New Book: What a Mouse Can Teach Us About Guppy Color

Yesterday I finally bundled the last of over 177 scientific papers into comb binders and put the finishing touches on a new PDF document, the "Guppy Research Abstracts."

The document lists all the scientific research papers on pigmentation and color pattern that I have found useful in the past decade or so. Included is each paper's abstract, a summary of the research and its findings.

The documents are sorted into four categories: general research, zebrafish specific research, guppy specific research and fin specific research. The table of contents lists all the papers alphabetically. You can also use the Acrobat PDF search tool to find papers on specific subjects (like melanophore or the blond gene). A wonderful tool for others who want to explore the science of fish color and pattern.

The document will be expanded in three ways over the next few years. I will be adding more papers of course, especially those suggested by my friends. I will also be expanding the comments underneath each extract. Comments remind me of why I think the paper is important. Or they record parallels I find in my own research. Or they add clarity to the paper by supplying background.

One of the first papers listed in this first edition of the book is a study I discovered recently that gelled ideas I have been playing with for the past few years about guppy color and pattern.

The paper is called "Adaptive Variation in Beach Mice Produced by Two Interacting Pigmentation Genes," by Cynthia C. Steiner, Jesse N. Weber and Hopi E. Hoekstra. It was published in PloS Biology 5(9) in 2007. Here is the abstract:

Little is known about the genetic basis of ecologically important morphological variation such
as the diverse color patterns of mammals. Here we identify genetic changes contributing to an adaptive
difference in color pattern between two subspecies of oldfield mice (Peromyscus polionotus). One mainland
subspecies has a cryptic dark brown dorsal coat, while a younger beach-dwelling subspecies has a lighter
coat produced by natural selection for camouflage on pale coastal sand dunes. Using genome-wide linkage
mapping, we identified three chromosomal regions (two of major and one of minor effect) associated
with differences in pigmentation traits. Two candidate genes, the melanocortin-1 receptor (Mc1r) and its
antagonist, the Agouti signaling protein (Agouti), map to independent regions that together are responsible
for most of the difference in pigmentation between subspecies. A derived mutation in the coding region of
Mc1r, rather than change in its expression level, contributes to light pigmentation. Conversely, beach mice
have a derived increase in Agouti mRNA expression but no changes in protein sequence. These two genes
also interact epistatically: the phenotypic effects of Mc1r are visible only in genetic backgrounds containing
the derived Agouti allele. These results demonstrate that cryptic coloration can be based largely on a few
interacting genes of major effect.

You might wonder what the color pattern on a terrestrial rodent could possibly have to do with a guppy. Well, first of all it is a well known fact that color and pattern is widely conserved across the animal kingdom. And indeed this is what we find here. The mouse and the guppy both have the Mc1r gene. And a version of the agouti gene has also been found in fish. In mice, the two genes act together as a switch to produce lighter or darker fur. In fish the two genes act together to create the dorsal-ventral pigment pattern, dark on the dorsal and silver on the ventrum. (I define both these terms in the dictionary at the back of the Guppy Research Abstracts.)

What is interesting about the mouse paper is that it shows that the Mc1r gene and the agouti gene do not have a simple relationship. "The distribution of phenotypic scores among F₂ individuals was not consistent with simple Mendelian inheritance." (p. 0002). "Thus, the effect of each of the two genes on phenotype clearly depends on the genotype at the other locus." What the authors are describing is the interaction of two different genes, i.e. genes that are not alleles. This is a very important concept because it means you cannot say that one simply suppresses the other or is dominant over the other. Mendel is little help to us here. What the authors describe is a fairly elaborate switch that allows beach mice to turn down their dark color against the light sand dunes by switching between the production of the two types of light and dark pigments in the mouse black color cell. I found this fascinating because the parallel situation in guppies is between a guppy with lots of black color cells versus a guppy with a lot of iridophores. Is a metallic guppy equivalent to a light colored mouse? I think so... In any event, this is a very interesting model to keep in mind when thinking about how two genes might interact to produce color in the guppy...the subject of a good part of my last blog (A New Tool for the Guppy Color Investigator). See how the scientific research begins to expand and support observations that start right in your fish room?

Why is it important to read the scientific literature? Well I have had the suspicion for some time that the deeper we dig into guppy color genetics the fewer genes we will find. That is exactly what the authors of the mouse paper say about their discovery (p. 0004). "Thus, a small number of chromosomal regions--and perhaps only a few genes--are responsible for most of the difference in color pattern between subspecies." Was I prescient? Nope. I must have absorbed that view from reading other scientific papers. This paper just shows rather elegantly how such a view works on the molecular level. Best of all, it is backed up by solid scientific research.

Like many of the papers listed in the Guppy Research Abstracts, this one can be found and downloaded from the Internet for free: http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.0050219. So you can read it to find out exactly how these two genes (along with a third, the kit gene) act to produce dramatic changes in the phenotype of the mouse. Look at the pictures. You will see that the light colored underbelly of the mouse (equivalent to the silver belly of the guppy) moves up toward the dark dorsal, somewhat overtaking it. It made me wonder if full metallic guppies were due to the same mechanism described by the scientists. Is the key to a full metal guppy found buried in this paper?

I think you can see where I get most of my ideas when I am speculating about guppy color genetics. In the newly published Guppy Research Abstracts I am sharing the results of my search through the literature in the past ten years. There is a lot of value in collecting 177 papers relevant to guppy color genetics into a book. But I think the real added value over time will be the comments I add underneath the abstracts themselves.

Right now I am giving away the first copies of the abstracts to people who buy my genetics book "The Theory and Practice of Guppy Breeding." But I am thinking the book will be more useful to people who read the new book when I have it done, the Guppy Color Manual. By then hopefully I will have the abstracts thoroughly annotated. The Abstracts actually began as an appendix to that book, and grew too large to include in it. I will probably give it free to people who buy that book. Given that it is really the first draft of the book and will be expanded, I do not want to release it widely. Except to a few fanatics...