Penetrance.

PZ Myers has another excellent post, this time on the role of temperature in regulating development. This got me to thinking about a few things:

1. Developmental plasticity probably makes for great difficulties in comparing experimental results obtained by different labs, different experimenters in the same lab, or even the same experimenter at different times. I bet this causes problems when comparing gene expression data obtained in different experiments (yes, I know there are internal controls for microarrays/rtPCR).
2. More practically, rearing temperature can affect the penetrance of certain Drosophila mutants. For example, the mutation eyeless (ey) causes the eyes to appear much smaller than usual (similar phenotype as Drop). When mutant flies are reared at higher temperatures they have nearly wild-type eyes. This is a drastic phenotypic difference! I learned this while TAing a biology course in which this mutation was used as a phenotypic marker. In order to ensure that the students had enough flies to score every two weeks they were reared at above 25C. This changed the penetrance of the mutant phenotype and gave the students fits scoring their crosses.
3. Phenotypic plasticity also plays a role in evolution. This is most famously described in West-Eberhard's Developmental Plasticity and Evolution. It also provides for a nice way in which the genotype can "interact" with the environment. Because temperature influences the phenotypic expression of alleles, a genotype may manifest itself differently in different environments. I'm sure this is nothing new, but it provides a more tangible way to understand why fitness depends on the environment.

Go read PZ's post for yourself, as it's another great example of his ability to distill a scientific publication for lay consumption.