ScienceWeek

EVOLUTION: GENETICS AND CONVERGENT EVOLUTION

The following points are made by Gregory A. Wray (Nature 2006 440:1001):

1) Similarities between species can arise in two ways: either each species has retained the trait in question from their common ancestor, or each has acquired it independently [1]. Although the first possibility might seem far more likely, convergence is sufficiently common and detailed in its manifestations for more than one unwary biologist to have been duped. Indeed, life abounds with cases of convergence: the wings of birds and bats, and the eyes of octopuses and humans are among the most familiar examples from an immense and varied casebook [2]. But although evolutionary biologists can usually identify independent origins of a trait, the genetic basis of this process is less clear. New work [3] describes the identification of mutations responsible for the convergent evolution of a black spot on the wings of various species of fruitfly. The results reveal much about the way in which simple mutations can modify developmental processes to produce convergent traits.

2) The new work began by reconstructing genealogical relationships among 77 fruitfly species in the genus Drosophila. The researchers then used this information to identify gains and losses of a pigment spot on the flies' wings, and found that the spot had evolved independently at least twice, producing convergent similarity in D. biarmipes and D. tristis. But the spot was subsequently lost on several occasions as well. For instance, the unadorned wings of the most famous species in the genus, D. melanogaster, derive from a distant spotted ancestor.

3) Prud'homme et al [1] took advantage of this checkered history to study the genetic basis for convergence. To validate their approach, they examined two spotted species that derive from the same spotted ancestor and whose spots would therefore be expected to share the same genetic basis. Their previous work with one of these species, D. biarmipes, identified mutations in a part of the regulatory region of the yellow gene (called the spot element) as responsible for one origin of the wing spot [4]. This gene encodes an enzyme involved in pigment synthesis [5], and mutations affecting its expression have contributed to the evolution of certain pigmentation patterns in other fruitfly species. During wing development in D. biarmipes and D. elegans, the yellow gene is expressed specifically in a region that prefigures the black spot of the adult wing, implying that there is probably a similar underlying genetic basis for the spots in these species.

4) So much for similarity by common descent; what about convergent similarity? The repeated spot losses and gains turn out to have a surprisingly similar genetic basis. In two independent cases of spot loss, represented by D. gunungcola and D. mimetica, different sets of mutations in the spot element of the yellow regulatory region abolish expression of yellow specifically in the spot region of the wing. The spot element comprises just a few hundred of the roughly 180 million base pairs of the fruitfly genome, making this a "similar" genetic basis by any reasonable criterion. Even more striking is the case of D. tristis, a species that underwent an independent, convergent acquisition of the wing spot. Here, the spot regulatory element does not harbor the mutations responsible for spot production in this species. Instead, Prud'homme et al [1] discovered a different regulatory element that activates yellow expression in the spot region of the wing. The convergently similar wing spots of D. biarmipes and D. tristis are therefore the product of mutations in the same gene, but involve co-option of different regulatory elements.

References (abridged):

1. Hall, B. K. (ed.) Homology: The Hierarchical Basis of Comparative Biology (Academic, London, 1994)

2. Conway Morris, S. Life's Solution: Inevitable Humans in a Lonely Universe (Cambridge Univ. Press, 2003)

3. Prud'homme, B. et al. Nature 440, 1050-1053 (2006)

4. Gompel, N. et al. Nature 433, 481-487 (2005)

5. Wittkopp, P. J. , True, J. & Carroll, S. B. Development 129, 1849-1858 (2002)

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