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First published online February 15, 2006
Journal of Experimental Biology 209, v-a (2006)
Copyright © 2006 The Company of Biologists Limited
doi: 10.1242/jeb.02098

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Outside JEB

JAWS OF LIFE

Douglas Fudge

University of Guelph

dfudge{at}uoguelph.ca

If Charles Darwin had visited the African rift lakes instead of the Galapagos when he was developing his theory of biological evolution, we might now refer to the myriad species of cichlid fishes there as `Darwin's Fishes'. Like the finches of the Galapagos, African cichlids are a diverse group that has evolved fairly recently from a common ancestor, and they have been remarkably creative in the ways that they have carved up the adaptive landscape with regard to feeding. The diversity of feeding styles among cichlids includes algal scrapers, plankton eaters, fish eaters and even a group that makes a living eating fish scales. Not surprisingly, these groups exhibit dramatic differences in the functional morphology of their jaws, just as Darwin's finches differ conspicuously in the size and shape of their beaks.

The power output of the jaws is a product of the force they exert and their velocity of opening and closing. Because the muscle power available for moving the jaws is limited, species that favor biting force do so at the expense of biting velocity. Similarly, suction feeders that require their jaws to open and close at high velocity are not able to close their jaws with much force. This evolutionary trade-off can be neatly summed up by a quantitative trait called the `mechanical advantage,' with strong biters and fast suckers having high and low values of this trait, respectively. Although the functional morphology of this system is now well understood, its genetic and molecular underpinnings have been less certain.

In a recent paper, R. Craig Albertson and colleagues report findings that shed new light on the mechanisms involved in these adaptive radiation events in cichlids. To find out more about the genes involved, they conducted a genetic linkage study using two wild cichlid species, one an algal scraper (with high mechanical advantage jaws) and one a more typical suction feeder (with lower mechanical advantage jaws). By hybridizing these two species in two generations of crosses, they were able to identify candidate genes that correlate with differences in mechanical advantage. One of the genes that correlated with the mechanical advantage of jaw closing is bmp4 (or `bone morphogenetic protein'), which had previously been identified as an important player in vertebrate craniofacial development.

The researchers probed further by examining bmp4 expression in developing embryos and found that in those regions giving rise to the jaws, the timing and magnitude of bmp4 expression differed markedly between the strong biters and the fast biters. While these data further supported the link between bmp4 and mechanical advantage, the evidence was still circumstantial. Demonstrating a causal relationship required manipulating bmp4 expression and measuring the effects on the mechanical advantage of the jaws. For these experiments, they turned to the zebrafish model. Indeed, by overexpressing bmp4 in developing zebrafish embryos, they were able to effect profound changes in jaw morphology and mechanical advantage, which confirms bmp4 as a central player in the development and evolution of jaw morphology, especially with regard to mechanical advantage. Interestingly, other studies suggest that changes in the sequence and expression of bmp4 were probably important in the adaptive radiation of several other vertebrate groups, including, you guessed it, Darwin's finches.

References

Albertson, R. C., Streelman, J. T., Kocher, T. D. and Yelick, P. C. (2005). Integration and evolution of the cichlid mandible: the molecular basis of alternate feeding strategies. Proc. Natl. Acad. Sci. USA 102,16287 -16292.[Abstract/Free Full Text]

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This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Fudge, D. Search for Related Content PubMed Articles by Fudge, D. Social Bookmarking                         What's this?