In my last post I started a new short series on some biologists’ attempts to solve what they call an “adaptive recursion” or in other words, to know the full story of a trait from the bottom level of the gene to the top levels of ecology and differential fitness. Ecological descriptions frequently become “just-so stories” – claims of adaptations and how they arose but with little evidence. All levels of detail should be known before any such arguments can be proclaimed and this is exactly what Uwe Stolz, Jeffrey Feder, and Sebastian Velez, and others are attempting to do with the bioluminescence of Jamaican click beetles.
One of the first steps towards solving the adaptive recursion is to quantify the phenotype in terms of its nature (in this case, wavelength), allele count, and population frequencies.After capturing 219 beetles across Jamaica, the authors were able to use a spectrophotometer on their bioluminescent organs and record the lights’ wavelengths. They found three dorsal phenotypes (green, lime, and yellow-green) and five ventral phenotypes (yellow-green, green-yellow, yellow-orange, and orange). The frequencies and wavelengths of the five ventral phenotypes are shown in Fig 1.
How could the two organs have different ranges of phenotypes? When the authors asked if there was any correlation between the two organs, they found none (r = 0.01, P = 0.88, 219 df). This indicates that there may be a separate gene for each organ. The authors also found that the luciferase sequences cloned from each organ differed. Furthermore, the isolated mRNA sequences matched genomic DNA sequences – there was not a single locus that produced two different mRNA transcripts. Each organ is controlled by a different gene.
So the click beetles have two luciferase genes, one for each type of organ, dorsal and ventral. The authors further discovered that the dorsal organ has two alleles, green and yellow-green, and the ventral organ has three alleles, yellow-green, yellow, and orange. As discussed in the previous post, these alleles can mix when the beetle is heterozygous. If a beetle has an orange allele and a yellow allele, a yellow-orange color results. In the light of other combinations, this trait is probably Mendelian: a single locus with multiple alleles and no epistatic or pleiotropic interactions. However, because the beetles cannot be bred in the lab, the Mendelian hypothesis cannot be 100% confirmed although it is likely so. Furthermore, there could be more than one gene for each trait, but the authors state that the methods for doing so could not work with the beetle species.
Jamaican click beetles have two separate genes for each kind of organ, dorsal or ventral, that most likely follow simple Mendelian rules. A potentially interesting point not brought up by the authors is that the two genes are probably a result of a gene duplication and that the phenotypes are not merely differentially regulated (as would be supposed by evo-devo advocates). Which method is simpler, I wonder? Evolution does not always proceed down the simpler path, however…
The link between gene and phenotype is quickly being resolved in this specific trait and the authors are one step closer towards completing the “adaptive recursion.” The next post will detail more of the gene – phenotype link by examining how these different alleles are related and how these alleles arose. After that we will look at how evolutionary processes such as natural selection have affected these genes.
Stolz U, Velez S, Wood KV, Wood M, & Feder JL (2003). Darwinian natural selection for orange bioluminescent color in a Jamaican click beetle. Proceedings of the National Academy of Sciences of the United States of America, 100 (25), 14955-9 PMID: 14623957