Thursday, February 09, 2006

The amazing polyphenic Manduca

(Note: Many thanks to PZ Myers, whose own blog gave me the heads up on this one.)

Yuichiro Suzuki and H. Frederik Nijhout have just published the results of a very interesting study of evolutionary change observed in the laboratory. If you have an institutional subscription to Science, you can read it here. Since non-biologists might find the article extremely technical, I'll try to summarize it very briefly:

Every organism has a genotype, which is its (usually) unique complement of genetic material, and a phenotype, which is the way that its genotype is expressed in physical traits. Sometimes we use the terms "genotype" and "phenotype" to describe characteristics of the whole organism or of the interaction of genes; sometimes we use those terms to describe the structure and effect of a single gene. As an example: I have type A blood. Because my mother also has type A blood but my father had type O, I could have inherited the allele for the A antigen only from my mother, and had to have inherited the O allele from my father. So, my genotype with regard to AB blood groups has to be AO. But, because my A allele allows me to make the A antigen, a quick blood test (ouch!) will reveal that my blood phenotype is A.

A polyphenism is a slightly more complicated situation. Most of our visible, developmental, or biochemical traits depend not on single genes, but on interactions of multiple genes with one another -- and often with the environment. In many of the Lepidoptera (butterflies and moths), caterpillars or pupae in a summer brood will have different colors from those in an overwintering brood. One example is the tomato hornworm, Manduca quinquemaculata. M. quinquemaculata caterpillars that develop at 20ºC are black; those that develop at 28ºC are green. This color difference doesn't depend on a genetic difference. A black caterpillar can mature into a moth whose own offspring develop in a warmer seasonal environment and will be green. This is a true polyphenism.

M. quinquemaculata has a close relative, the tobacco hornworm M. sexta. M. sexta has natural color differences as well, but in this case, the differences seen in nature are genetic, and not a polyphenism. M. sexta caterpillars will be green regardless of developmental temperature -- unless they carry only a recessive allele called black, which makes them -- you guessed it -- black. This genetic mutation alters the level of a hormone which regulates, among other things, how pigmentation develops.

However, just as there's plenty of other variation among people who happen to have type A blood, there's other genetic variation among M. sexta individuals that happen to share a larval color phenotype. Reasoning that M. sexta might share some underlying genetic similarity with its close relative M. quinquemaculata, Suzuki and Nijhout set out to expose this variation by exposing black M. sexta to heat shocks more extreme than it would normally encounter in nature. When they applied temperatures of up to 42ºC to developing caterpillars, they discovered that some of the caterpillars would develop green coloration instead, while others changed much less or not at all. So, although it wasn't obvious under natural conditions, there was still variation among black-mutant individuals in how they responded to heat shock. (The color of the green "wild type" caterpillars was not sensitive to heat shock.)

Suzuki and Nijhout then carried their experiment further. They selectively bred a line of moths whose caterpillars were strong responders to heat shock, and a line that didn't respond to heat shock. (Like all good experiments, it had a control -- a line that was heat-shocked but not selectively bred to individuals that responded the same way.) After 13 generations, Suzuki and Nijhout had produced one line of M. sexta that responded to developmental temperature differences with a color change, much like M. quinquemaculata -- and another line that was always black, regardless of heat exposure.

In short, in M. sexta, the black genotype was required for temperature response -- but whether or not a temperature response could be induced in the black caterpillars depended on other genetic variation that was already present in the population. The more technical published details of this experiment describe the physiology of the hormone response, and some other experiments that the authors did to confirm their results. But, my favorite part of the story was revealed by a quick search of the web site of Duke University, where the team did its research. Prof. Nijhout is a well-known researcher in the field of insect developmental biology, but Yuichiro Suzuki, the first author of the paper, is a graduate student. There can't be many things more exciting to a young researcher than a first-authored paper in Science. As Alice learned in Wonderland, it can take real patience to pry answers out of a caterpillar.

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