Experimental stress during molt suggests the evolution of condition‐dependent and condition‐independent ornaments in the king penguin

Sexual selection and social selection are two important theories proposed for explaining the evolution of colorful ornamental traits in animals. Understanding signal honesty requires studying how environmental and physiological factors during development influence the showy nature of sexual and social ornaments. We experimentally manipulated physiological stress and immunity status during the molt in adult king penguins (Aptenodytes patagonicus), and studied the consequences of our treatments on colourful ornaments (yellow‐orange and UV beak spots and yellow‐orange auricular feather patches) known to be used in sexual and social contexts in this species. Whereas some ornamental features showed strong condition‐dependence (yellow auricular feather chroma, yellow and UV chroma of the beak), others were condition‐independent and remained highly correlated before and after the molt (auricular patch size and beak UV hue). Our study provides a rare examination of the links between ornament determinism and selection processes in the wild. We highlight the coexistence of ornaments costly to produce that may be honest signals used in mate choice, and ornaments for which honesty may be enforced by social mediation or rely on genetic constraints.     

Phenotypic quality and molt in the barn swallow, Hirundo rustica

Phenotypic quality may determine the development and expressionof secondary sexual characters. We studied the relationshipbetween molt and several measures of phenotypic quality in thesexually size-dimorphic barn swallow (Hirundo rustica) in itswinter quarters in Namibia. Males were in a more advanced stageof molt than females and juveniles, and the speed of molt asdetermined from the residual of the regression of the size ofthe gap in wings caused by missing and growing feathers on wingmolt score (residual wing raggedness) was also higher in malesthan in females and juveniles. Male barn swallows with longand symmetric tail feathers had a more advanced stage of moltand molted at a higher speed than males with short and asymmetrictails. Long-tailed females had a delayed molt, and females withasymmetric tails had less advanced molt and lower rates of feathergrowth than females with symmetric tails. Molt of secondariesin juveniles also appeared to be less advanced if they had longtails. Adult barn swallows molted their tail feathers in anirregular sequence with the longest, outermost tail featherusually replaced before the second or the third outermost feathers.Good body condition was positively associated with a high moltscore for some feather tracts and a rapid wing molt in adultfemales and tail molt in juveniles. Mallophaga were only weaklynegatively associated with primary and secondary molt scorein adult females and speed of wing molt in adult males. In conclusion,phenotypic quality of adult male barn swallows as reflectedby the expression of their secondary sexual character duringthe previous molt reliably reflected stage and speed of currentmolt.     

Temporal and spatial patterns of flight and body feather molt of Bank,Barn, and Cliff swallows in North and South America

Molt is energetically demanding and various molt strategies (i.e., molt series, duration, intensity, timing, and location) have evolved to reduce the negative fitness consequences of this process. As such, molt varies considerably among species. Identifying where and when specific feathers are molted is also crucial to inform species‐specific studies using stable isotope markers to assign individuals to geographical regions where they molt. Using museum specimens, we examined the molt of three species of migratory swallows in the Americas: Bank Swallows (Riparia riparia), Barn Swallows (Hirundo rustica), and Cliff Swallows (Petrochelidon pyrrhonota). All three species have one primary and two secondary molt series. Bank and Cliff swallows had one rectrix molt series, and Barn Swallows molted the outer rectrix (R6) separately from the inner five rectrices (R1‐5). All three species have a relatively long flight feather molt duration (i.e., 140–183 days) and low molt intensity. Barn Swallows initiated flight feather molt in the fall, about 2 months later than Bank and Cliff swallows. Barn Swallows likely delay molt because of constraints associated with double brooding. For all three species, molt started with the primaries and inner secondaries and was closely followed by the rectrices and, finally, the outer secondaries. For those that began and then interrupted molt either in breeding areas or during fall migration, the first feathers molted were predominantly S8 and P1. All three species underwent body molt throughout the year, but most individuals molted their body plumage in wintering areas. We recommend that the most appropriate feathers for stable isotope research examining migratory connectivity and habitat use are either R2‐R4 or S2‐S4.     

