As a toxin dies a prion comes to life: A tentative natural history of the [Het-s] prion

A variety of signaling pathways, in particular with roles in cell fate and host defense, operate by a prion-like mechanism consisting in the formation of open-ended oligomeric signaling complexes termed signalosomes. This mechanism emerges as a novel paradigm in signal transduction. Among the proteins forming such signaling complexes are the Nod-like receptors (NLR), involved in innate immunity. It now appears that the [Het-s] fungal prion derives from such a cell-fate defining signaling system controlled by a fungal NLR. What was once considered as an isolated oddity turns out to be related to a conserved and widespread signaling mechanism. Herein, we recall the relation of the [Het-s] prion to the signal transduction pathway controlled by the NWD2 Nod-like receptor, leading to activation of the HET-S pore-forming cell death execution protein. We explicit an evolutionary scenario in which formation of the [Het-s] prion is the result of an exaptation process or how a loss-of-function mutation in a pore-forming cell death execution protein (HET-S) has given birth to a functional prion ([Het-s]).     

Arch and beam: deployment of microstructural fabrics in relation to loads exerted on the shells of bivalved molluscs in different functional regimes

Two apparently contradictory models have been proposed, relating the disposition of microstructural fabrics of the bivalve shell to stresses they experience. These models are here shown to apply to the function of the shell in different circumstances. In its day-to-day operation, the shell acts as a pair of beams, loaded under opposing stresses exerted by the adductor muscles and the ligament. The resulting strain is built into the shell as new layers are added to its growing interior surface (Wainwright 1969). The distribution of stresses induced by attempts to crush the shell, or by violent adduction intended to prevent it from being opened, is different. Here, the shell acts as a dome, a ‘shell’ in the architect's sense. Compressional stress develops in the outer layer and tension in the inner layer. The distribution of shell microstructures in many bivalves is biomechanically consistent with the need to resist these latter stresses. The shell is prestressed in the right direction to resist this deformation. However, the built-in strain is an exaptation in relation to this function. Any added resistance to crushing it provides is fortuitously advantageous, since it becomes prestressed as an unavoidable consequence of shell growth and articulation.     

Multiple origins of secondary temporal fenestrae and orbitozygomatic junctions in birds

Orbitozygomatic junctions (bony connections between the postorbital and zygomatic processes) and secondary temporal fenestrae have long been known to occur in a few avian species, but no comprehensive study of this phenomenon has ever been published. Having surveyed all non‐passerine and most passerine families, we established that the orbitozygomatic junction evolved 18–20 times independently in Cracidae, Phasianoidea (Odontophoridae and Phasianidae), Gastornis, Columbidae (three times), Pteroclidae (Syrrhaptes), Aptornis, Thinocoridae (Thinocorus), Scolopacidae (possibly twice), Ciconiidae (twice), Brachypteraciidae (Uratelornis), Picidae (Picus spp.), Psittacidae (Melopsittacus), Cacatuidae, and Alaudidae (twice). The junction arises in evolution as a result of either elongation of the two processes that meet at angles or the appearance of a bony cross‐bridge in place of a ligament or aponeurosis. In the first case, the junction is initially non‐adaptive, as indicated by its extreme variation (e.g., in Cracinae and Ciconiidae), and may or may not prove functional as an exaptation. Whenever adaptive, the junction supports an expansion of the adductor mandibulae externus (primarily its pars media). In addition, a rostral extension of the tympanic wing has come to cross the temporal fossa in Strigidae (at least twice) and probably Podargus. Altogether, secondary bony connections across the temporal fossa evolved independently at least 21–23 times in neornithine birds.     

Convergent domestication of pogo-like transposases into centromere-binding proteins in fission yeast and mammals

The mammalian centromere-associated protein B (CENP-B) shares significant sequence similarity with 3 proteins in fission yeast (Abp1, Cbh1, and Cbh2) that also bind centromeres and have essential function for chromosome segregation and centromeric heterochromatin formation. Each of these proteins displays extensive sequence similarity with pogo-like transposases, which have been previously identified in the genomes of various insects and vertebrates, in the protozoan Entamoeba and in plants. Based on this distribution, it has been proposed that the mammalian and fission yeast centromeric proteins are derived from "domesticated" pogo-like transposons. Here we took advantage of the vast amount of sequence information that has become recently available for a wide range of fungal and animal species to investigate the origin of the mammalian CENP-B and yeast CENP-B-like genes. A highly conserved ortholog of CENP-B was detected in 31 species of mammals, including opossum and platypus, but was absent from all nonmammalian species represented in the databases. Similarly, no ortholog of the fission yeast centromeric proteins was identified in any of the various fungal genomes currently available. In contrast, we discovered a plethora of novel pogo-like transposons in diverse invertebrates and vertebrates and in several filamentous fungi. Phylogenetic analysis revealed that the mammalian and fission yeast CENP-B proteins fall into 2 distinct monophyletic clades, each of which includes a different set of pogo-like transposons. These results are most parsimoniously explained by independent domestication events of pogo-like transposases into centromeric proteins in the mammalian and fission yeast lineages, a case of "convergent domestication." These findings highlight the propensity of transposases to give rise to new host proteins and the potential of transposons as sources of genetic innovation.     

