pteridines which have similar spectral qualities (i.e. can make similar colors).  The fact that carotenoids are used so frequently by animals may indicate a substantial metabolic cost to synthesizing pteridines.

A really cool recent study was looking at color variation in an aphid species - both red and green aphids are known in this species.  Much to the researchers' surprise the aphid was find to be able to synthesize a carotenoid, something previously unknown in animals.  When the gene responsible was sequenced it was found to be very similar to a gene in fungi, indicating that the aphid got the gene through lateral gene transfer (very common in bacteria - the transfer was most likely accomplished through the action of bacteria and/or viruses).

There are a number of other groups of pigments made by animals that are less well known.  Ommochromes and Papiliochromes are pigments only known to be made by insects.  They are mostly reds and yellows.  Ommochromes occur in different parts of insects but are best known for producing the often quite striking colors in compound eyes.  Papiliochromes are only known from butterflies where they produce wing colors.

Animals cannot produce green or blue pigments. These colors in animals are known as structural colors as they are produced by light interacting with the physical shape of the surface of the animal.  One example would be the use of crystals to diffract light before it hits a pigment layer underneath.

III. Some simple examples of color variation - melanism and albinism.

The rat snake Pantherophis obseleta is a widespread, large snake in the eastern and central US and extreme southern Canada.  In the northern part of its range it is known as the black rat snake.

Juveniles hatch out as snakes with a pale background with distinct blotches of darker color.  As individuals age the background color darkens until the snake's back and sides are black with the difference between the blotches and the background barely visible if at all.

In areas to the southwest, the snakes also darken with age but they never become solid black and the blotches are always visible.

 

Here in the southeast the snakes retain the juvenile coloration throughout their entire life.

This variation is caused by differences in the amount of melanin laid down in the cells over time.  It seems likely that the variation is driven by a tradeoff between body temperature and camouflage.  In the north the dark color is advantageous for absorbing heat but it is less cryptic (more easily seen).  In the south absorbing heat is less important and a more cryptic color pattern is retained throughout life.  Note that this is a hypothesis on my part, I don't know if anyone has actually tested this.

The 'overproduction' of melanin such that it obscures the 'normal' color is known as melanism.  It is a very common phenomenon.  Anyone who has visited Toronto will note that all the squirrels there are completely black.  These are melanistic eastern grey squirrels.  Black squirrels occur in greater or smaller numbers in various areas throughout the range of eastern grey squirrels.  No one seems to have done a systematic study but the frequency of melanism is said to be higher in northern and urban populations.

Melanism is a widespread phenomenon.  The North American black bear has two common color phases: black and cinnamon, which presumably larger differ in the amount of melanin.  Leopards and jaguars and other spotted cats very commonly have melanistic individuals.  In these cases the animal appears solid black but the spots can still be seen.  The persistence of the spotting indicates that the dark color is due to melanin obscuring the pattern rather than a genetic change that removes the pattern.

It is thought that melanism is adaptive in leopards and jaguars either due to improved ability to ambush prey in dense forest habitats or improved resistance to viral diseases.  Melanism in these species is due the same gene (locus) in both species but different alleles in the two cases (see discussion of cat color genetics below).

The most famous example of adaptive melanism is in the peppered moth in Britain.  This species occurs in a light form that is off white with darker speckles and a melanistic form that is mostly dark gray/black.  Prior to the industrial revolution the light form predominated.  With the advent of factories, coal soot blackened the tree trunks and branches near urban areas.  The melanistic form rapidly became more common.  With environmental regulation, soot was reduced, the trees gradually became less dark and the light form of the moth returned to dominance.

Melanistic and pale versions of the moth on differently colored bark

Work in the mid 20th century by Bernard Kettlewell demonstrated that the change in moth color frequency was due to bird predation, an idea that had originally been proposed in the late 19th century.  More recently Michael Majerus expanded and refined Kettlewell's experiments to conclusively demonstrate that moths were more vulnerable to predation when their color does not match the tree bark color.

The opposite of melanism is albinism.  Albinos are indivdiuals who do not produce melanin.  Their coloration depends on any other pigments that are produced.  Albino mammals typically have all white fur but albino reptiles, while pale, often have color patterns produced by other pigments

IV. Genetics of Animal Color.

A) Fruit fly eye color.  We'll start out with a consideration of the genetics of eye color in the 'fruit' fly, Drosophila melanogaster (I've put the word fruit in quotes because Drosophila feed on yeast associated with over-ripe or rotting fruit rather than on the fruit itself and entomologists use the term fruit fly for an entirely different group of flies whose larvae do in fact eat fruit).  This is a fairly simple example in which the genetics is well understood.

The fly eyes are a brick red color in wild type individuals.  The term wild type refers to the form normally seen in natural (i.e. wild) populations.

The brick red color is the result of two different pigments occurring in the eye: drosopterin which is bright red and ommochrome which is brown.  Each of these pigments is synthesized by the fly and transported into the eye.  Mutations in a variety of genes can affect the eye color.