Population differences in the structure and coloration of great tit contour feathers

Contour feathers cover most of the avian body and play critical roles in insulation, social communication, aerodynamics, and water repellency. Feather production is costly and the development of the optimum characteristics for each function may be constrained by limited resources or time, and possibly also lead to trade‐offs among the different characteristics. Populations exposed to different environmental conditions may face different selective pressures, resulting in differences in feather structure and coloration, particularly in species with large geographical distributions. Three resident populations of great tit Parus major L. from different latitudes differed in feather structure and coloration. Individuals from the central population exhibited less dense and longer contour feathers, with a higher proportion of plumulaceous barbs than either northern or southern birds, which did not differ in their feather structure. Ultraviolet reflectance and brightness of the yellow of the contour feathers of the breast was higher for the southern than for the northern population. Birds with greener plumage (higher hue) had less dense but longer feathers, independently of the population of origin. Differences in feather structure across populations appear to be unrelated to the contour feather colour characteristics except for hue. Nutritional and time constraints during molt might explain the pattern of feather structure, whereas varying sexual selection pressure might underlie the coloration patterns observed. Our results suggest that different selective pressures or constraints shape contour feather traits in populations exposed to varying environmental conditions. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 114 , 82–91.     

“Jack‐of‐all‐trades” is parthenogenetic

Sex is evolutionarily more costly than parthenogenesis, evolutionary ecologists therefore wonder why sex is much more frequent than parthenogenesis in the majority of animal lineages. Intriguingly, parthenogenetic individuals and species are as common as or even more common than sexuals in some major and putative ancient animal lineages such as oribatid mites and rotifers. Here, we analyzed oribatid mites (Acari: Oribatida) as a model group because these mites are ancient (early Paleozoic), widely distributed around the globe, and include a high number of parthenogenetic species, which often co‐exist with sexual oribatid mite species. There is evidence that the reproductive mode is phylogenetically conserved in oribatid mites, which makes them an ideal model to test hypotheses on the relationship between reproductive mode and species'' ecological strategies. We used oribatid mites to test the frozen niche variation hypothesis; we hypothesized that parthenogenetic oribatid mites occupy narrow specialized ecological niches. We used the geographic range of species as a proxy for specialization as specialized species typically do have narrower geographic ranges than generalistic species. After correcting for phylogenetic signal in reproductive mode and demonstrating that geographic range size has no phylogenetic signal, we found that parthenogenetic lineages have a higher probability to have broader geographic ranges than sexual species arguing against the frozen niche variation hypothesis. Rather, the results suggest that parthenogenetic oribatid mite species are more generalistic than sexual species supporting the general‐purpose genotype hypothesis. The reason why parthenogenetic oribatid mite species are generalists with wide geographic range sizes might be that they are of ancient origin reflecting that they adapted to varying environmental conditions during evolutionary history. Overall, our findings indicate that parthenogenetic oribatid mite species possess a widely adapted general‐purpose genotype and therefore might be viewed as “Jack‐of‐all‐trades.”     

Evolution of the structure of tail feathers: implications for the theory of sexual selection