Adaptation or exaptation? An experimental test of hypotheses on the origin of salinity tolerance in Bufo calamita

The natterjack toad (Bufo calamita) shows variation in embryonic and larval salinity tolerance across populations in southern Spain. However, its aquatic/terrestrial biphasic life cycle, together with remarkable differences in salinity tolerance between Spanish and UK freshwater populations suggest an alternative hypothesis to local adaptation. Drought resistance during the terrestrial phase and salinity tolerance during the aquatic phase are both related to osmotic stress tolerance, and if there were an association between them, one could have evolved as an exaptation from the other. To test such an association, we reared B. calamita juveniles from three populations known to differ genetically in their salinity tolerance, under either dry or humid conditions. Drought decreased growth rate, enhanced burying behaviour, and decreased foraging activity and efficiency. No significant population x treatment interaction was found for any variable, i.e. populations were equally affected by drought. These results do not support the hypothesis of a genetic association between salinity and drought tolerance.     

Homosexual behavior in primates: A review of evidence and theory

Homosexual behavior is defined as genital contact, genital manipulation or both between same-sex individuals. Available data indicate that this behavior is phylogenetically widespread among the anthropoid primates, but totally absent among prosimians. The majority of the 33 species that demonstrate homosexual behavior do so rarefy, but for a substantial number (N =12) it appears to be a more common pattern under free-ranging conditions. I summarize data on homosexual behavior as it relates to form, living condition, age, sex, social organization, and ecological context, and discuss hormonal, demographic, and sociosexual theories for primate homosexual behavior. Among adult primates, the behavior is not the product of abnormal excesses or deficiencies in androgens. Prenatal excesses of androgens may have some effect on the expression of female homosexual behavior, but these effects might vary over the life span, and data are equivocal at present. Demographic processes that result in skewed sex ratios can favor the expression of homosexual behavior in a population, which causes intraspecific variation. I examine several sociosexual explanations, including (a) proceptivity enhancement, (b) receptivity reduction, (c) dominance assertion, (d) practice for heterosexual copulation, (e) tension regulation, (f) reconciliation, and (g) alliance formation. An evolutionary scenario highlights the transformations this behavior underwent during the evolution of the anthropoid primates. I suggest exaptation as a theoretical framework for interpreting homosexual behavior and conclude that future consideration of sexual selection among primates should address homosexual components of this process.     

A conceptual model for the origin of worker behaviour and adaptation of eusociality

In a model based on the wasp family Vespidae, the origin of worker behaviour, which constitutes the eusociality threshold, is not based on relatedness, therefore the origin of eusociality does not depend on inclusive fitness, and workers at the eusociality threshold are not altruistic. Instead, incipient workers and queens behave selfishly and are subject to direct natural selection. Beyond the eusociality threshold, relatedness enables 'soft inheritance' as the framework for initial adaptations of eusociality. At the threshold of irreversibility, queen and worker castes become fixed in advanced eusociality. Transitions from solitary to facultative, facultative to primitive, and primitive to advanced eusociality occur via exaptation, phenotypic accommodation and genetic assimilation. Multilevel selection characterizes the solitary to highly eusocial transition, but components of multilevel selection vary across levels of eusociality. Roles of behavioural flexibility and developmental plasticity in the evolutionary process equal or exceed those of genotype.     

Intronic hammerhead ribozymes are ultraconserved in the human genome

Small ribozymes have been regarded as living fossils of a prebiotic RNA world that would have remained in the genomes of modern organisms. In this study, we report the ultraconserved occurrence of hammerhead ribozymes in Amniota genomes (reptiles, birds and mammals, including humans), similar to those described previously in amphibians and platyhelminth parasites. The ribozymes mapped to intronic regions of different genes, such as the tumour suppressor RECK in birds and mammals, a mammalian tumour antigen and the dystrobrevin beta in lizards and birds. In vitro characterization confirmed a high self-cleavage activity, whereas analysis of RECK-expressed sequence tags revealed fusion events between the in vivo self-cleaved intron and U5 or U6 small nuclear RNA fragments. Together, these results suggest a conserved role for these ribozymes in messenger RNA biogenesis.