Brood‐parasite‐induced female‐biased mortality affects songbird demography: negative implications for conservation

Parasites, of all sorts, can profoundly affect host population dynamics. Parasites commonly cause sex‐biased mortality and this can add to their impact. Female‐biased mortality in particular can destabilize dynamics and promote population collapse. We previously reported in a correlative study that brown‐headed cowbird Molothrus ater brood parasitism of song sparrows Melospiza melodia appears to cause female‐biased host nestling mortality. Here, we report results from ‘infestation’ and ‘de‐infestation’ experiments designed to test whether brood parasitism causes female‐biased mortality, and we document the resulting demographic impact using a simulation model. Experimental cowbird infestation of song sparrow nests halved the proportion of female host nestlings (0.31±0.07 vs 0.59±0.06; infested vs unparasitized nests at day 6) replicating the halving reported in naturally cowbird‐parasitized nests (0.28±0.01 vs 0.57±0.05; parasitized vs unparasitized). De‐infestation of naturally cowbird‐parasitized nests in turn wholly eliminated any effect on the proportion of female host nestlings (0.53±0.13 vs 0.54±0.06; de‐infested vs unparasitized) confirming that brood parasitism is the cause. This halving of the proportion of females fledging is likely to be as significant as nest predation in affecting population dynamics, based on the elasticities derived from our demographic model (–0.50 vs –0.59). Experimental infestation reduced the testosterone levels, begging behaviour, and body mass of six day old female host nestlings, whereas males were largely unaffected, suggesting that it is the exacerbation of intra‐brood competition that may be primarily responsible for the resulting female‐biased mortality. The brown‐headed cowbird is invasive in most of North America and has been implicated in regional population declines of many native species. We suggest that female‐biased host offspring mortality is likely to be commonplace among the 144 host species the cowbird successfully parasitizes, and we discuss the negative implications for songbird conservation, given the projected demographic impact.     

Cascading trait‐mediation: disruption of a trait‐mediated mutualism by parasite‐induced behavioral modification

Trait‐mediated indirect interactions (TMII) are important driving‐forces causing trophic cascades in aquatic and terrestrial food webs. Furthermore, since most biological communities are not simple food chains but complex networks of interactions, one TMII within a community might easily be influenced by another TMII. In other words, TMII themselves can be cascades with potential implications for community dynamics. Here we report on one of such cascade, where a parasitic fly induces behavioral changes that disrupt a trait‐mediated ant–hemipteran mutualism. We show that during parasite‐induced low‐activity periods, the ant Azteca instabilis fails to protect its mutualistic scale‐insect partner against predatory ladybeetles. Thus, in the presence of the parasite, ladybeetles ate as many scales in ant‐patrolled plants as they did in ant‐free plants. These results demonstrate how, through a cascade of trait‐mediated interactions, associations between members of a community can be drastically altered.     

Interpreting phenotypic variation in plants

Plant ecologists and evolutionary biologists frequently examine patterns of phenotypic variation across variable environments or genetic identities. Too often, we ignore the fact that most phenotypic traits change throughout growth and development of individual plants, and that rates of growth and development are highly variable. Plants growing in different environments are likely to grow at different rates, and will be of different sizes and stages of development at a particular age. When we compare plants as a function of plant size or developmental stage, as well as a function of age, we broaden our understanding of phenotypic variation between plants.     

Effects of antagonistic coevolution on parasite‐mediated host coexistence

Parasites can promote diversity by mediating coexistence between a poorer and superior competitor, if the superior competitor is more susceptible to parasitism. However, hosts and parasites frequently undergo antagonistic coevolution. This process may result in the accumulation of pleiotropic fitness costs associated with host resistance, and could breakdown coexistence. We experimentally investigated parasite‐mediated coexistence of two genotypes of the bacterium Pseudomonas fluorescens, where one genotype underwent coevolution with a parasite (a virulent bacteriophage), whereas the other genotype was resistant to the evolving phages at all time points, but a poorer competitor. In the absence of phages, the resistant genotype was rapidly driven extinct in all populations. In the presence of the phages, the resistant genotype persisted in four of six populations and eventually reached higher frequencies than the sensitive genotype. The coevolving genotype showed a reduction in the growth rate, consistent with a cost of resistance, which may be responsible for a decline in its relative fitness. These results demonstrate that the stability of parasite‐mediated coexistence of resistant and susceptible species or genotypes is likely to be affected if parasites and susceptible hosts coevolve.     