Bird tails are extraordinarily variable in length and functionality. In some species, males have evolved exaggeratedly long tails as a result of sexual selection. Changes in tail length should be associated with changes in feather structure. The study of the evolution of feather structure in bird tails could give insight to understand the causes and means of evolution in relation to processes of sexual selection. In theory, three possible means of tail length evolution in relation to structural components might be expected: (1) a positive relationship between the increase in length and size of structural components maintaining the mechanical properties of the feather; (2) no relationship; that is, enlarging feather length without changes in the structural components; and (3) a negative relationship; that is, enlarging feather length by reducing structural components. These hypotheses were tested using phylogenetic analyses to examine changes in both degree of exaggeration in tail length and structural characteristics of tail feathers (rachis width and density of barbs) in 36 species, including those dimorphic and nondimorphic in tail length. The degree of sexual dimorphism in tail length was negatively correlated with both rachis width and density of barbs in males but not in females. Reinforcing this result, we found that dimorphism in tail length was negatively associated with dimorphism in tail feather structure (rachis width and density of barbs). These results support the third hypothesis, in which the evolution of long feathers occurs at the expense of making them simpler and therefore less costly to produce. However, we do not know the effects of enfeeblement on the costs of bearing. If the total costs increased, the enfeeblement of feathers could be explained as a reinforcement of the honesty of the signal. Alternatively, if total costs were reduced, the strategy could be explained by cheating processes. The study of female preferences for fragile tail feathers is essential to test these two hypotheses. Preferences for fragile tails would support the evolution of reinforcement of honesty, whereas female indifference would indicate the existence of cheating in certain stages of the evolutionary process.     

Melanin‐based ornament darkness positively correlates with across‐season nutritional condition

Sexually dimorphic ornamental traits are widely regarded as indicators of nutritional condition. However, variation of nutritional condition outside the reproductive and the ornament production seasons has rarely been considered, although it affects the generality of information content, especially for ornaments that may be used across the year. We measured several indicators of migratory and molt condition in male and female blackcaps (Sylvia atricapilla) during their autumn migration, and quantified their crown reflectance. We detected robust correlations between migratory and molt condition indices, and the correlation structure was similar in the two sexes. Furthermore, the across‐season measure of body condition was positively related to the darkness of the black crown in males, while being unrelated to reflectance traits of the reddish crown in females. Our results reinforce the possibility that some melanin‐based ornaments may be year‐round indicators of individual quality via their dependence on nutritional condition.     

A structural inventory of native ribosomal ABCE1‐43S pre‐initiation complexes

In eukaryotic translation, termination and ribosome recycling phases are linked to subsequent initiation of a new round of translation by persistence of several factors at ribosomal sub‐complexes. These comprise/include the large eIF3 complex, eIF3j (Hcr1 in yeast) and the ATP‐binding cassette protein ABCE1 (Rli1 in yeast). The ATPase is mainly active as a recycling factor, but it can remain bound to the dissociated 40S subunit until formation of the next 43S pre‐initiation complexes. However, its functional role and native architectural context remains largely enigmatic. Here, we present an architectural inventory of native yeast and human ABCE1‐containing pre‐initiation complexes by cryo‐EM. We found that ABCE1 was mostly associated with early 43S, but also with later 48S phases of initiation. It adopted a novel hybrid conformation of its nucleotide‐binding domains, while interacting with the N‐terminus of eIF3j. Further, eIF3j occupied the mRNA entry channel via its ultimate C‐terminus providing a structural explanation for its antagonistic role with respect to mRNA binding. Overall, the native human samples provide a near‐complete molecular picture of the architecture and sophisticated interaction network of the 43S‐bound eIF3 complex and the eIF2 ternary complex containing the initiator tRNA.     

Green‐up selection by red deer in heterogeneous,human‐dominated landscapes of Central Europe

The forage maturation hypothesis (FMH) assumes that herbivores cope with the trade‐off between digestibility and biomass in forage by selecting vegetation at intermediate growth. The green wave hypothesis (GWH) extends the FMH to suggest how spatiotemporal heterogeneity in plant quality shapes migratory movements of herbivores. Growing empirical support for these hypotheses mainly comes from studies in vast landscapes with large‐scale habitat heterogeneity. It is unclear, however, to what extent ungulates surf green waves in human‐altered landscapes with small‐scale heterogeneity in terms of land use and topography. We used plant phenological proxies derived from Sentinel 2 satellite data to analyze the habitat selection of 93 collared red deer (Cervus elaphus) in montane and alpine habitats. Using a step selection analysis, we investigated how plant phenology, that is, the instantaneous rate of green‐up (IRG) and normalized difference vegetation index (NDVI), and a set of variables describing topography and human presence influenced red deer resource selection in open habitats. We learned that red deer selected areas with high biomass at green‐up and avoided habitats with possible exposure to human activity. Additionally, landscape structure and topography strongly influenced spatial behavior of red deer. We further compared cumulative access to high‐quality forage across migrant strategies and found migrants gained better access than residents. Many migratory individuals surfed the green wave, and their surfing behavior, however, became less pronounced with decreasing distance to settlements. Within the constraints of topography and human land use, red deer track spring green‐up on a fine spatiotemporal scale and follow the green wave across landscapes in migration movements. Thus, they benefit from high‐quality forage even in human‐dominated landscapes with small‐scale heterogeneity and vegetation emerging in a heterogenic, dynamic mosaic.     

