Why does the immune system of Atlantic cod lack MHC II?

MHC II, a major feature of the adaptive immune system, is lacking in Atlantic cod, and there are different scenarios (metabolic cost hypothesis or functional shift hypothesis) that might explain this loss. The lack of MHC II coincides with an increased number of genes for MHC I and Toll-like receptors (TLRs).     

Why do some animals mate with one partner rather than many? A review of causes and consequences of monogamy

Why do some animals mate with one partner rather than many? Here, I investigate factors related to (i) spatial constraints (habitat limitation, mate availability), (ii) time constraints (breeding synchrony, length of breeding season), (iii) need for parental care, and (iv) genetic compatibility, to see what support can be found in different taxa regarding the importance of these factors in explaining the occurrence of monogamy, whether shown by one sex (monogyny or monandry) or by both sexes (mutual monogamy). Focusing on reproductive rather than social monogamy whenever possible, I review the empirical literature for birds, mammals and fishes, with occasional examples from other taxa. Each of these factors can explain mating patterns in some taxa, but not in all. In general, there is mixed support for how well the factors listed above predict monogamy. The factor that shows greatest support across taxa is habitat limitation. By contrast, while a need for parental care might explain monogamy in freshwater fishes and birds, there is clear evidence that this is not the case in marine fishes and mammals. Hence, reproductive monogamy does not appear to have a single overriding explanation, but is more taxon specific. Genetic compatibility is a promising avenue for future work likely to improve our understanding of monogamy and other mating patterns. I also discuss eight important consequences of reproductive monogamy: (i) parentage, (ii) parental care, (iii) eusociality and altruism, (iv) infanticide, (v) effective population size, (vi) mate choice before mating, (vii) sexual selection, and (viii) sexual conflict. Of these, eusociality and infanticide have been subject to debate, briefly summarised herein. A common expectation is that monogamy leads to little sexual conflict and no or little sexual selection. However, as reviewed here, sexual selection can be substantial under mutual monogamy, and both sexes can be subject to such selection. Under long‐term mutual monogamy, mate quality is obviously more important than mate numbers, which in turn affects the need for pre‐mating mate choice. Overall, I conclude that, despite much research on genetic mating patterns, reproductive monogamy is still surprisingly poorly understood and further experimental and comparative work is needed. This review identifies several areas in need of more data and also proposes new hypotheses to test.     

Why does liver fibrosis occur in schistosomiasis?

Schistosomiasis is one of the most prevalent of several chronic inflammatory diseases in which morbidity results primarily from tissue scarring. New concepts regarding the molecular pathogenesis of scar formation are being applied in research efforts to define the basis of liver fibrosis in schistosomiasis. Such investigations have led to the identification of an apparently novel lymphokine, fibroblast stimulating factor-I (FsF-I), produced in the egg granulomas. FsF-1 and other granulomo-derived fibrogenic cytokines may represent the molecular links between periovular granulomotous inflammation and hepatic fibrosis. Here, David Wyler postulates that the unmodified production of these f brogenic signals may be responsible for the development of severe hepatic fibrosis in the subpopulotion of infected individuals who develop this complication.     

Why does remyelination fail in multiple sclerosis?

Multiple sclerosis is a common cause of neurological disability in young adults. The disease is complex -- its aetiology is multifactorial and largely unknown; its pathology is heterogeneous; and, clinically, it is difficult to diagnose, manage and treat. However, perhaps its most frustrating aspect is the inadequacy of the healing response of remyelination. This regenerative process generally occurs with great efficiency in experimental models, and sometimes proceeds to completion in multiple sclerosis. But as the disease progresses, the numbers of lesions in which demyelination persists increases, significantly contributing to clinical deterioration. Understanding why remyelination fails is crucial for devising effective methods by which to enhance it.     