Tropics,trophics and taxonomy: the determinants of parasite‐associated host mortality

Empirical studies often reveal deleterious effects of parasites on host survival, but the ecological and environmental processes modulating parasite‐associated host mortality are not well understood. We conducted meta‐analysis of experimental studies assessing parasite‐associated mortality (n = 52) to evaluate broad‐scale patterns in host mortality risk relative to host or parasite taxon, parasite life cycle, or local environmental conditions. Overall, likelihood of host mortality was ～2.6 times higher among infected individuals when compared with hosts that either lacked parasites or had experimentally‐reduced parasite burdens. Parasites with complex life cycles reliant on predation‐mediated transmission generally were associated with higher mortality risk than those exploiting other transmission strategies. We also detected a negative relationship between parasite‐associated host mortality and latitude; host mortality risk declined by ～2.6% with each degree increase in latitude. This result indicated the likely importance of abiotic factors in determining parasite effects. Host taxonomy further influenced parasite‐associated mortality risk, with amphibian, fish, and mollusc hosts generally having higher hazard than arthropod, mammal, and bird hosts. Our results suggest patterns that conform to the predicted link between host mortality and parasite transmissibility, and pathogenicity. The relationship between host mortality and latitude in particular may portend marked shifts in host–parasite relationships pursuant to ongoing and projected global climate change.     

Thymic involution and thymocyte phenotypic alterations induced by murine mammary adenocarcinomas

A profound thymic atrophy has been observed in mice bearing large adenocarcinomas of the mammary gland. Only 2 to 5% of thymocytes remained 4 wk after tumor implantation. Although there is a slight decrease in the overall percentages of Thy-1+ cells in tumor bearers, the majority of the remaining cells are of a Thy-1 low phenotype. There was a lower percentage of double positive (CD4+, CD8+) cells, an increase of CD4+ CD8- thymocytes, similar percentages of CD4- CD8+ cells and double negative (CD4- CD8-) thymocytes in tumor-bearing mice. In addition, an increased percentage of CD3 cells could be detected in these animals. These results indicate that proportionally less immature thymocytes are present in the atrophic thymuses of mammary tumor bearers. Enhanced levels of glucocorticoids are known to produce similar effects on the thymus. However, adrenalectomy of mice followed by tumor implantation did not result in reversal of the thymic atrophy. Furthermore, a study of serum corticosterone levels in tumor bearers indicated no significant changes during tumorigenesis. A study of several parameters of bone marrow (BM) populations indicate that there is an increase in cells of the granulocyte-macrophage lineage and a decrease in lymphocytes induced by tumor-derived granulocyte macrophage-CSF. An alteration of prothymocytes in the BM is not the main cause of the thymic atrophy because BM cells from normal and tumor-bearing mice reconstituted irradiated normal mice equally well. There was no preferential recruitment of double positive cells to the spleen as indicated by no significant differences in the levels of T cells of immature phenotype including the CD4+ CD8+ population in the spleens of tumor bearers. Because no major changes were observed in tumor bearers, either at their capacity to repopulate the thymus or at the patterns of subsequent redistribution of thymocytes, it was postulated that the thymic atrophy may be caused by a direct or indirect effect of the tumor or tumor-associated factor(s). Intrathymic injections of tumor cells into young normal recipient mice resulted in a significant reduction of the thymus weight and cellularity. These data suggest that mammary tumors can secrete factor(s) that are capable of severely impairing the normal development of cells of the T cell lineage.     

Interpreting phenotypic variation in plant allelochemistry: problems with the use of concentrations

Ecologists often use concentrations of defensive compounds as measures of plant allocation to defence and/or allelochemical production. I demonstrate that this practice may lead to erroneous conclusions because plants produce and allocate molecules (quantities) of compounds whereas concentrations reflect the distribution of these quantities in plant tissues and are, therefore, functions of plant biomass. As a tool for distinguishing between shifts in allelochemical production versus changes in plant biomass in determining allelochemical concentrations, I suggest using a technique known as graphical vector analysis (GVA) which has been developed for diagnosing nutrient limitations in forest stands, but has seldom been applied by researchers studying plant allelochemicals. I used data from several published studies to demonstrate how GVA can be applied to interpret ontogenetic and environmental effects on allelochemical levels and to compare the results obtained for different allelochemical types, plant species, treatments and experiments. These examples show that changes in plant biomass per se are an important source of variation in allelochemical concentrations and, therefore, concentration data can be easily misinterpreted if changes in absolute content and plant biomass are not considered simultaneously. Because studies reporting variation in allelochemical concentrations have been considered as tests for general theories of plant chemical defence, evidence in support of or against these theories should be re-examined using multivariate techniques such as analysis of covariance, allometric analysis and GVA. Received: 16 September 1998 / Accepted: 4 March 1999     