Context‐dependency in carnivore co‐occurrence across a multi‐use conservation landscape

Carnivore intraguild dynamics depend on a complex interplay of environmental affinities and interspecific interactions. Context‐dependency is commonly expected with varying suites of interacting species and environmental conditions but seldom empirically described. In South Africa, decentralized approaches to conservation and the resulting multi‐tenure conservation landscapes have markedly altered the environmental stage that shapes the structure of local carnivore assemblages. We explored assemblage‐wide patterns of carnivore spatial (residual occupancy probability) and temporal (diel activity overlap) co‐occurrence across three adjacent wildlife‐oriented management contexts—a provincial protected area, a private ecotourism reserve, and commercial game ranches. We found that carnivores were generally distributed independently across space, but existing spatial dependencies were context‐specific. Spatial overlap was most common in the protected area, where species occur at higher relative abundances, and in game ranches, where predator persecution presumably narrows the scope for spatial asymmetries. In the private reserve, spatial co‐occurrence patterns were more heterogeneous but did not follow a dominance hierarchy associated with higher apex predator densities. Pair‐specific variability suggests that subordinate carnivores may alternate between pre‐emptive behavioral strategies and fine‐scale co‐occurrence with dominant competitors. Consistency in species‐pairs diel activity asynchrony suggested that temporal overlap patterns in our study areas mostly depend on species'' endogenous clock rather than the local context. Collectively, our research highlights the complexity and context‐dependency of guild‐level implications of current management and conservation paradigms; specifically, the unheeded potential for interventions to influence the local network of carnivore interactions with unknown population‐level and cascading effects.     

Complex postbreeding molt strategies in a songbird migrating along the East Asian Flyway,the Pallas’s Grasshopper Warbler Locustella certhiola

Molt strategies have received relatively little attention in current ornithology, and knowledge concerning the evolution, variability and extent of molt is sparse in many bird species. This is especially true for East Asian Locustella species where assumptions on molt patterns are based on incomplete information. We provide evidence indicating a complex postbreeding molt strategy and variable molt extent among the Pallas's Grasshopper Warbler Locustella certhiola, based on data from six ringing sites situated along its flyway from the breeding grounds to the wintering areas. Detailed study revealed for the first time that in most individuals wing feather molt proceeds from the center both toward the body and the wing‐tip, a molt pattern known as divergent molt (which is rare among Palearctic passerines). In the Russian Far East, where both breeding birds and passage migrants occur, a third of the adult birds were molting in late summer. In Central Siberia, at the northwestern limit of its distribution, adult individuals commenced their primary molt partly divergently and partly with unknown sequence. During migration in Mongolia, only descendantly (i.e., from the body toward the wing‐tip) molting birds were observed, while further south in Korea, Hong Kong, and Thailand the proportion of potential eccentric and divergent feather renewal was not identifiable since the renewed feathers were already fully grown as expected. We found an increase in the mean number of molted primaries during the progress of the autumn migration. Moderate body mass levels and low‐fat and muscle scores were observed in molting adult birds, without any remarkable increase in the later season. According to optimality models, we suggest that an extremely short season of high food abundance in tall grass habitats and a largely overland route allow autumn migration with low fuel loads combined with molt migration in at least a part of the population. This study highlights the importance of further studying molt strategy as well as stopover behavior decisions and the trade‐offs among migratory birds that are now facing a panoply of anthropogenic threats along their flyways.     

