Solving the “adaptive recursion” in Jamaican click beetles (I)

One of evolutionary biology’s old and ongoing problems is demystifying the link between genotypic and phenotypic changes. Scientists frequently know the changes in one of the two categories, but they infrequently know how a single change affects both. One great example we do know is the mutation that causes sickle cell anemia, but such knowledge can be rare, especially for more complex traits. While evo-devo aims to solve this problem, one of the more interesting criticisms of the field is its reliance on model organisms in the laboratory. Model organisms like zebrafish have been deliberately bred to exhibit as little genetic variation as possible – they all develop the same! When discussing evolution through studies of zebrafish, this feature – minimal genetic variation – hinders that very discussion. One has to go out to the field to see how evolution works.

Molecular biologists may rarely study any level higher than DNA and proteins, and ecologists may rarely study any level lower than a morphological trait. (This is a gross generalization.) These levels are different steps what some biologists call the “adaptive recursion”,

gene – phenotype – ecology – fitness – evolutionary change.

(I assume “recursion” is used as each element is embedded in another element.) In understanding the entire picture, we can get a better sense of how natural selection works on all levels of biology.

The reason we want to understand all levels of this recursion was outlined in the famous 1979 spandrels paper by SJ Gould and Richard Lewontin. In that paper, Gould and Lewontin warned against the “adaptationist program” in which biologists created just-so stories of how selection created this trait for that reason using minimal evidence. If that scenario were disproved, another story was proposed. Little regard was given to non-adaptive, or null, hypotheses like genetic drift, exaptations, or evolutionary byproduct…ism(?). Nowadays, selection has to be shown to have occurred at the genetic level before declarations of its influence on a certain traits are made. Before that even, we need to ask more questions. What changes are being made at the genetic level? How do those changes specifically affect phenotype? What is the natural history of the trait? What are its effects on fitness? Why are changes in allele frequencies occurring – selection, drift, or migration? That is, we need to study the entirety of the adaptive recursion.

These are hard and time-consuming questions to ask but they are important for a full (and not just-so) scientific story. Right now, biologists Uwe Stolz, Jeffrey Feder and Sebastian Velez are hoping to complete such a story, a full adaptive recursion, about the bioluminescent Jamaican click beetle, Pyrophorus plagiophtalamus.

The Jamaican click beetle is unique in the bioluminescent world in that it varies in the color it luminesces. Apparently other species have only a single color, including other members of the genus Pyrophorus, but Jamaican click beetles have multiple colors, ranging from green to orange (Figure 1). (Note that these colors can be mixed which will be discussed in the next post.) I do not mean an individual shows different colors, but that the species has multiple colors à la eye color in humans.

Fig 1. (A) Paired dorsal light organs of <i>P. plagiophthalamus</i>. (B) Allele colors in dorsal organs: green (dGR) and yellow–green (dYG). (C) Ventral light organ of a yellow bioluminescing beetle. The dorsal organ is also visible but does not bioluminesce during flight and is seen here due to the ‘‘irritated’’ state of the specimen. (D) Allele colors in ventral organ: green (vYG), yellow (vYE), and orange (vOR).

The beetles also have multiple bioluminescing organs, specifically a pair of dorsal organs and a single ventral organ. According to personal observations by the authors, the ventral organs are only visible during flight from below (in which the dorsal organs are not visible). Jamaican click beetles do not flash these lights either (like fireflies do), but are continuously lit during flight. The authors presume the ventral organ is what males use to attract mates but they carefully note that this is currently a just-so story for this part of the adaptive recursion. (One possible alternative is that the organs are used to confuse predators.) The authors have not reached this part, however.

Why study these beetles? The primary reason is that the bioluminescent trait is easy to identify and seems important to click beetle ecology. Furthermore, the biological mechanism of bioluminescence – a single genetic pathway involving the enzyme luciferase – is well understood. There is no detected pleiotropy or epistasis; the trait is easy to understand and seems to follow Mendelian rules. This way the colors can mix – two different alleles produce a unique color à la snapdragon color (red allele + white allele = pink phenotype), expanding possible phenotypes by mixing green with yellow, or yellow with orange.

The reason to even study the beetles in the first place, however, is that color variation is not static throughout Jamaica. Instead, some populations show more orange and others more yellow and green. Feder, Stolz and Velez have hypothesized that natural selection is currently favoring orange. Why?

Subsequent posts will discuss the following:

What are these bioluminsecent colors? How were they identified?

What is happening to the frequencies of the colors? Is there change? Is there evidence of selection?

How did these different colors arise in terms of mutation, recombination, and gene flow?
Stolz, U. (2003). Darwinian natural selection for orange bioluminescent color in a Jamaican click beetle Proceedings of the National Academy of Sciences, 100 (25), 14955-14959 DOI: 10.1073/pnas.2432563100

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