Symbiont‐mediated phenotypic variation without co‐evolution in an insect–fungus association

Recent studies have shown that symbionts can be a source of adaptive phenotypic variation for their hosts. It is assumed that co‐evolution between hosts and symbionts underlies these ecologically significant phenotypic traits. We tested this assumption in the ectosymbiotic fungal associate of the gall midge Asteromyia carbonifera. Phylogenetic analysis placed the fungal symbiont within a monophyletic clade formed by Botryosphaeria dothidea, a typically free‐living (i.e. not associated with an insect host) plant pathogen. Symbiont isolates from four divergent midge lineages demonstrated none of the patterns common to heritable microbial symbioses, including parallel diversification with their hosts, substitution rate acceleration, or A+T nucleotide bias. Amplified fragment length polymorphism genotyping of the symbiont revealed that within‐lineage genetic diversity was not clustered along host population lines. Culture‐based experiments demonstrated that the symbiont‐mediated variation in gall phenotype is not borne out in the absence of the midge. This study shows that symbionts can be important players in phenotypic variation for their hosts, even in the absence of a co‐evolutionary association.     

Evaluation of phenotypic and genotypic alterations induced by long periods of subculturing of Cryptococcus neoformans strains

Cryptococcus neoformans is an encapsulated fungal organism that can cause disease in apparently immunocompetent, as well as immunocompromised, hosts. Since 1930, successive subculture has been used to preserve C. neoformans isolates in our Fungus Collection. In the 1970s, some of these Fungus Collection samples were selected to be subjected to a different methods of maintenance--that of lyophilized. Our objective was to analyze C. neoformans isolates in order to make a comparative evaluation between these two methods of preservation. The overall aim of this study was to qualify the preservation technique used in our mycology laboratory since the technique used might affect the survival, stability and purity of the primary isolates in culture. The samples were analyzed using classical mycology methods and using the randomly amplified polymorphic DNA technique In the analysis of phenotypes and genotypes, the typical characteristics of C. neoformans were found to differ in relation to the different methods of preservation employed. The aim of this study was to demonstrate the importance of selecting the appropriate method of preservation for fungus collections. This selection can affect the survival and purity of the cultures, and preserve the stability of their physiological, biochemical, and genetic characteristics.     

Che‐1‐induced inhibition of mTOR pathway enables stress‐induced autophagy

Mammalian target of rapamycin (mTOR) is a key protein kinase that regulates cell growth, metabolism, and autophagy to maintain cellular homeostasis. Its activity is inhibited by adverse conditions, including nutrient limitation, hypoxia, and DNA damage. In this study, we demonstrate that Che‐1, a RNA polymerase II‐binding protein activated by the DNA damage response, inhibits mTOR activity in response to stress conditions. We found that, under stress, Che‐1 induces the expression of two important mTOR inhibitors, Redd1 and Deptor, and that this activity is required for sustaining stress‐induced autophagy. Strikingly, Che‐1 expression correlates with the progression of multiple myeloma and is required for cell growth and survival, a malignancy characterized by high autophagy response.     

The division of labor: genotypic versus phenotypic specialization

A model of the division of labor in simple evolving systems is explored to compare two strategies evident in natural populations: phenotypic specialization (such as differentiation by regulated gene expression) and genotypic specialization (such as co-infection by complementary covirus populations). While genotypic specialization is vulnerable to the chance extinction of an essential specialist type and to parasitism, phenotypic specialization is able to overcome these hurdles. When simple spatial effects are included, phenotypic specialization has further benefits, protecting against destructive dynamic patterns. Many of the advantages of phenotypic specialization, however, can only be realized when a high degree of relatedness within groups is ensured.     

On divide-and-conquer strategies for parsimony analysis of large data sets: Rec-I-DCM3 versus TNT

Roshan et al. recently described a divide-and-conquer technique for parsimony analysis of large data sets, Rec-I-DCM3, and stated that it compares very favorably to results using the program TNT. Their technique is based on selecting subsets of taxa to create reduced data sets or subproblems, finding most-parsimonious trees for each reduced data set, recombining all parts together, and then performing global TBR swapping on the combined tree. Here, we contrast this approach to sectorial searches, a divide-and-conquer algorithm implemented in TNT. This algorithm also uses a guide tree to create subproblems, with the first-pass state sets of the nodes that join the selected sectors with the rest of the topology; this allows exact length calculations for the entire topology (that is, any solution N steps shorter than the original, for the reduced subproblem, must also be N steps shorter for the entire topology). We show here that, for sectors of similar size analyzed with the same search algorithms, subdividing data sets with sectorial searches produces better results than subdividing with Rec-I-DCM3. Roshan et al.'s claim that Rec-I-DCM3 outperforms the techniques in TNT was caused by a poor experimental design and algorithmic settings used for the runs in TNT. In particular, for finding trees at or very close to the minimum known length of the analyzed data sets, TNT clearly outperforms Rec-I-DCM3. Finally, we show that the performance of Rec-I-DCM3 is bound by the efficiency of TBR implementation for the complete data set, as this method behaves (after some number of iterations) as a technique for cyclic perturbations and improvements more than as a divide-and-conquer strategy.     

