Saturday, February 06, 2010
Two new papers are out in PLoS Genetics which make inferences about adaptation using butterfly species which exhibit Mullerian mimicry. I'll give the author summaries instead of the abstracts.
Genomic Hotspots for Adaptation: The Population Genetics of Mullerian Mimicry in the Heliconius melpomene Clade:
The diversity of wing patterns in Heliconius butterflies is a longstanding example of both Mullerian mimicry and adaptive radiation. The genetic regions controlling such patterns are "hotspots" for adaptive evolution, with small regions of the genome controlling major changes in wing pattern. Across multiple hybrid zones in Heliconius melpomene and related species, we no find no strong population signal of recent selection. Nonetheless, we find significant associations between genetic variation and wing pattern at multiple sites. This suggests patterning alleles are relatively old, and might be a better model for most natural adaptation, in contrast to the simple genetic basis of recent human-induced selection such as pesticide resistance. Strikingly, across the region controlling the red forewing band, a very strong association with phenotype implicates three genes as potentially being involved in control of wing pattern. One of these, a kinesin gene, shows parallel differences in expression levels between divergent forms in the two mimetic species, making it a strong candidate for control of wing pattern. These results show that mimicry involves parallel changes in gene expression and strongly suggest a role for this gene in control of wing pattern.
Genomic Hotspots for Adaptation: The Population Genetics of Mullerian Mimicry in Heliconius erato:
Identifying the genetic changes responsible for beneficial variation is essential for understanding how organisms adapt. Here, we use a combination of mapping, population genetic analysis, and gene expression studies to identify the genomic regions responsible for phenotypic evolution in the Neotropical butterfly Heliconius erato. H. erato, together with its co-mimic H. melpomene, have undergone parallel and concordant radiations in their warningly colored wing patterns across Central and South America. The "genes" underlying the H. erato color pattern radiation are classic examples of Mendelian loci of large effect and are under strong natural selection. Nonetheless, we do not see a clear molecular signal of recent natural selection, suggesting that the H. erato color pattern radiation, or the alleles that underlie it, may be quite old. Moreover, rather than being single locus, the genetic patterns suggest that multiple, widely dispersed loci may underlie pattern variation in H. erato. One of these loci, a kinesin gene, shows parallel expression differences between races during wing pattern formation in both H. erato and H. melpomene, suggesting that it plays an important role in pattern variation. High rates of recombination within naturally occurring H. erato hybrid zones mean that finer genetic dissection will allow us to localize causative sites and better understand the history and molecular basis of this extraordinary adaptive radiation.
Here's a section from the first paper which I found intriguing:
The results therefore appear to support the 'shifting balance' model for the evolution of Heliconius colour pattern races...whereby novel wing patterns arise and spread through otherwise continuous populations behind moving hybrid zones...The 'Pleistocene refuge' model seems less likely, as recent contact after extended periods of geographic isolation would presumably have left a stronger signal of genetic differentiation between divergent races, perhaps across the genome but especially more strongly in regions linked to patterning loci...
I have no idea why they necessarily think this validates the shifting balance. You can see David's critique of the model, but reading Will Provine's intellectual biography of Sewall Wright it seems that the shifting balance sometimes becomes the evolutionary genetic version of "it's complicated."* What they seem to have done here though is refute a simple model of powerful selective sweeps giving rise to these morphs recently. Rather, these seem to be ancient local adaptations, whose frequencies and genetic architectures are perhaps perturbed by long term exogenous (e.g., environment) and endogenous (e.g., complex frequency dependencies) dynamics.
Despite my lack of clarity on a few theoretical issues, I found the papers very interesting, and haven't really processed them fully.
Baxter SW, Nadeau NJ, Maroja LS, Wilkinson P, Counterman BA, et al. 2010 Genomic Hotspots for Adaptation: The Population Genetics of Mullerian Mimicry in the Heliconius melpomene Clade. PLoS Genet 6(2): e1000794. doi:10.1371/journal.pgen.1000794
Counterman BA, Araujo-Perez F, Hines HM, Baxter SW, Morrison CM, et al. 2010 Genomic Hotspots for Adaptation: The Population Genetics of Mullerian Mimicry in Heliconius erato. PLoS Genet 6(2): e1000796. doi:10.1371/journal.pgen.1000796
* I see one reference to epistasis in both papers, and that concept is very important in the shifting balance. Though I assume the LD and supergenes might point to that.
Labels: Population genetics