Why do females mate multiply? A review of the genetic benefits

The aim of this review is to consider the potential benefits that females may gain from mating more than once in a single reproductive cycle. The relationship between non-genetic and genetic benefits is briefly explored. We suggest that multiple mating for purely non-genetic benefits is unlikely as it invariably leads to the possibility of genetic benefits as well. We begin by briefly reviewing the main models for genetic benefits to mate choice, and the supporting evidence that choice can increase offspring performance and the sexual attractiveness of sons. We then explain how multiple mating can elevate offspring fitness by increasing the number of potential sires that compete, when this occurs in conjunction with mechanisms of paternity biasing that function in copula or post-copulation. We begin by identifying cases where females use pre-copulatory cues to identify mates prior to remating. In the simplest case, females remate because they identify a superior mate and 'trade up' genetically. The main evidence for this process comes from extra-pair copulation in birds. Second, we note other cases where pre-copulatory cues may be less reliable and females mate with several males to promote post-copulatory mechanisms that bias paternity. Although a distinction is drawn between sperm competition and cryptic female choice, we point out that the genetic benefits to polyandry in terms of producing more viable or sexually attractive offspring do not depend on the exact mechanism that leads to biased paternity. Post-copulatory mechanisms of paternity biasing may: (1) reduce genetic incompatibility between male and female genetic contributions to offspring; (2) increase offspring viability if there is a positive correlation between traits favoured post-copulation and those that improve performance under natural selection; (3) increase the ability of sons to gain paternity when they mate with polyandrous females. A third possibility is that genetic diversity among offspring is directly favoured. This can be due to bet-hedging (due to mate assessment errors or temporal fluctuations in the environment), beneficial interactions between less related siblings or the opportunity to preferentially fertilise eggs with sperm of a specific genotype drawn from a range of stored sperm depending on prevailing environmental conditions. We use case studies from the social insects to provide some concrete examples of the role of genetic diversity among progeny in elevating fitness. We conclude that post-copulatory mechanisms provide a more reliable way of selecting a genetically compatible mate than pre-copulatory mate choice. Some of the best evidence for cryptic female choice by sperm selection is due to selection of more compatible sperm. Two future areas of research seem likely to be profitable. First, more experimental evidence is needed demonstrating that multiple mating increases offspring fitness via genetic gains. Second, the role of multiple mating in promoting assortative fertilization and increasing reproductive isolation between populations may help us to understand sympatric speciation.     

Consciousness in dolphins? A review of recent evidence

For millennia, dolphins have intrigued humans. Scientific study has confirmed that bottlenose dolphins are large-brained, highly social mammals with an extended developmental period, flexible cognitive capacities, and powerful acoustic abilities including a sophisticated echolocation system. These findings have led some to ask if dolphins experience aspects of consciousness. Recent investigations targeting self-recognition/self-awareness and metacognition, constructs tied to consciousness on some accounts, have analyzed the dolphin’s ability to recognize itself in a mirror or on a video as well as to monitor its own knowledge in a perceptual categorization task. The current article reviews this work with dolphins and grapples with some of the challenges in designing, conducting, and interpreting these studies as well as with general issues related to studying consciousness in animals. The existing evidence does not provide a convincing case for consciousness in dolphins. For productive scientific work on consciousness in dolphins (and other animals including humans), we need clearer characterizations of consciousness, better methods for studying it, and appropriate paradigms for interpreting outcomes. A current focus on metamemory in animals offers promise for future discovery in this area.     

Why does biparental plastid inheritance revive in angiosperms?

It is widely believed that plastid and mitochondrial genomes are inherited through the maternal parent. In plants, however, paternal transmission of these genomes is frequently observed, especially for the plastid genome. A male gametic trait, called potential biparental plastid inheritance (PBPI), occurs in up to 20% of angiosperm genera, implying a strong tendency for plastid transmission from the male lineage. Why do plants receive organelles from the male parents? Are there clues in plastids that will help to elucidate the evolution of plants? Reconstruction of the ancestral state of plastid inheritance patterns in a phylogenetic context provides insights into these questions. In particular, a recent report demonstrated the unilateral occurrence of PBPI in angiosperms. This result implies that nuclear cytoplasmic conflicts, a basic driving force for altering the mode of organelle inheritance, might have arisen specifically in angiosperms. Based on existing evidence, it is likely that biparental inheritance may have occurred to rescue angiosperm species with defective plastids.     

Why are some human disease-associated mutations fixed in mice?

A recent comparative genomic analysis revealed the presence of nucleotide sequences in mouse that are known to be disease-associated in humans, yet the mouse appears normal. In this article we formulate and test several hypotheses in an attempt to explain why these apparently deleterious mutations become fixed in mice. We find that except for one case, the fixations of the disease-associated mutations occurred before the separation of Mus musculus and Mus spretus at least 1 million years ago and that the fixations are not attributable to a founder effect during the recent history of mouse breeding. About 80% of the cases involve diseases that occur before reproductive age in humans and these substitutions are unlikely to have been fixed because of the inefficiency of natural selection against late-onset diseases. We conclude that the compensatory mutation hypothesis remains the most probable explanation for the majority of the fixations of disease mutations in mice.     

Why are some microorganisms boring?

Microorganisms from diverse environments actively bore into rocks, contributing significantly to rock weathering. Carbonates are the most common substrate into which they bore, although there are also reports of microbial borings into volcanic glass. One of the most intriguing questions in microbial evolutionary biology is why some microorganisms bore. A variety of possible selection pressures, including nutrient acquisition, protection from UV radiation and predatory grazing could promote boring. None of these pressures is mutually exclusive and many of them could have acted in concert with varying strengths in different environments to favour the development of microorganisms that bore. We suggest that microbial boring might have begun in some environments as a mechanism against entombment by mineralization.     

