RESUMO
PREMISE OF THE STUDY: Reproductive isolation between sympatric species pairs may be maintained by both pre- and postmating barriers. Here we evaluate potential barriers to mating between the outcrossing Mimulus luteus and its more highly selfing sympatric congener, M. cupreus, two members of the South American luteus complex of Mimulus. METHODS: Seed set was compared following autonomous self-pollination, manual pollination, conspecific outcrossing, and sympatric and allopatric hybridization, for laboratory-maintained inbred lines and wild-collected accessions. Survival and reproductive fitness of hybrids relative to parental species were examined across environments that differed with respect to temperature and soil nutrients, two factors that vary across the ranges of M. luteus and M. cupreus. KEY RESULTS: Mimulus luteus was minimally capable of autonomous self-fertilization, consistent with reliance on an animal pollinator, whereas M. cupreus was a successful selfer across all tested accessions. Postmating barriers to hybridization are negligible, in both low- and high-stress environments, across multiple sympatric and allopatric populations. CONCLUSION: As in the North American M. guttatus-M. nasutus species pair, postmating barriers contribute little to isolation between M. luteus and M. cupreus. This result reinforces the importance of premating barriers, specifically species differences in reliance on, and accessibility to, animal pollinators. A unique aspect of the M. luteus-M. cupreus pair is the recent gain of red floral anthocyanin pigmentation in M. cupreus. On the basis of species differences in vegetative anthocyanin production, a facultative stress-protective response, we propose a potential stress-protective role for the constitutive floral anthocyanins of M. cupreus.
Assuntos
Cruzamentos Genéticos , Mimulus/fisiologia , Autofertilização/fisiologia , Chile , Secas , Ecótipo , Aptidão Genética , Temperatura Alta , Hibridização Genética , Endogamia , Folhas de Planta/fisiologia , Polinização/fisiologia , Isolamento Reprodutivo , Sementes/fisiologia , Especificidade da EspécieRESUMO
Identifying the genetic basis of parallelism reveals the means by which evolution repeats itself and shows what aspects-if any-may be predictable. The recently tetraploid luteus group of Mimulus contains five species native to central Chile, three of which have evolved extensive red floral pigmentation using at least two distinct loci . Here we show that the parallel evolution of petal lobe anthocyanin (PLA) pigmentation in M. cupreus and M. luteus var. variegatus occurred via separate yet strikingly similar mechanisms. In each case, a dominant, single-locus gain of pigmentation maps to a genomic region (pla1 and pla2, respectively) containing adjacent, apparently recently duplicated paralogs of MYB anthocyanin-regulating transcription factors. Interestingly, candidate genes in pla1 and pla2 are themselves related by an older duplication. In both cases, pla genotype cosegregates with expression of multiple genes in the anthocyanin biosynthetic pathway, revealing a mechanism of coordinated trans-regulatory expression changes across functionally related enzyme-encoding genes. We conclude that in this instance, evolution has repeated itself with marked consistency. Duplication has enabled that repetition to occur using two physically independent but functionally similar loci, highlighting the importance of genomic complexity to the evolutionary process.
Assuntos
Evolução Biológica , Flores/genética , Duplicação Gênica , Mimulus/genética , Pigmentação , Antocianinas/química , Antocianinas/genética , Chile , Cromossomos de Plantas/genética , Flores/química , Regulação da Expressão Gênica , Mimulus/química , Especificidade da Espécie , Tetraploidia , Fatores de Transcrição/genéticaRESUMO
Deciphering the genetic architecture of phenotypic change provides a framework for understanding how evolution proceeds at a genetic level, and paves the way for work at the molecular level. A series of intra- and interspecific crosses were used to investigate the genetic control of recently evolved floral pigmentation phenotypes in a group of closely related Mimulus species from central Chile. An intraspecific polymorphism was found to be controlled by a single Mendelian locus. Differences between species, by contrast, were composed of multiple independent patterning elements, including both Mendelian and polygenic traits. The most striking phenotypic novelty in this group, anthocyanin pigmentation in the petal lobes, has evolved three times independently. The results illustrate how genetically simple modular elements can interact with polygenic or quantitative traits to create complex new phenotypes. The repeated evolution of petal lobe anthocyanins suggests that natural selection may have played a role in the evolution of red coloration in the Chilean Mimulus, and shows that red coloration has been achieved via different genetic pathways in these closely related species.