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.
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.
Abstract
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 cactus’s 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.
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Reference
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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