Divergence in brain composition during the early stages of ecological specialization in Heliconius butterflies.

During speciation across ecological gradients, diverging populations are exposed to contrasting sensory and spatial information that present new behavioural and perceptive challenges. These challenges may be met by heritable or environmentally induced changes in brain function which mediate behaviour. However, few studies have investigated patterns of neural divergence at the early stages of speciation, inhibiting our understanding of the relative importance of these processes. Here, we provide a novel case study. The incipient species pair, Heliconius erato and H. himera, are parapatric across an environmental and altitudinal gradient. Despite ongoing gene flow, these species have divergent ecological, behavioural and physiological traits. We demonstrate that these taxa also differ significantly in brain composition, in particular in the relative levels of investment in structures that process sensory information. These differences are not explained solely by environmentally-induced plasticity, but include heritable, nonallometric shifts in brain structure. We suggest these differences reflect divergence to meet the demands of contrasting sensory ecologies. This conclusion would support the hypothesis that the evolution of brain structure and function play an important role in facilitating the emergence of ecologically distinct species.

The diversification of Heliconius butterflies: what have we learned in 150 years?

Research into Heliconius butterflies has made a significant contribution to evolutionary biology. Here, we review our understanding of the diversification of these butterflies, covering recent advances and a vast foundation of earlier work. Whereas no single group of organisms can be sufficient for understanding life's diversity, after years of intensive study, research into Heliconius has addressed a wide variety of evolutionary questions. We first discuss evidence for widespread gene flow between Heliconius species and what this reveals about the nature of species. We then address the evolution and diversity of warning patterns, both as the target of selection and with respect to their underlying genetic basis. The identification of major genes involved in mimetic shifts, and homology at these loci between distantly related taxa, has revealed a surprising predictability in the genetic basis of evolution. In the final sections, we consider the evolution of warning patterns, and Heliconius diversity more generally, within a broader context of ecological and sexual selection. We consider how different traits and modes of selection can interact and influence the evolution of reproductive isolation.

Parallel episodes of phyletic dwarfism in callitrichid and cheirogaleid primates.

The Callitrichidae are the smallest anthropoids, whereas the Cheirogaleidae include the smallest of all primates. Using species-level analyses, we show that these are derived conditions; both neonatal and adult body mass decreased in a gradual, phyletic manner in parallel across callitrichids, and across cheirogaleids. We identify lineages with particularly rapid decreases and highlight the pygmy marmoset, Callithrix pygmaea, as a phenotypic outlier. The life-history traits associated with body-mass reduction in each clade suggest that the convergent evolution of small body size was achieved by changes in different ontogenetic stages. Body-size reduction in callitrichids appears to be almost exclusively due to alterations in prenatal growth rate, whereas body-size reduction in cheirogaleids may have been largely due to reduced duration of growth phases. Finally, we use these results to discuss some of the debates surrounding the evolution of Homo floresiensis and suggest potential parallels between the evolution of H. floresiensis and callitrichids.

Microcephaly genes and the evolution of sexual dimorphism in primate brain size.

Microcephaly genes are amongst the most intensively studied genes with candidate roles in brain evolution. Early controversies surrounded the suggestion that they experienced differential selection pressures in different human populations, but several association studies failed to find any link between variation in microcephaly genes and brain size in humans. Recently, however, sex-dependent associations were found between variation in three microcephaly genes and human brain size, suggesting that these genes could contribute to the evolution of sexually dimorphic traits in the brain. Here, we test the hypothesis that microcephaly genes contribute to the evolution of sexual dimorphism in brain mass across anthropoid primates using a comparative approach. The results suggest a link between selection pressures acting on MCPH1 and CENPJ and different scores of sexual dimorphism.

Positive selection on NIN, a gene involved in neurogenesis, and primate brain evolution.

A long-held dogma in comparative neurobiology has been that the number of neurons under a given area of cortical surface is constant. As such, the attention of those seeking to understand the genetic basis of brain evolution has focused on genes with functions in the lateral expansion of the developing cerebral cortex. However, new data suggest that cortical cytoarchitecture is not constant across primates, raising the possibility that changes in radial cortical development played a role in primate brain evolution. We present the first analysis of a gene with functions relevant to this dimension of brain evolution. We show that NIN, a gene necessary for maintaining asymmetric, neurogenic divisions of radial glial cells (RGCs), evolved adaptively during anthropoid evolution. We explored how this selection relates to neural phenotypes and find a significant association between selection on NIN and neonatal brain size in catarrhines. Our analyses suggest a relationship with prenatal neurogenesis and identify the human data point as an outlier, possibly explained by postnatal changes in development on the human lineage. A similar pattern is found in platyrrhines, but the highly encephalized genus Cebus departs from the general trend. We further show that the evolution of NIN may be associated with variation in neuron number not explained by increases in surface area, a result consistent with NIN's role in neurogenic divisions of RGCs. Our combined results suggest a role for NIN in the evolution of cortical development.

De novo synthesis of carbohydrates by stereoselective aldol reaction: L-cladinose.

Aldol reactions of methyl 2-methoxypropanoate (4), the corresponding ester of 2-methoxypropanoic acid with 4-methyl-2,6-di-(tert-butyl)phenol (13), and silylketene acetals 14 and 15 with (S)-2-(phenyl-methoxy)propanal (17) have been investigated. The lithium enolate of 4 reacts with 17 to give primarily beta-hydroxy ester 18a. If the reaction is carried out with the bis-silylketene acetal 14 under the influence of stannic chloride, beta-hydroxy acid 20c is produced. Compound 20c is cleanly inverted, via the beta-lactone 26, to provide beta-hydroxy acid 19c. Compound 18a has been converted into L-cladinose by the sequence of steps: 18a----35----39----40----41----42----1.