Journal club: Choosy females speed speciation of poison dart frogs
SUMMARY: Female poison dart frogs' distinct preferences for mates with particular colours may be a first step in the process of speciation
Strawberry dart frog, Dendrobates pumilio, photographed at La Loma Jungle Lodge on Isla Bastimentos, Panama.
Image: Cori Richards-Zawacki.
Speciation is often a subtle thing, starting with a tiny difference here or there, and after a few generations, these changes begin to add up into something obvious. But speciation only happens after a portion of a population becomes reproductively isolated from the larger group.
Many events can lead to reproductive isolation. Solid barriers such as mountains or oceans are the most common, but other factors can also be powerful isolating mechanisms. Female choice is particularly influential, especially in populations where there are two obviously different types of males to choose from. One such example is a Panamanian population of poison dart frogs that has two distinct colour morphologies ("morphs") living side-by-side.
The strawberry dart frog, Dendrobates pumilio (pictured above), is a brilliantly-coloured dynamo that can sit comfortably on a €2 coin -- if it would only sit still. These diminutive frogs are found on the islands of the Bocas del Toro archipelago and the adjacent mainland of Panama (see map):
Map of Bocas del Toro Archipelago, Panama.
Image: Alexander Karnstedt (Creative Commons Attribution 3.0 Unported license.)
This frog species has an amazing variety of vibrant colours, including red, yellow, blue, green, orange and white, along with various combinations and patterns, known as colour morphs. Yet surprisingly, all of these colour morphs comprise the same species: the males all have similar mating calls and lab studies have found minimal genetic divergence in a number of genes that are known to change rapidly.
Depending upon where in their range they are seen, these frogs have different colour combinations and patterns that are characteristic for that area. But in a few areas, such as the Panamanian island of Bastimentos, frogs of two colour morphs, red and yellow, live side by side.
Laboratory studies indicated that female choice is the driving force that maintains both colours in this population.
"Both red and yellow females from this population preferred to court with males of their own phenotype when given a choice between red or yellow", according to a paper published by Corinne Richards-Zawacki, an assistant professor of ecology and evolutionary biology at Tulane University, Harvard University postdoctoral researcher, Ian Wang, and Kyle Summers, an associate professor of biology at East Carolina University. But it has been pointed out that behavioural studies in the lab may be misleading because constraints on time, memory and mobility prevent females from visiting all possible mates -- so what are female frogs actually doing in the wild?
To learn more about these frogs' natural history, the researchers conducted an 18-month mark–recapture study designed to provide insight into the genetic basis of these frogs' colour traits and to identify their breeding patterns from their pedigrees.
The researchers captured 677 wild strawberry dart frogs (312 males, 274 females, and 91 juveniles) on the northwest tip of the island of Bastimentos. Each frog was classified by sex, measured, weighed and photographed. The tip of the fourth toe of each hind leg was clipped off and saved for later DNA analysis (frogs regenerate their digits, so they were not permanently damaged).
The team constructed a pedigree analysis -- a Punnett square for all you genetics students -- to show the observed frequency of yellow versus red coloration (figure 1):
Figure 1. Expectations for offspring phenotypes if red is completely dominant over yellow.
According to their data, 35 percent of male and female frogs had a yellow back. This frequency is consistent with two alleles of one gene are competing for a single locus where the allele for one trait, red colour in this case, is completely dominant over the other, yellow colour in this case -- a textbook example of a Mendelian dominant (X = red, x = yellow; figure 1).
To identify whether female frogs had chosen their mates based on colour, the team examined the DNA from their 677 genotyped frogs to find mother-father-offspring trios. They identified 32 mother–father–offspring trios with 90% confidence, and 24 of those trios were also identified with 95% confidence. Since the scientists had already estimated all their study frogs' colours from photographs, they analysed this colour data and found that nearly all wild red morph females (91%) mated with a red male.
What is interesting is that no clear pattern was identified for wild yellow morph females. This could be due to a number of reasons, but other researchers' findings indicate that females of both colour morphs actually do prefer males of their own colour -- when given a choice. But since there are far fewer yellow males to be found in any given location, bypassing breeding opportunities with a red male to actively seek out a yellow male that may not be found in a reasonable span of time imposes a greater cost to yellow females than it does for red females. So in this population, wild red female frogs can "afford" to be choosy, whereas the wild yellow females cannot.
These findings provide more insight into the role of sexual selection as well as the importance of quantifying the patterns of female mate choice.
Of course, this little paper will lead the thinking reader to ask how a given female knows what colour she is. It's not known, but could be based in either the animal's physiology or genetics. I am not a frog expert, but I am curious to know how scientists will design convincing experiments that will get at the answer to this important question.
Corinne L. Richards-Zawacki, Ian J. Wang & Kyle Summers (2012). Mate choice and the genetic basis for colour variation in a polymorphic dart frog: inferences from a wild pedigree. Molecular Ecology, 21, 3879-3892. doi:10.1111/j.1365-294X.2012.05644.x
Corrine Richards-Zawacki, emails: 28 & 29 August 2012.
Tulane University press release
NOTE: this story is slightly reformatted from the original for republication here.
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