Plant species distribution across two contrasting treeline ecotones in the Spanish Pyrenees

We describe the structure of two contrasting (elevation, topography,climate, vegetation, soil) alpine forest–pasture ecotones located in theCentral Pyrenees (sites Ordesa, O, and Tessó, T). We define ecotonestructure as the spatial distribution of trees of different size classes andgrowth-forms and the relationship between these aspects and the spatialdistribution of understory vegetation and substrate. The studied ecotones aredominated by Pinus uncinata Ram. and have been littleaffected by anthropogenic disturbances (logging, grazing) during this century.One rectangular plot (30 × 140 m) was located within eachsite with its longest side parallel to the slope and encompassing treeline andtimberline. The distribution of size and growth-form classes at site O followeda clear sequence of increasing size downslope from shrubby multistemmedkrummholz individuals to bigger arborescent trees. At site O, regeneration wasconcentrated near the krummholz area and over rocky substrates. This suggeststhat krummholz may modify microenvironment conditions and increase seedlingsurvival. At site T, regeneration was abundant above the treeline where thecover of the dominant understory shrub (Rhododendronferrugineum) decreased. In both ecotones the diversity of plants washigher above the treeline than in the forest and decreased going downslopecoinciding with the increase of P. uncinata cover. Thereduction of plant diversity appeared above the current timberline. At site O,the decrease was steep and spatially heterogeneous what may be due in part tothe edaphic heterogeneity. At site T the change was abrupt though smaller. Therelationships between the plant community and tree regeneration should be takeninto account in future ecological studies of treeline pattern.     

Genetic heterogeneity among intertidal habitats in the flat periwinkle, Littorina obtusata

Comparisons among patterns exhibited by functionally distinct genetic markers have been widely used to infer the impacts of demography and selection in structuring genetic variation in natural populations. However, such multilocus comparisons remain an indirect evaluation of selection at particular candidate loci; ideally, the identification of a candidate gene by comparative genetic methodologies should be complemented by functional analyses and experimental manipulations of genotypes in the laboratory or field. We examined genotype frequency variation among replicated intertidal habitats at two spatial scales in the grazing snail Littorina obtusata. Both of the candidate allozyme markers varied predictably with environment, and these patterns were consistent at both spatial scales. Three of four reference loci were spatially homogeneous, but one microsatellite exhibited significant structure at both geographical and mesoscales. To initiate a direct examination of whether the observed genotype frequency variation at one of the candidate markers, mannose-6-phosphate isomerase (MPI), was impacted by differential survivorship of genotypes, we conducted a series of laboratory-based thermal stress assays using snails from two geographically disparate source populations. When snails were exposed to bouts of thermal/desiccation stress, patterns of mortality were nonrandom with respect to MPI genotype. Furthermore, patterns of mortality in the laboratory manipulation coincided with the observed distribution of genotypes in the field. The data suggest the operation of selection at the Mpi or a linked locus, but functional studies and further experimentation are required to establish the relationship between MPI genotype and fitness across heterogeneous intertidal environments.     

Divergent trophic levels in two cryptic sibling bat species

Changes in dietary preferences in animal species play a pivotal role in niche specialization. Here, we investigate how divergence of foraging behaviour affects the trophic position of animals and thereby their role for ecosystem processes. As a model, we used two closely related bat species, Myotis myotis and M. blythii oxygnathus, that are morphologically very similar and share the same roosts, but show clear behavioural divergence in habitat selection and foraging. Based on previous dietary studies on synanthropic populations in Central Europe, we hypothesised that M. myotis would mainly prey on predatory arthropods (i.e., secondary consumers) while M. blythii oxygnathus would eat herbivorous insects (i.e., primary consumers). We thus expected that the sibling bats would be at different trophic levels. We first conducted a validation experiment with captive bats in the laboratory and measured isotopic discrimination, i.e., the stepwise enrichment of heavy in relation to light isotopes between consumer and diet, in insectivorous bats for the first time. We then tested our trophic level hypothesis in the field at an ancient site of natural coexistence for the two species (Bulgaria, south-eastern Europe) using stable isotope analyses. As predicted, secondary consumer arthropods (carabid beetles; Coleoptera) were more enriched in ¹⁵N than primary consumer arthropods (tettigoniids; Orthoptera), and accordingly wing tissue of M. myotis was more enriched in ¹⁵N than tissue of M. blythii oxygnathus. According to a Bayesian mixing model, M. blythii oxygnathus indeed fed almost exclusively on primary consumers (98%), while M. myotis ate a mix of secondary (50%), but also, and to a considerable extent, primary consumers (50%). Our study highlights that morphologically almost identical, sympatric sibling species may forage at divergent trophic levels, and, thus may have different effects on ecosystem processes.     