Late breeding season definitive prebasic molt in males,and late breeding season brood care by females,in central California Wilson’s warblers

I made observations of a central California population of Wilson''s Warbler, Cardellina pusilla, after July 1 over 10 breeding seasons. I sighted males in definitive prebasic molt from July 4 (in 2007) to September 1 (in 1999). Most territorial males molted on their breeding territories, and individual molt lasted up to 46 days. Following prebasic molt, territorial males engaged in subdued “post‐molt singing,” which lasted about 7 days in some males, and which I first heard on August 13 (in 2004) and last heard on September 6 (in 1999). I sighted no female in definitive prebasic molt, or in fresh basic plumage, during the study. Of 13 females sighted ≥ July 21, 11 were in late breeding season uniparental brood care, and I could not rule out late brood care for the other two. Most, and possibly all, females not engaged in late season uniparental brood care apparently vacated their breeding territories before July 21. This departure was much earlier than for resident males, the last of which I sighted on September 10 (in 1999). Early‐departing females presumably underwent prebasic molt after July 21 at locations not known. Remaining late‐nesting females must have molted much later than resident males and likely later than early‐departing females, and at locations unknown. I last sighted two uniparental brood‐tending females, still in worn plumage, on August 26 and 29, respectively. Two unique findings of this study are a male/female difference in location of prebasic molt, and a likely dichotomy of prebasic molt timing between females leaving their breeding territories early and those remaining in uniparental brood care. Another finding, post‐molt singing in most and possible all territorial males, is a largely unrecognized behavior, but one previously reported in several passerine species. Post‐molt singing may reliably indicate completion of prebasic molt.     

Evolution of breeding plumages in birds: A multiple‐step pathway to seasonal dichromatism in New World warblers (Aves: Parulidae)

Many species of birds show distinctive seasonal breeding and nonbreeding plumages. A number of hypotheses have been proposed for the evolution of this seasonal dichromatism, specifically related to the idea that birds may experience variable levels of sexual selection relative to natural selection throughout the year. However, these hypotheses have not addressed the selective forces that have shaped molt, the underlying mechanism of plumage change. Here, we examined relationships between life‐history variation, the evolution of a seasonal molt, and seasonal plumage dichromatism in the New World warblers (Aves: Parulidae), a family with a remarkable diversity of plumage, molt, and life‐history strategies. We used phylogenetic comparative methods and path analysis to understand how and why distinctive breeding and nonbreeding plumages evolve in this family. We found that color change alone poorly explains the evolution of patterns of biannual molt evolution in warblers. Instead, molt evolution is better explained by a combination of other life‐history factors, especially migration distance and foraging stratum. We found that the evolution of biannual molt and seasonal dichromatism is decoupled, with a biannual molt appearing earlier on the tree, more dispersed across taxa and body regions, and correlating with separate life‐history factors than seasonal dichromatism. This result helps explain the apparent paradox of birds that molt biannually but show breeding plumages that are identical to the nonbreeding plumage. We find support for a two‐step process for the evolution of distinctive breeding and nonbreeding plumages: That prealternate molt evolves primarily under selection for feather renewal, with seasonal color change sometimes following later. These results reveal how life‐history strategies and a birds' environment act upon multiple and separate feather functions to drive the evolution of feather replacement patterns and bird coloration.     

