RESUMEN
The history of life offers plentiful examples of convergent evolution, the independent derivation of similar phenotypes in distinct lineages. The emergence of convergent phenotypes among closely related lineages (frequently termed "parallel" evolution) is often assumed to result from changes in similar genes or developmental pathways, but the genetic origins of convergence remains poorly understood. Ninespine (Pungitius pungitius) and threespine (Gasterosteus aculeatus) stickleback fish provide many examples of convergent evolution of adaptive phenotypes, both within and between genera. The genetic architecture of several important traits is now known for threespine sticklebacks; thus, ninespine sticklebacks provide a unique opportunity to critically test whether similar or different chromosome regions control similar phenotypes in these lineages. We have generated the first genome-wide linkage map for ninespine sticklebacks and used quantitative trait locus mapping to identify chromosome regions controlling several skeletal traits and sex determination. In ninespine sticklebacks, these traits mapped to chromosome regions not previously known to control the corresponding traits in threespine sticklebacks. Therefore, convergent morphological evolution in these related, but independent, vertebrate lineages might have different genetic origins. Comparative genetics in sticklebacks provides an exciting opportunity to study the mechanisms controlling similar phenotypic changes in different animal groups.
Asunto(s)
Evolución Biológica , Procesos de Determinación del Sexo , Smegmamorpha , Animales , Ligamiento Genético , Pelvis/anatomía & histología , Fenotipo , Cromosomas Sexuales , Smegmamorpha/anatomía & histología , Smegmamorpha/genéticaRESUMEN
The distribution of effect sizes of genes underlying adaptation is unknown (Orr 2005). Are suites of traits that diverged under natural selection controlled by a few pleiotropic genes of large effect (major genes model), by many independently acting genes of small effect (infinitesimal model), or by a combination, with frequency inversely related to effect size (geometric model)? To address this we carried out a quantitative trait loci (QTL) study of a suite of 54 position traits describing body shapes of two threespine stickleback species: an ancestral Pacific marine form and a highly derived benthic species inhabiting a geologically young lake. About half of the 26 detected QTL affected just one coordinate and had small net effects, but several genomic regions affected multiple aspects of shape and had large net effects. The distribution of effect sizes followed the gamma distribution, as predicted by the geometric model of adaptation when detection limits are taken into account. The sex-determining chromosome region had the largest effect of any QTL. Ancestral sexual dimorphism was similar to the direction of divergence, and was largely eliminated during freshwater adaptation, suggesting that sex differences may provide variation upon which selection can act. Several shape QTL are linked to Eda, a major gene responsible for reduction of lateral body armor in freshwater. Our results are consistent with predictions of the geometric model of adaptation. Shape evolution in stickleback results from a few genes with large and possibly widespread effects and multiple genes of smaller effect.
Asunto(s)
Adaptación Fisiológica/genética , Tamaño Corporal/genética , Smegmamorpha/anatomía & histología , Smegmamorpha/genética , Animales , Femenino , Regulación de la Expresión Génica , Hibridación Genética , Masculino , Sitios de Carácter Cuantitativo/genética , Caracteres SexualesRESUMEN
Major phenotypic changes evolve in parallel in nature by molecular mechanisms that are largely unknown. Here, we use positional cloning methods to identify the major chromosome locus controlling armor plate patterning in wild threespine sticklebacks. Mapping, sequencing, and transgenic studies show that the Ectodysplasin (EDA) signaling pathway plays a key role in evolutionary change in natural populations and that parallel evolution of stickleback low-plated phenotypes at most freshwater locations around the world has occurred by repeated selection of Eda alleles derived from an ancestral low-plated haplotype that first appeared more than two million years ago. Members of this clade of low-plated alleles are present at low frequencies in marine fish, which suggests that standing genetic variation can provide a molecular basis for rapid, parallel evolution of dramatic phenotypic change in nature.