Local distribution and thermal ecology of two intertidal fishes

Geographic variability in the physiological attributes of widely distributed species can be a result of phenotypic plasticity or can reflect evolutionary responses to a particular habitat. In the field, we assessed thermal variability in low and high intertidal pools and the distribution of resident fish species Scartichthys viridis and transitory Girella laevifrons along this vertical intertidal gradient at three localities along the Chilean coast: Antofagasta (the northernmost and warmest habitat), Carrizal Bajo (central coast) and Las Cruces (the southernmost and coldest habitat). In the laboratory, we evaluated the thermal sensitivity of fish captured from each locality. The response to temperature was estimated as the frequency of opercular movements and as thermal selectivity in a gradient; the former being a indirect indicator of energy costs in a particular environment and the latter revealing differential occupation of habitat. Seawater temperature in intertidal pools was greatest at Antofagasta, and within each site was greatest in high intertidal pools. The two intertidal fish species showed opposite patterns of local distribution, with S. viridis primarily inhabiting the lower sectors of the intertidal zone, and G. laevifrons occupying the higher sectors of the intertidal zone. This pattern was consistent for all three localities. Locality was found to be a very important factor determining the frequency of opercular movement and thermal selectivity of both S. viridis and G. laevifrons. Our results suggest that S. viridis and G. laevifrons respond according to: (1) the thermal history of the habitat from which they came, and (2) the immediate physical conditions of their habitat. These results suggest local adaptation rather than plasticity in thermoregulatory and energetic mechanisms.     

Conservation of locus-specific microsatellite variability across species: a comparison of two Drosophila sibling species, D. melanogaster and D. simulans

Fifteen microsatellite loci were studied in Drosophila melanogaster andDrosophila simulans, two closely related sibling species which split 2- 3.5MYA. Within-species variances in repeat number were found to differ up to1,000-fold among individual microsatellite loci. A significant correlationof log variances between both species indicated a locus- specific mutationrate of microsatellites. Hence, locus-specific effects are apparently amongthe major forces influencing microsatellite variation and deserve moreconsideration in microsatellite analysis.     

The distribution of B chromosomes across species

In this review we look at the broad picture of how B chromosomes are distributed across a wide range of species. We review recent studies of the factors associated with the presence of Bs across species, and provide new analyses with updated data and additional variables. The major obstacle facing comparative studies of B chromosome distribution is variation among species in the intensity of cytogenetic study. Because Bs are, by definition, not present in all individuals of a species, they may often be overlooked in species that are rarely studied. We give examples of corrections for differences in study effort, and show that after a variety of such corrections, strong correlations remain. Several major biological factors are associated with the presence of B chromosomes. Among flowering plants, Bs are more likely to occur in outcrossing than in inbred species, and their presence is also positively correlated with genome size and negatively with chromosome number. They are no more frequent in polyploids than in diploids, nor in species with multiple ploidies. Among mammals, Bs are more likely to occur in species with karyotypes consisting of mostly acrocentric chromosomes. We find no evidence for an association with chromosome number or genome size in mammals, although the sample for genome size is small. The associations with breeding system and acrocentric chromosomes were both predicted in advance, but those with genome size and chromosome number were discovered empirically and we can offer only tentative explanations for the very strong associations we have uncovered. Our understanding of why B chromosomes are present in some species and absent in others is still in its infancy, and we suggest several potential avenues for future research.     

Inferring species abundance distribution across spatial scales

A long-standing problem in ecology is to understand how the species–abundance distribution (SAD) varies with sampling scale. The problem was first characterized by Preston as the veil line problem. Although theoretical and empirical studies have now shown the nonexistence of the veil line, this problem has generated much interest in scaling biodiversity patterns. However, research on scaling SAD has so far exclusively focused on the relationship between the SAD in a smaller sampling area and a known SAD assumed for a larger area. An unsolved challenge is how one may predict species–abundance distribution in a large area from that of a smaller area. Although upscaling biodiversity patterns (e.g. species–area curve) is a major focus of macroecological research, upscaling of SAD across scale is, with few exceptions, ignored in the literature. Methods that directly predict SAD in a larger area from that of a smaller area have just started being developed. Here we propose a Bayesian method that directly answers this question. Examples using empirical data from tropical forests of Malaysia and Panama are employed to demonstrate the use of the method and to examine its performance with increasing sampling area. The results indicate that only 10-15% of the total census area is needed to adequately predict species abundance distribution of a region. In addition to species abundance distributions, the method also predicts well the regional species richness.     

Differences in oviposition behaviour of two sympatric sibling species of the genus Ostrinia

Changes in host preferences are thought to be a major source of genetic divergence between phytophagous insect taxa. In western Europe, two sympatric taxa, O. nubilalis (the European corn borer) and O. scapulalis, feed mainly on maize and hop or mugwort, respectively. These two species may have diverged without geographic isolation after a host shift of ancestral populations onto maize or another cultivated species (e.g. sorghum). A previous study using inbred laboratory strains revealed that the two species differ in their oviposition choices in maize-mugwort tests. We sampled four natural populations in France (two of each taxon) and tested their oviposition behaviour toward four of their main host plant species: maize, sorghum, mugwort and hop. O. nubilalis females showed a very high preference for laying their eggmasses on maize, whereas O. scapulalis females displayed a more balanced range of preferences. O. nubilalis females were attracted slightly to sorghum, suggesting that this plant is an accidental, rather than a regular and ancestral host plant of O. nubilalis. One important result arising from this study is the significant proportion of eggs laid by both Ostrinia species on hop. This may explain why some stands of hop are sometimes not only infested by O. scapulalis but also by O. nubilalis larvae, a situation preventing assortative mating based on microallopatry. Hence, further studies must be conducted to see whether the host preference in the genus Ostrinia might be linked to assortative mating by a mechanism that is not mediated by the host plant.     

