title: Insights into the development and evolution of exaggerated traits using de novo transcriptomes of two species of horned scarab beetles.
abstract: Scarab beetles exhibit an astonishing variety of rigid exo-skeletal outgrowths, known as "horns". These traits are often sexually dimorphic and vary dramatically across species in size, shape, location, and allometry with body size. In many species, the horn exhibits disproportionate growth resulting in an exaggerated allometric relationship with body size, as compared to other traits, such as wings, that grow proportionately with body size. Depending on the species, the smallest males either do not produce a horn at all, or they produce a disproportionately small horn for their body size. While the diversity of horn shapes and their behavioural ecology have been reasonably well studied, we know far less about the proximate mechanisms that regulate horn growth. Thus, using 454 pyrosequencing, we generated transcriptome profiles, during horn growth and development, in two different scarab beetle species: the Asian rhinoceros beetle, Trypoxylus dichotomus, and the dung beetle, Onthophagus nigriventris. We obtained over half a million reads for each species that were assembled into over 6,000 and 16,000 contigs respectively. We combined these data with previously published studies to look for signatures of molecular evolution. We found a small subset of genes with horn-biased expression showing evidence for recent positive selection, as is expected with sexual selection on horn size. We also found evidence of relaxed selection present in genes that demonstrated biased expression between horned and horn-less morphs, consistent with the theory of developmental decoupling of phenotypically plastic traits.
authors.affiliation: Department of Entomology, Washington State University, Pullman, Washington, United States of America ; School of Biological Sciences, University of Bristol, Bristol, United Kingdom.
authors.forename: Ian A
authors.initials: IA
authors.lastname: Warren
authors.affiliation: Department of Biology, Pennsylvania State University, State College, Pennsylvania, United States of America.
authors.forename: J Cristobal
authors.initials: JC
authors.lastname: Vera
authors.affiliation: Division of Biological Sciences, The University of Montana, Missoula, Montana, United States of America.
authors.forename: Annika
authors.initials: A
authors.lastname: Johns
authors.affiliation: Department of Entomology, Washington State University, Pullman, Washington, United States of America.
authors.forename: Robert
authors.initials: R
authors.lastname: Zinna
authors.affiliation: Department of Biology, Pennsylvania State University, State College, Pennsylvania, United States of America.
authors.forename: James H
authors.initials: JH
authors.lastname: Marden
authors.affiliation: Division of Biological Sciences, The University of Montana, Missoula, Montana, United States of America.
authors.forename: Douglas J
authors.initials: DJ
authors.lastname: Emlen
authors.affiliation: Program in Ecology, Evolutionary Biology and Behavior, Michigan State University, East Lansing, Michigan, United States of America ; Department of Zoology, Michigan State University, East Lansing, Michigan, United States of America.
authors.forename: Ian
authors.initials: I
authors.lastname: Dworkin
authors.affiliation: Department of Entomology, Washington State University, Pullman, Washington, United States of America.
authors.forename: Laura C
authors.initials: LC
authors.lastname: Lavine
mesh_terms: D060105:Animal Shells / Q000254:growth & development*; D000818:Animals; D001483:Base Sequence; D049628:Body Size; D001517:Coleoptera / Q000254:growth & development*; D020451:Contig Mapping; D018076:DNA, Complementary / Q000235:genetics; D005260:Female; D008297:Male; D058977:Molecular Sequence Annotation; D008969:Molecular Sequence Data; D010641:Phenotype; D020641:Polymorphism, Single Nucleotide / Q000235:genetics; D012641:Selection, Genetic; D017422:Sequence Analysis, DNA; D012727:Sex Characteristics; D013045:Species Specificity; D059467:Transcriptome
chemical_list: D018076:DNA, Complementary