Why do some small islands lack vegetation? Evidence from long‐term introduction experiments

Classic island biogeography theory predicts that very small islands, near the extreme lower end of the species–area relationship, should support very few species. At times no species may be present, however, due to randomness in the immigration–extinction dynamics. Alternatively, a lack of vegetation on very small islands may indicate that such islands do not contain the appropriate habitat for the establishment or long‐term survival of plants, or that disturbances are too frequent or intense. These potential mechanisms were evaluated in the central Exumas, Bahamas, where surveys of 117 small islands revealed that over a third of the islands supported no terrestrial plant life. Area and exposure were significant predictors of whether a small island was vegetated or not in multiple logistic regressions. No islands naturally devoid of vegetation were colonized over a 17‐yr period, and only two naturally vegetated islands lost all vegetation. Experimental introductions of two species –Sesuvium portulacastrum and Borrichia arborescens– revealed that a number of islands naturally lacking vegetation were able to sustain introduced populations over the long term (up to 15 yr). Drought and hurricanes appeared to have reduced the establishment success and possibly long‐term survival of the introductions, although some populations survived four major hurricanes. Turnover rates of both introduced species were often an order of magnitude higher on the experimental introduction islands than on other islands in the archipelago. It appears many of the islands in this system that naturally lack vegetation may be physically capable of supporting terrestrial plant life, yet have no plants primarily due to barriers to colonization.     

Why does the larval integument of some sawfly species disrupt so easily? The harmful hemolymph hypothesis

The larvae of several sawfly species belonging to the Tenthredinidae (Hymenoptera) have such a low mechanical resistance in the integument that slight mechanical damage to the integument is enough to provoke the release of hemolymph at a given spot. We quantified this phenomenon, which we call "easy bleeding", by measuring the pressure needed to pierce dissected sawfly integument. We also investigated the feeding deterrance of ethanolic extracts of the hemolymph by laboratory bioassays using Myrmica rubra ant workers. These traits, integument resistance and hemolymph deterrence, were inversely related, considering 22 tenthredinid species. A negative correlation was obtained by only taking into account the species of one tenthredinid tribe, namely the Phymatocerini (nine species studied). Our results support the "harmful hemolymph hypothesis" that we present here and that assumes a functional link between these morphological and chemical traits, jointly acting as a chemical defense strategy. We suspect hemolymph deterrence to be often due to sequestration of plant secondary metabolites. We discuss the role of easy bleeding, considering the fact that sawfly larvae are frequently the prey of invertebrate and vertebrate predators. It is suggested that invertebrates such as ants were more important than vertebrates in the evolution of easy bleeding.     

Why do some adult birds skip breeding? A hormonal investigation in a long-lived bird

Skipping reproduction is often observed in long-lived organisms, but proximate mechanisms remain poorly understood. Since young and/or very old snow petrels (Pagodroma nivea) commonly skip breeding, we tested whether they are physiologically able to breed during the pre-laying stage. To do so, we measured the ability of known-age (11-45 years old) petrels to release luteinizing hormone (LH, a crucial driver for breeding), by injecting exogenous gonadotropin-releasing hormone (GnRH). Although young petrels exhibited low baseline LH levels, they were able to elevate LH levels after a GnRH challenge. Moreover, young and very old petrels showed a stronger decrease in LH levels after the 10 min post-GnRH injection compared with middle-aged petrels. Birds that skipped breeding were as able as breeders to release LH after a GnRH challenge, indicating that they had functional pituitaries. However, the decision to skip reproduction was linked to a strong LH decrease after the 10 min post-GnRH injection. Our result suggests that the youngest and the oldest petrels fail to maintain elevated baseline LH levels, thereby do not initiate reproductive activities. Skipping reproduction in long-lived birds probably results from age-related changes in the dynamics of the hypothalamic-pituitary-gonadal (HPG) axis function.     

Buzz-pollination in Neotropical bees:genus-dependent frequencies and lack of optimal frequency for pollen release

