In the April issue of Scientific American, I present recent insights into the extraordinary evolution of cichlid fish. Although many factors have probably contributed to the stunning success of this group, one presumably very important one is their unusual feeding apparatus: In addition to the usual mouth jaws that other fish have, cichlids possess an extra set of jaws in their throats—like the creature in the movie Alien. These throat jaws, which are highly modified gill bones, with associated ligaments and muscles, serve as an additional food processer that can crush or pierce food after it passes through the mouth. The development of this second pair of jaws meant that cichlids could evolve all sorts of specializations of their mouth jaws for particular food items without danger of becoming too dependent on a given food source, because they had the throat jaws to fall back on.
What makes the throat jaws all the more amazing is that, at least in some species, they can adapt to different food sources within an individual’s lifetime. My colleagues and I investigated the morphological flexibility of the throat jaws in a series of experiments with a generalist river-living species (Astatoreochromis alluaudi) that in different populations either feeds on snails or doesn’t depending on how common this food source is. In these fish the teeth and the bone on the throat jaws that holds them change in response to feeding on hard prey items. Fish teeth are not permanent like ours. Rather they are replaced continually with new ones from below every six weeks or so. The cells in the bones of the throat jaws apparently can sense the mechanical strain that arises from the physical stress of feeding on such hard foods.
Our experiments have identified a number of genes and gene interactions that might be involved in bringing about these profound changes in the shapes of the bones and teeth that allow the fish to feed on these hard snails more efficiently. As a result of this work, we are beginning to better understand the molecular developmental basis of phenotypic plasticity—the ability of an individual to cope with environmental changes by altering its morphology accordingly.
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One idea that we are currently testing is whether more specialized species than this river cichlid, such as the extreme specialists that live in Africa’s Lake Victoria, have lost their ability to respond to environmental changes. Have they become more hardwired so to speak? And are the same genes and gene pathways that are initially responsible for plasticity later recruited in a more rigid way for permanent species-specific differences? Has specialization evolved at the cost of plasticity? Food for thought.
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