The purpose of this forum is to introduce notable papers and books published by you and other persons. The work can be new or old, but it should be of wide interest and high quality. A brief comment on the significance of the work should be attached. The current categories of the subjects are (1) adaptation, (2) behavioral evolution, (3) dosage compensation, (4) evo-devo, (5) gene evolution, (6) genomic evolution, (7) molecular phylogeny, (8) natural selection, (9) phenotypic evolution, (10) sensory receptors, (11) sex chromosomes, (12) sex determination, (13) speciation, (14) symbiosis and evolution, and (15) horizontal gene transfer. However, new categories can be added if necessary. Emphasis will be given on the biological work rather than on the mathematical. Any person may post a paper by sending it to one of the editors listed below. We also welcome your comments on posted work, but we moderate all the comments to control spam. This forum is primarily for scientific discussion and to construct a database for good molecular evolution papers.

Wednesday, October 31, 2012

Evolution of Obligate Specialist Species: How Changes to a Single Gene Restricted Drosophila pachea to a Single Species of Cactus

Contributed by: Steve Schaeffer

             The hormone ecdysone regulates many developmental processes during metamorphosis in insects.  In most Drosophila species, cholesterol is the initial substrate for the pathway that synthesizes ecdysone.  A recent study by Lang et al. (2012) provides evidence that amino acid substitutions in the neverland gene are responsible for an adaptive host shift of a Drosophila species. The enzyme produced by the neverland gene converts cholesterol to 7-dehydrocholesterol in the initial step in the ecdysone pathway.  Drosophila pachea lives exclusively on senita cactus (Lophocereus schottii).  The senita cactus lacks appreciable levels of cholesterol, but does have other sterols such as lathosterol that could be used to make ecdysone.  The neverland enzyme of Drosophila pachea is unable to make 7-dehydrocholesterol from cholesterol, which results from two to four amino acid changes in residues that are strongly conserved among Drosophila species. Lang et al. (2012) used transgenic experiments in Drosophila melanogaster neverland knockdown mutants to test the activity of the D. pachea neverland extant and inferred proteins with a variety of sterols.  The ancestral neverland proteins were able to convert both cholesterol and lathosterol into 7-dehydrocholesterol while only lathosterol is used by the extant D. pachea enzyme to make 7-dehydrocholesterol.  A population genetics analysis shows that nucleotide diversity in the neverland gene region is consistent with a recent selective sweep.  This study is a nice example of an integrative study that used molecular and population genetic analysis to show how changes to a single enzyme can result in a species being an obligate specialist.  In this case, D. pachea became an obligate specialist on senita cactus.  This change may have allowed D. pachea to have fewer competitors on its new host.  While this change may currently be advantageous, if availability of the host is reduced, D. pachea could be an evolutionary deadend.


Most living species exploit a limited range of resources. However, little is known about how tight associations build up during evolution between such specialist species and the hosts they use. We examined the dependence of Drosophila pachea on its single host, the senita cactus. Several amino acid changes in the Neverland oxygenase rendered D. pachea unable to transform cholesterol into 7-dehydrocholesterol (the first reaction in the steroid hormone biosynthetic pathway in insects) and thus made D. pachea dependent on the uncommon sterols of its host plant. The neverland mutations increase survival on the cactuss unusual sterols and are in a genomic region that faced recent positive selection. This study illustrates how relatively few genetic changes in a single gene may restrict the ecological niche of a species.

Lang, M., S. Murat, A. G. Clark, G. Gouppil, C. Blais, L. M. Matzkin, É. Guittard, T. Yoshiyama-Yanagawa, H. Kataoka, R. Niwa, R Lafont, C. Dauphin-Villemant, and V. Orgogozo. 2012 Mutations in the neverland gene turned Drosophila pachea into an obligate specialist species. Science 337: 1658-1661.

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