Demographic variation in timing and intensity of feather molt in migratory Fork‐tailed Flycatchers (Tyrannus s. savana)

Understanding the annual cycle of migratory birds is imperative for evaluating the evolution of life‐history strategies and developing effective conservation strategies. Yet, we still know little about the annual cycle of migratory birds that breed at south‐temperate latitudes of South America. We aged, sexed, and determined the progression and intensity of body, remige, and rectrix molt of migratory Fork‐tailed Flycatchers (Tyrannus s. savana) at breeding sites in southern South America and at wintering sites in northern South America. Molt of both body and flight feathers occurred primarily during the winter. In early winter, a similar proportion of young and adult flycatchers molted remiges and rectrices, but remige molt intensity (number of remiges molting) was greater and primary molt progression (mean primary feather molting) more advanced in adults. In late winter, remige molt intensity and primary molt progression did not differ between age groups. We found no difference between males and females either in the proportion of individuals molting in winter or in the intensity or progress of remige molt. Our results suggest that the nominate subspecies of Fork‐tailed Flycatcher undergoes one complete, annual molt on the wintering grounds, and represents the first comprehensive evaluation of molt timing of a migratory New World flycatcher that overwinters in the tropics. Given that breeding, molt, and migration represent three key events in the annual cycle of migratory birds, knowledge of the timing of these events is the first step toward understanding the possible tradeoffs migratory birds face throughout the year.     

Body condition and feather molt of a migratory shorebird during the non‐breeding season

Migratory shorebirds have some of the highest fat loads among birds, especially species which migrate long distances. The upland sandpiper Bartramia longicauda makes long‐distance migrations twice a year, but variation in body condition or timing of feather molt during the non‐breeding season has not been studied. Molt is an important part of the annual cycle of migratory birds because feather condition determines flight performance during migration, and long‐distance movements are energetically costly. However, variation in body condition during molt has been poorly studied. The objective of our field study was to examine the timing and patterns of feather molt of a long distance migratory shorebird during the non‐breeding season and test for relationships with body size, fat depots, mass, and sex. Field work was conducted at four ranches in the Northern Campos of Uruguay (Paysandú and Salto Departments). We captured and marked 62 sandpipers in a 2‐month period (Nov–Jan) during four non‐breeding seasons (2008–2012). Sex was determined by genetic analyses of blood samples taken at capture. Molt was measured in captured birds using rank scores based on published standards. Body mass and tarsus length measurements showed female‐biased sexual size dimorphism with males smaller than females. Size‐corrected body mass (body condition) showed a U‐shaped relationship with the day of the season, indicating that birds arrived at non‐breeding grounds in relatively good condition. Arriving in good body condition at non‐breeding grounds is probably important because of the energetic demands due to physiological adjustments after migration and the costs of feather molt.     

Tail autotomy works as a pre‐capture defense by deflecting attacks

Caudal autotomy is a dramatic antipredator adaptation where prey shed their tail in order to escape capture by a predator. The mechanism underlying the effectiveness of caudal autotomy as a pre‐capture defense has not been thoroughly investigated. We tested two nonexclusive hypotheses, that caudal autotomy works by providing the predator with a “consolation prize” that makes it break off the hunt to consume the shed tail, and the deflection hypothesis, where the autotomy event directs predator attacks to the autotomized tail enabling prey escape. Our experiment utilized domestic dogs Canis familiaris as model predator engaged to chase a snake‐like stimulus with a detachable tail. The tail was manipulated to vary in length (long versus short) and conspicuousness (green versus blue), with the prediction that dog attacks on the tail should increase with length under the consolation‐prize hypothesis and conspicuous color under the deflection hypothesis. The tail was attacked on 35% of trials, supporting the potential for pre‐capture autotomy to offer antipredator benefits. Dogs were attracted to the tail when it was conspicuously colored, but not when it was longer. This supports the idea that deflection of predator attacks through visual effects is the prime antipredator mechanism underlying the effectiveness of caudal autotomy as opposed to provision of a consolation prize meal.     