Over 50 genera of bees release pollen from flower anthers using thoracic vibrations,a phenomenon known as buzz-pollination.The efficiency of this process is directly affected by the mechanical properties of the buzzes,namely the duration,amplitude,and frequency.Nonetheless,although the effects of the former two properties are well described,the role of buzz frequency on pollen release remains unclear.Furthermore,nearly all of the existing studies describing vibrational properties of natural buzz-pollination are limited to bumblebees(Bombus)and carpenter bees(Xvlocopa)constraining our current understanding of this behavior and its evolution.Therefore,we attempted to minimize this shortcoming by testing whether flower anthers exhibit optimal frequency for pollen release and whether bees tune their buzzes to match these(optimal)frequencies.If true,certain frequencies will trigger more pollen release and lighter bees will reach buzz frequencies closer to this optimum to compensate their smaller buzz amplitudes.Two strategies were used to test these hypotheses:(i)the use of(artificial)vibrational playbacks in a broad range of buzz frequencies and amplitudes to assess pollen release by tomato plants(Solarium Ivcopersicum L.)and(ii)the recording of natural buzzes of Neotropical bees visiting tomato plants during pollination.The playback experiment indicates that although buzz frequency does affect pollen release,no optimal frequency exists for that.In addition,the recorded results of natural buzz-pollination reveal that buzz frequencies vary with bee genera and are not correlated with body size.Therefore,neither bees nor plants are tuned to optimal pollen release frequencies.Bee frequency of buzz-pollination is a likely consequence of the insect flight machinery adapted to reach higher accelerations,while flower plant response to buzz-pollination is the likely result of its pollen granular properties.     

Why do extracurricular activities prevent dropout more effectively in some high schools than in others? A mixed-method examination of organizational dynamics

Abstract

This study describes policies and practices implemented in 12 high schools (Quebec, Canada) that more or less effectively leveraged extracurricular activities (ECA) to prevent dropout among vulnerable students. Following an explanatory sequential mixed design, three school profiles (Effective, Ineffective, and Mixed) were derived based on quantitative student-reported data. Qualitative interviews with frontline staff revealed that in Effective schools, ECA had a unique overarching goal: to support school engagement and perseverance among all students, including vulnerable ones. Moreover, in these schools staff had access to sufficient resources—human and material—and implemented inclusive practices. In Ineffective schools, ECA were used as a means to attract well-functioning students from middle-class families, and substantial resources were channeled toward these students, with few efforts to include vulnerable ones. Schools with a Mixed profile had both strengths and weakness. Recommendations for school-level policies that bolster ECA’s ability to support students’ perseverance are provided.     

Why dengue and yellow fever coexist in some areas of the world and not in others?

Urban yellow fever and dengue coexist in Africa but not in Asia and South America. In this paper, we examine four hypotheses (and various combinations thereof) to explain the absence of yellow fever in urban areas of Asia and South America. In addition, we examine an additional hypothesis that offers an explanation of the coexistence of the infections in Africa while at the same time explaining their lack of coexistence in Asia. The hypotheses we tested to explain the nonexistence of yellow fever in Asia are the following: (1) the Asian Aedes aegypti is relatively incompetent to transmit yellow fever; (2) there would exist a competition between dengue and yellow fever viruses within the mosquitoes, as suggested by in vitro studies in which the dengue virus always wins; (3) when an A. aegypti mosquito that is infected by or latent for yellow fever acquires dengue, it becomes latent for dengue due to internal competition within the mosquito between the two viruses; (4) there is an important cross-immunity between yellow fever and other flaviviruses, dengue in particular, such that a person recovered from a bout of dengue exhibits a diminished susceptibility to yellow fever. This latter hypothesis is referred to below as the "Asian hypothesis." Finally, we hypothesize that: (5) the coexistence of the infections in Africa is due to the low prevalence of the mosquito Aedes albopictus in Africa, as it competes with A. aegypti. We will refer to this latter hypothesis as the "African hypothesis." We construct a model of transmission that allows all of the above hypotheses to be tested. We conclude that the Asian and the African hypotheses can explain the observed phenomena, whereas other hypotheses fail to do so.     

Why are some genetic diseases common?

Various processes (selection, mutation, migration and genetic dirft) are known to determine the frequency of genetic disease in human populations, but so far it has proved almost impossible to decide to what extent each is responsible for the presence of a particular genetic disease. The techniques of gene and haplotype analysis offer new hope in addressing this issue, and we review relevant studies of three haemoglobinopathies: sickle cell anaemia, and and thalassaemia. We show how for each disease it is possible to recognize a pattern of regionally specific mutations, found in association with one or a few haplotypes, that is best explained as the result of selection; other patterns are due to population migration and genetic drift. However, we caution that such conclusions can be drawn in special circumstances only. In the case of the haemoglobinopathies it is possible because a selective agent (malaria) was already suspected, and the investigations could be carried out in relatively genetically homogenous populations whose migratory histories are known. Moreover, some data reviewed here suggest that gene conversion and the haplotype composition of a population may affect the frequency of a mutation, making interpretation of gene frequencies difficult on the basis of standard population genetics theory. Hence attempts to use the same approaches with other genetic diseases are likely to be frustrated by a lack of suitably untrammelled populations and by difficulties accounting for poorly understood genetic processes. We conclude that although this combination of molecular and population genetics is successful when applied to the study of haemoglobinopathies, it may not be so easy to apply it to the study of other genetic diseases.