Jaw jaw

The spectacular displays of adaptive radiation by cichlid flocks in Africa's Rift Valley lakes have received their fair share of attention from hobbyists as well as evolutionary biologists and ecologists. Darwin's dreamponds offer a snapshot of the faunal diversity that a small founder population can achieve over geological time. Lake Victoria in its present state is estimated at just over 14,000 years and in this span, at least 300 species of Haplochromine cichlids known locally as furu have been identified. The introduction of the Nile perch to the lake during the 1960s, however, has happily made things much simpler, as this gargantuan predator has eradicated about 200 species since. Lake Malawi, with at least 500 described species, is reckoned at 4-5 million years, while old fogey Lake Tanganyika, at 9-12 million years, harbours at least 300 endemic cichlids from 12 lineages, including the world's largest cichlid, Boulengerochromis microlepis, which reaches over three feet in length and looks to me more

Each lake offers a unique living laboratory in which phylogenetic histories and morphological adaptations can be examined and analysed to obtain ideas on how the physical environment, sexual selection, feeding competition and geographical isolation (rocky reefs in the lakes are separated by vast oceans of sand and serve as underwater islands that spawn new populations and species) impose their respective pressures on natural selection, resulting in the blooming of an incredible diversity of shapes, sizes and behavioural traits from generalised ancestral types (thought to be a creature resembling Haplochromis elegans). For sheer variety of forms, Lake Tanganyika is probably the most notable, with cichlids that resemble gobies (below right), jacks and sardines, as well as shell-dwellers (right), algae-scrapers, pelagic piscivores, planktivores, sand divers, scale rippers, pack hunters and even one that mimics a corpse in order to draw prey (pictured below left). One of the most popular species amongst aquarists is Lamprologus brichardi (left), which practices sibling brood care. In Lake Malawi, two broad clades are well-known: the rock-hugging mbuna and a sand-loving tribe.

Palaeoblog pointed me to this press release that shed light on the genetic map of cichlid jaws. Now, a key anatomical feature of cichlids that has arguably contributed to the family's success in African and American tropical waters is their jaw-in-jaw construction. Behind the external set of jaws lies a second pair of often powerful and toothy pharyngeal jaws that make short work of whatever morsel the fish can capture. Think of them as resembling little Aliens ala Giger.

Tijs Goldschmidt has noted that "an incredible number of furu variations have emerged from a single theme" and that the head structure is vital in identifying a species. "The shape of the skull, the teeth, and the mouth differs from species to species and is sometimes so bizarre that it is almost inconceivable that such a head exists," he writes. "Wide mouths, protruding jaws, or, by contrast, small, short, highly undeveloped ones; shrunken mouths or parrot-like beaks; long protruding teeth or a painfully misformed set of bicuspids; lips of all sizes, from tight, thin ones to thick, fleshy ones; round, straight and receding foreheads; a glazed stare fixed in large, bulging eyes; heavy jaw muscles embedded in a corpulent face." In addition, the Victorian cichlids are now thought to all belong to a single species flock, i.e. they are monophyletic, and Goldschmidt remarks that "the genetic differences between the furu species were extremely small, even smaller than the differences between certain human populations" and he wonders why, despite the explosive speciation and radiation, "that molecular evolution appeared not to have kept pace with morphological evolution."

Goldschmidt's further thoughts are on the role of sexual selection as an isolation mechanism in Victoria's cichlids. But now I return to the paper by J. Todd Streelman et al which examines the genetic foundation of fish jaws and show how altering the expression of a particular gene in an embryo can lead to physical changes in the adult fish. There is growing suspicion that genes work in more subtle yet powerful ways that previously thought, e.g. pleiotropic genes; at the same time, it has been shown that certain gene clusters (e.g. the Hox genes) regulate the same organisational structures and ontogenic development in nearly all animals, from flatworms to Fats Domino. Thus, an adaptive feature such as a longer neck and legs, or webbed feet and winged limbs, may require much less genetic reshuffling than thought, as certain gene groups help to build an organism that corresponds with its adaptive trait.

In the case of the cichlids studied by Streelman's team, it was found that the jaw opening function utilises different genetic components from the jaw-closing system. Two Malawian cichlid species were compared: one adapted for biting prey (left) and the other for sucking in plankton (right). The length (and thus, strength) of the levers that controlled the 'in' or closing (biting) process and the 'out' or opening (sucking) process were discovered to be negatively correlated as well as governed by genes on different chromosomes. The team demonstrated the function of the gene bpm4 in controlling the closing process by injecting its protein into embryonic zebra fish which developed enhanced biting power in their jaws.

Besides the light shed on the developmental biology of fishes achieved by this study, could it be possible to see the findings as an indication of how the varied feeding strategies of Rift Lake cichlids might have evolved? In times of intense competition amongst say, biting fish, is it plausible that individuals in which this single gene is expressed in greater strength achieve a biological advantage that sparks the route to genetic drift and eventual speciation? Goldschmidt observes that scale-biters have emerged in Lake Victoria, but they have yet to reach the extremes of this adaptation compared to convergent scale-biting cichlids in Lake Malawi, which have radiated into left- and right-handed populations (i.e. the fish's jaw are oriented to a particular direction). What other morphological features might harbour the promise of discovery in the interplay of their proteomic triggers and the great game of survival?