Gregor Mendel meets the Aridlands Tiger Beetle
In Arizona a couple of weeks back, I had a lot of fun photographing the Aridlands Tiger Beetles (Cicindela marutha). I first encountered this species three years ago when visiting Willcox Playa with my collecting buddy John. We were astonished at the diversity and abundance of tiger beetles on the playa, though there was a bit less diversity than we first thought. Most of the C. marutha that we encountered were bright green, however a few were coppery red. We initially thought them a separate species.
The color variants of C. marutha are well known. From Pearson's Field Guide to the Tiger Beetles of the United States and Canada:
Bright green individuals are most common in the southern parts of its range, and bright rusty-red individuals are more common in the northern parts of its range, but both forms occur together in many areas.Willcox Playa is in the south of the species' range, so no surprise that we are seeing mostly green color forms (morphs). What we observed, however, is that there are actually three color morphs of C. marutha on Willcox Playa. Some of the green individuals are the clear, bright green shown in the top photo. A smaller number of the green individuals are a brassy yellow-green.
I had wanted to get photos of all three color morphs during my recent visit, and I was only partly successful. Here is the brassy yellow-green color:
This is not just a feature of the angle from which you are viewing the beetle, some of the green beetles really are yellower than others. Unfortunately, I was not able to get a photo of a red beetle. It's far and away the least common of the three colors. In 2007, when we saw huge numbers of C. marutha, I managed to see three or four individuals, and collected one. Last year there were fewer C. marutha and I saw only one red one. This year, there were even fewer tiger beetles and I didn't see a single red form, hence no photo. Here is a picture of a pinned red specimen:
All of this leads into my hypothesis. Most likely, the elytral color is determined genetically. The simplest explanation of the observation of three colors with a common, relatively rare, and very rare form is inheritance involving two co-dominant alleles. Much as crossing a red flower with a white flower will produce pink offspring, crossing a green C. marutha with a red one produces the yellow-green offspring. Green individuals are homozygous for (have two copies of) the green allele, red individuals are homozygous for the red allele, and yellow-green individuals are heterozygous- they have one copy of each allele. The distribution of colors in the population suggests that at Willcox Playa, the red allele is rarer than the green allele.
This hypothesis would be very easily testable by making crossing individuals with various combinations of the elytral colors in the laboratory. For example, crossing two yellow-green individuals should result in 1/4 of the offspring being green, 1/4 of the offspring being red, and 1/2 of the offspring being yellow-green. If the reality of the inheritance pattern is more complicated than what I described in the hypothesis, then these proportions of offspring will not be observed.
Most of us probably learned this stuff in junior high school biology class, yet somehow I'm always amazed to see visible evidence of Mendalian genetics in the wild.