SOX2 plays a crucial role in cell proliferation and lineage segregation during porcine pre‐implantation embryo development

ObjectivesGene regulation in early embryos has been widely studied for a long time because lineage segregation gives rise to the formation of a pluripotent cell population, known as the inner cell mass (ICM), during pre‐implantation embryo development. The extraordinarily longer pre‐implantation embryo development in pigs leads to the distinct features of the pluripotency network compared with mice and humans. For these reasons, a comparative study using pre‐implantation pig embryos would provide new insights into the mammalian pluripotency network and help to understand differences in the roles and networks of genes in pre‐implantation embryos between species.Materials and methodsTo analyse the functions of SOX2 in lineage segregation and cell proliferation, loss‐ and gain‐of‐function studies were conducted in pig embryos using an overexpression vector and the CRISPR/Cas9 system. Then, we analysed the morphological features and examined the effect on the expression of downstream genes through immunocytochemistry and quantitative real‐time PCR.ResultsOur results showed that among the core pluripotent factors, only SOX2 was specifically expressed in the ICM. In SOX2‐disrupted blastocysts, the expression of the ICM‐related genes, but not OCT4, was suppressed, and the total cell number was also decreased. Likewise, according to real‐time PCR analysis, pluripotency‐related genes, excluding OCT4, and proliferation‐related genes were decreased in SOX2‐targeted blastocysts. In SOX2‐overexpressing embryos, the total blastocyst cell number was greatly increased but the ICM/TE ratio decreased.ConclusionsTaken together, our results demonstrated that SOX2 is essential for ICM formation and cell proliferation in porcine early‐stage embryogenesis.     

Haste makes waste but condition matters: molt rate-feather quality trade-off in a sedentary songbird

Background

The trade-off between current and residual reproductive values is central to life history theory, although the possible mechanisms underlying this trade-off are largely unknown. The ‘molt constraint’ hypothesis suggests that molt and plumage functionality are compromised by the preceding breeding event, yet this candidate mechanism remains insufficiently explored.

Methodology/Principal Findings

The seasonal change in photoperiod was manipulated to accelerate the molt rate. This treatment simulates the case of naturally late-breeding birds. House sparrows Passer domesticus experiencing accelerated molt developed shorter flight feathers with more fault bars and body feathers with supposedly lower insulation capacity (i.e. shorter, smaller, with a higher barbule density and fewer plumulaceous barbs). However, the wing, tail and primary feather lengths were shorter in fast-molting birds if they had an inferior body condition, which has been largely overlooked in previous studies. The rachis width of flight feathers was not affected by the treatment, but it was still condition-dependent.

Conclusions/Significance

This study shows that sedentary birds might face evolutionary costs because of the molt rate–feather quality conflict. This is the first study to experimentally demonstrate that (1) molt rate affects several aspects of body feathers as well as flight feathers and (2) the costly effects of rapid molt are condition-specific. We conclude that molt rate and its association with feather quality might be a major mediator of life history trade-offs. Our findings also suggest a novel advantage of early breeding, i.e. the facilitation of slower molt and the condition-dependent regulation of feather growth.     

Cryo‐EM structures of perforin‐2 in isolation and assembled on a membrane suggest a mechanism for pore formation

Perforin‐2 (PFN2, MPEG1) is a key pore‐forming protein in mammalian innate immunity restricting intracellular bacteria proliferation. It forms a membrane‐bound pre‐pore complex that converts to a pore‐forming structure upon acidification; but its mechanism of conformational transition has been debated. Here we used cryo‐electron microscopy, tomography and subtomogram averaging to determine structures of PFN2 in pre‐pore and pore conformations in isolation and bound to liposomes. In isolation and upon acidification, the pre‐assembled complete pre‐pore rings convert to pores in both flat ring and twisted conformations. On membranes, in situ assembled PFN2 pre‐pores display various degrees of completeness; whereas PFN2 pores are mainly incomplete arc structures that follow the same subunit packing arrangements as found in isolation. Both assemblies on membranes use their P2 β‐hairpin for binding to the lipid membrane surface. Overall, these structural snapshots suggest a molecular mechanism for PFN2 pre‐pore to pore transition on a targeted membrane, potentially using the twisted pore as an intermediate or alternative state to the flat conformation, with the capacity to cause bilayer distortion during membrane insertion.