Seasonal stability and species specificity of environmentally acquired chemical mating signals in orchid bees.

Traits that mediate reproductive isolation between species, such as those involved in mate choice and/or recognition, are predicted to experience stabilizing selection towards the species mean. Male orchid bees collect chemical compounds from many sources, such as plants and fungi, which they use as a perfume signal (pheromone) during courtship display, and are suggested to contribute to reproductive isolation between species. Environmentally acquired signals are more prone to variation as source availability can vary through space and time. If orchid bee perfumes are important for reproductive isolation between species, we expect them to exhibit stable species-specific differences in time and space. Here, we describe phenotypic patterns of inter- and intraspecific variation in the male perfumes of three sympatric species of Euglossa orchid bees across an entire year, investigating both their seasonality and species specificity. Our analysis revealed considerable within-species variation in perfumes. However, species specificity was maintained consistently throughout the year, supporting the idea that these perfumes could play an important role in reproductive isolation and are experiencing stabilizing selection towards a species mean. Our analysis also identified strong correlations in the abundance of some compounds, possibly due to shared collection sources between species. Our study suggests that orchid bee perfumes are robust in the face of environmental changes in resource availability and thus can maintain reproductive isolation between species.

El aislamiento reproductivo entre especies es usualmente controlado por rasgos que juegan un papel importante en la selección y/o reconocimiento de pareja. Estos rasgos usualmente están bajo la presión de selección estabilizadora, lo que hace que se mantengan cerca al promedio de la especie. Los machos de las abejas de las orquídeas recolectan compuestos químicos de plantas y hongos y los usan como señales de feromonas durante el cortejo. Se piensa que las feromonas de las abejas de las orquídeas contribuyen al aislamiento reproductivo entre especies. Al ser obtenidas del ambiente, estas señales químicas (perfumes) son más propensas a la variación, ya que la disponibilidad de la fuente puede variar en el espacio y el tiempo. Si los perfumes contribuyen al aislamiento reproductivo, se esperaría que las diferencias entre especies sean estables tanto en el tiempo como en el espacio. Aquí, investigamos la estacionalidad y la especificidad a nivel de especie de los perfumes durante todo un año en tres especies simpátricas de abejas del género Euglossa. Nuestro análisis reveló variación considerable en los perfumes de cada especie. Sin embargo, la especificidad de cada especie se mantuvo durante todo el año, lo cual sugiere que los perfumes juegan un papel importante en el aislamiento reproductivo y que experimentan selección estabilizadora hacia la media de la especie. Nuestro análisis también identificó fuertes correlaciones en la abundancia de algunos compuestos, posiblemente debido a que diferentes especies comparten las mismas fuentes de recolección. Nuestro estudio sugiere que los perfumes de las abejas de las orquídeas son robustos frente a los cambios ambientales y la variación en la disponibilidad de recursos y por lo tanto pueden contribuir al aislamiento reproductivo.

Light-regulated collective contractility in a multicellular choanoflagellate.

Collective cell contractions that generate global tissue deformations are a signature feature of animal movement and morphogenesis. However, the origin of collective contractility in animals remains unclear. While surveying the Caribbean island of Curaçao for choanoflagellates, the closest living relatives of animals, we isolated a previously undescribed species (here named Choanoeca flexa sp. nov.) that forms multicellular cup-shaped colonies. The colonies rapidly invert their curvature in response to changing light levels, which they detect through a rhodopsin-cyclic guanosine monophosphate pathway. Inversion requires actomyosin-mediated apical contractility and allows alternation between feeding and swimming behavior. C. flexa thus rapidly converts sensory inputs directly into multicellular contractions. These findings may inform reconstructions of hypothesized animal ancestors that existed before the evolution of specialized sensory and contractile cells.

The role of isoforms in the evolution of cryptic coloration in Peromyscus mice.

A central goal of evolutionary biology is to understand the molecular mechanisms underlying phenotypic adaptation. While the contribution of protein-coding and cis-regulatory mutations to adaptive traits has been well documented, additional sources of variation - such as the production of alternative RNA transcripts from a single gene, or isoforms - have been understudied. Here, we focus on the pigmentation gene Agouti, known to express multiple alternative transcripts, to investigate the role of isoform usage in the evolution of cryptic colour phenotypes in deer mice (genus Peromyscus). We first characterize the Agouti isoforms expressed in the Peromyscus skin and find two novel isoforms not previously identified in Mus. Next, we show that a locally adapted light-coloured population of P. maniculatus living on the Nebraska Sand Hills shows an upregulation of a single Agouti isoform, termed 1C, compared with their ancestral dark-coloured conspecifics. Using in vitro assays, we show that this preference for isoform 1C may be driven by isoform-specific differences in translation. In addition, using an admixed population of wild-caught mice, we find that variation in overall Agouti expression maps to a region near exon 1C, which also has patterns of nucleotide variation consistent with strong positive selection. Finally, we show that the independent evolution of cryptic light pigmentation in a different species, P. polionotus, has been driven by a preference for the same Agouti isoform. Together, these findings present an example of the role of alternative transcript processing in adaptation and demonstrate molecular convergence at the level of isoform regulation.

Convergence in feeding posture occurs through different genetic loci in independently evolved cave populations of Astyanax mexicanus.

When an organism colonizes a new environment, it needs to adapt both morphologically and behaviorally to survive and thrive. Although recent progress has been made in understanding the genetic architecture underlying morphological evolution, behavioral evolution is poorly understood. Here, we use the Mexican cavefish, Astyanax mexicanus, to study the genetic basis for convergent evolution of feeding posture. When river-dwelling surface fish became entrapped in the caves, they were confronted with dramatic changes in the availability and type of food source and in their ability to perceive it. In this setting, multiple independent populations of cavefish exhibit an altered feeding posture compared with their ancestral surface forms. We determined that this behavioral change in feeding posture is not due to changes in cranial facial morphology, body depth, or to take advantage of the expansion in the number of taste buds. Quantitative genetic analysis demonstrates that two different cave populations have evolved similar feeding postures through a small number of genetic changes, some of which appear to be distinct. This work indicates that independently evolved populations of cavefish can evolve the same behavioral traits to adapt to similar environmental challenges by modifying different sets of genes.

Loss of schooling behavior in cavefish through sight-dependent and sight-independent mechanisms.

BACKGROUND: Surface populations of Astyanax mexicanus, living in rivers like their common ancestors, school, while several, independently derived cave populations of the same species have lost schooling behavior. RESULTS: We quantify schooling behavior in individual A. mexicanus and identify quantitative trait loci (QTL) for this trait. We find that the evolutionary modulation of schooling has both vision-dependent and -independent components. We also quantify differences in the lateral line and vision between cavefish and surface fish and relate these differences to the evolutionary loss of schooling behavior. We provide evidence that a monoamine neurotransmitter may have played a role in the evolution of schooling behavior. CONCLUSIONS: We find that vision is essential for schooling tendency in A. mexicanus, while the lateral line has a small effect on this behavior. Schooling behavior in A. mexicanus has evolved both through changes in sensory systems and through changes in genetic loci that likely act downstream of sensory inputs.