Ceranib‐2‐induced suicidal erythrocyte death

Ceramide is known to trigger apoptosis of nucleated cells and eryptosis of erythrocytes. Eryptosis is characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. Besides ceramide, stimulators of eryptosis include increase of cytosolic Ca²⁺‐activity ([Ca²⁺]_i) and oxidative stress. Ceramide is degraded by acid ceramidase and inhibition of the enzyme similarly triggers apoptosis. The present study explored, whether ceramidase inhibitor Ceranib‐2 induces eryptosis. Flow cytometry was employed to quantify phosphatidylserine‐exposure at the cell surface from annexin‐V‐binding, cell volume from forward scatter, [Ca²⁺]_i from Fluo3‐fluorescence, reactive oxygen species (ROS) from DCF dependent fluorescence, and ceramide abundance utilizing specific antibodies. Hemolysis was estimated from hemoglobin concentration in the supernatant. A 48 h exposure of human erythrocytes to Ceranib‐2 significantly increased the percentage of annexin‐V‐binding cells (≥50 μM) and the percentage of hemolytic cells (≥10 μM) without significantly modifying forward scatter. Ceranib‐2 significantly increased Fluo3‐fluorescence, DCF fluorescence and ceramide abundance. The effect of Ceranib‐2 on annexin‐V‐binding was not significantly blunted by removal of extracellular Ca²⁺. Ceranib‐2 triggers phospholipid scrambling of the erythrocyte cell membrane, an effect at least in part due to increase of ceramide abundance and induction of oxidative stress, but not dependent on Ca²⁺ entry. Copyright © 2016 John Wiley & Sons, Ltd.     

Genome‐wide DNA methylation alterations of Alternanthera philoxeroides in natural and manipulated habitats: implications for epigenetic regulation of rapid responses to environmental fluctuation and phenotypic variation

Alternanthera philoxeroides (alligator weed) is an invasive weed that can colonize both aquatic and terrestrial habitats. Individuals growing in different habitats exhibit extensive phenotypic variation but little genetic differentiation in its introduced range. The mechanisms underpinning the wide range of phenotypic variation and rapid adaptation to novel and changing environments remain uncharacterized. In this study, we examined the epigenetic variation and its correlation with phenotypic variation in plants exposed to natural and manipulated environmental variability. Genome‐wide methylation profiling using methylation‐sensitive amplified fragment length polymorphism (MSAP) revealed considerable DNA methylation polymorphisms within and between natural populations. Plants of different source populations not only underwent significant morphological changes in common garden environments, but also underwent a genome‐wide epigenetic reprogramming in response to different treatments. Methylation alterations associated with response to different water availability were detected in 78.2% (169/216) of common garden induced polymorphic sites, demonstrating the environmental sensitivity and flexibility of the epigenetic regulatory system. These data provide evidence of the correlation between epigenetic reprogramming and the reversible phenotypic response of alligator weed to particular environmental factors.     

Survival selection on escape performance and its underlying phenotypic traits: a case of many‐to‐one mapping

Selection often operates not directly on phenotypic traits but on performance which is important as several traits may contribute to a single performance measure (many‐to‐one mapping). Although largely ignored in the context of selection, this asks for studies that link all relevant phenotypes with performance and fitness. In an enclosure experiment, we studied links between phenotypic traits, swimming performance and survival in two Enallagma damselflies. Predatory dragonflies imposed survival selection for increased swimming propensity and speed only in E. annexum; probably E. aspersum was buffered by the former species’ presence. Accordingly, more circular caudal lamellae, structures involved in generating thrust while swimming, were selected for only in E. annexum. Other phenotypic traits that contributed to swimming speed were apparently not under selection, probably because of many‐to‐one mapping (functional redundancy). Our results indicate that not only the phenotypic distributions of syntopic prey organisms but also many‐to‐one mapping should be considered when documenting phenotype–performance–fitness relationships.