Some genes code for (make) proteins that transport the pigments.  A mutation in a gene for a ommochrome transport protein that produces a non-functional protein means that no ommochrome gets transported.  So the eyes contain only drosopterin and are bright red.  Because of a tradition that genes are typically named based on the results of mutations these gene is known as scarlet.

The converse is a gene that codes for a drosopterin transport protein.  A non-functional mutant means that the eye contains only ommochrome and is brown.

A third mutation affects a gene coding for a protein involved in the transport of both pigments.  Flies with this mutation have white eyes (no pigments) and are blind.

Other mutations affect the synthesis of the pigments and can result in different variants of the pigments and novel colors (e.g. sepia).

So what we see are a variety of ways to affect the color by affecting both the transport and the synthesis of pigments.

B. Cat Coat Color.  Now we'll switch to discussing coat color in domestic cats (aka 'Pooties') without worrying so much about what the genes actually do and considering more of the 'big picture.'  We'll start out by focusing on two main genes: agouti and tabby.

Both agouti and tabby are loci (singular locus).  A locus is a particular location on a chromosome containing a gene that has a particular function.  For example the scarlet locus in the example above coded for a particular protein that transported ommochrome into the eye.  An allele is a particular variant of the locus.  Again in the example above the wild-type allele produces the normal functional transport protein while the allele that results in the scarlet eye produces a non-functional transport protein.

The tabby and agouti loci produce the main elements of a cat's coat color pattern (that is if the cat resembles the wild cat ancestor at all closely).

The agouti locus controls the deposition of melanin in individual hairs.  The dominant allele (A) produces hairs that have three different colors occurring in bands on each individual hair. This gives the fur a grizzled appearance (i.e. it doesn't appear a solid color).  The recessive allele (a) produces solid colored hairs.  The default color of these hairs is blacks (depends on other genes).  This is similar to the situation in leopards.

The tabby locus causes the alternation of areas of solid black fur with areas of agouti fur.  If a cat has two copies of the recessive a allele at the agouti locus then it has black fur and the tabby locus is irrelevant.  If the cat has at least one A allele then the tabby locus controls the pattern of black vs. agouti.  There are three alleles.

T produces what is known as the mackerel tabby pattern.  Roughly parallel stripes of black run down the sides of the animal, along with bands on the legs and tail and facial striping.

Ta produces the Abyssinian color pattern.  All of the fur is agouti.

t produces the classic tabby pattern.  The black color occurs in thick curving and swirling stripes.

There are many additional genes that modify the basic patterns that I have shown.  I'm only going to mention one other because it illustrates  a couple of points and is especially interesting; the point color pattern found in Siamese cats (and in some other animals as well).

The genotype (genetic combination) in Siamese cats makes the production of melanin temperature dependent.  The torso of the body is too warm and the fur does not develop the 'normal' color.  Only regions far from the body core such as the ends of the limbs, tail, ears, and snout develop color.

Typically and historically siamese cats have had very specific colors for their points with solid colored fur only.  This is not a necessary requirement of the siamese genotype - it reflects the absence of the A allele at the agouti locus in the ancestral populations of these cats.  More recently the A allele has been bred into some siamese resulting in the 'lynx-point' color pattern.

The important point I want to make here is that genes don't operate in a vacuum.  They interact with one another and with the environment.  Creating a new color pattern doesn't require changing everything.  Just changing one piece can create something new.

Examples of Color Evolution:  Humans and Mice

Human skin color is one of our most obvious external characteristics.  It has a long and sorry history associated with bigotry.  Why do humans vary so much in skin color?

The mechanistic answer is that people differ in genes that control the amount of melanin lying under the skin.  More melanin = darker color.

The evolutionary answer is that skin color is the result of an evolutionary trade off.  There is (was) a geographical pattern (before the large scale movements of people in the last few centuries) in which human populations close to the equator tend to have darker skin and human populations at high latitudes tend to have lighter skin.

Melanin provides protection from UV damage and is beneficial in the intense sunlight of the tropics.  However melanin also interferes with the synthesis of vitamin D (which requires sunlight) and is disadvantageous in areas with less sunlight.

A recent (and local) hot area of evolutionary color research is on populations of mice living in coastal sand dune habitats in Florida.  These mice belong to a widespread species but they are strikingly different in coloration from inland populations.  Inland mice are fairly dark brown while the dune mice are almost white.  It seems fairly clear that the difference in color is driven by predation and the advantage of resembling your background in not getting eaten.

Beach and inland mice for comparison

Research on mice in populations along the gulf coast differ from inland populations consistently at two loci (i.e. the beach mice have different alleles for both genes than inland mice).  One is our friend from above, the agouti locus and the other is something called a melanocortin receptor.  I'm not going to get into the technical details here but one important point is that the white color of the beach mice depends on having the specific combination of both 'beach' alleles.

Another interesting fact is that Atlantic coast beach mice are also pale but use different alleles to get the same color.

Hopefully this introduction has given you the notion that from a relatively small number of building blocks a wide range of colors are possible and that interactions between genes can produces lots of color variation.

We haven't really touched on how color patterns get produced very much. That's for next time when we will also start looking at more sophisticated ecological explanations for color.

Originally posted to matching mole on Sun Dec 05, 2010 at 03:44 PM PST.

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