Antennal sensilla of two female anopheline sibling species with differing host ranges

Background

Malaria control strategies emphasize the need for prompt and effective treatment of malaria episodes. To increase treatment efficacy, Tanzania changed its first-line treatment from chloroquine to sulphadoxine-pyrimethamine (SP) in 2001. The effect of this policy change on the availability of antimalarials was studied in rural south-eastern Tanzania.

Methods

In 2001 and 2004, the study area was searched for commercial outlets selling drugs and their stocks were recorded. Household information was obtained from the local Demographic Surveillance System.

Results

From 2001 to 2004, the number of general shops stocking drugs increased by 15% and the number of drug stores nearly doubled. However, the proportion of general shops stocking antimalarials dropped markedly, resulting in an almost 50% decrease of antimalarial selling outlets. This led to more households being located farther from a treatment source. In 2004, five out of 25 studied villages with a total population of 13,506 (18%) had neither a health facility, nor a shop as source of malaria treatment.

Conclusion

While the change to SP resulted in a higher treatment efficacy, it also led to a decreased antimalarial availability in the study area. Although there was no apparent impact on overall antimalarial use, the decline in access may have disproportionately affected the poorest and most remote groups. In view of the imminent policy change to artemisinin-based combination therapy these issues need to be addressed urgently if the benefits of this new class of antimalarials are to be extended to the whole population.     

Phenotypic homogeneity of two intertidal snails across a wave exposure gradient in South Australia

Dislodgement by the large drag forces imparted by breaking waves is an important cause of mortality for intertidal snails. The risk of drag-induced dislodgement can be reduced with: (1) a smaller shell of lower maximum projected surface area (MPSA); (2) a streamlined shell shape characterized by a squatter shell; and/or (3) greater adhesive strength attained through a larger foot area or increased foot tenacity. Snails on exposed coasts tend to express traits that increase dislodgement resistance. Such habitat-specific differences could result from direct selection against poorly adapted phenotypes on exposed shores but may reflect gastropod adaptation to high wave action achieved through phenotypic plasticity or genetic polymorphism. With this in mind, we examined the size, shape and adhesive strength of populations of two gastropod species, Austrocochlea constricta (Lamarck) and Nerita atramentosa (Reeve), from two adjacent shores representing extremes in wave exposure. Over a 5 day period, maximum wave forces were more than 10 times greater on the exposed than sheltered shore. Size-frequency distributions indicate that a predator consuming snails within the 1.3-1.8 cm length range regulates sheltered shore populations of both snail species. Although morphological scaling considerations suggest that drag forces should not place physical limits on the size of these gastropods, exposed shore populations of both snails were small relative to the maximum size documented for these species. Therefore, selective forces at the exposed site might favour smaller individuals with increased access to microhabitat refuges. Unexpectedly, however, neither snail species exhibited between-shore differences in shape, foot area or foot tenacity, which are likely to have adaptive explanations. Hence, it is possible that these snails are incapable of adaptive developmental responses to high wave action. Instead, the homogeneous and wave-exposed nature of Australia's southern coastline may have favoured the evolution of generalist strategies in these species.     

Abrupt evolutionary change in the Balbiani ring gene family in two sibling species ofChironomus

Summary We have investigated interspecific divergence in a eucaryotic gene family, the Balbiani ring genes of the dipteran genusChironomus. In general, our data show thatC. pallidivittatus andC. tentans possess virtually identical Balbiani ring gene sequences, whereasC. thummi represents a more distantly related member of the genus. These results are consistent with phylogenetic relationships for the three species based on polytene chromosome banding patterns. However, one Balbiani ring gene sequence displays an evolutionarily anomalous behavior. A 16-kilobase-pair Balbiani ring gene 1 sequence inC. pallidivittatus that we show to be transcribed is absent in the sibling speciesC. tentans. We propose that this sequence was deleted inC. tentans after divergence of the species. Considering the importance of Balbiani ring genes for feeding and protection of theChironomus larvae, we suggest that the pronounced interspecific genetic difference contributes to a difference in ecological niche between the two sympatrically distributed species.     

Sexual maturity and intraspecific mating success of two sibling species of the Bactrocera dorsalis complex

Bactrocera carambolae Drew & Hancock and B. papayae Drew & Hancock (Diptera: Tephritidae), two closely related sibling species in the B. dorsalis (Hendel) complex were shown to have different rates of sexual maturity. The response of B. carambolae males to methyl eugenol was observed to begin 10 days after emergence and increased with age. The attractancy peaked at 28 days after emergence and above. The males were also found to require a higher dosage of methyl eugenol (1 mg) for optimal response. These results are in contrast with those of B. papayae reported previously. The response to methyl eugenol was also observed to correspond with the age when first mating was performed.After eclosion, both sexes of B. carambolae mated approximately 2 weeks later and showed significantly lower matings/individual when compared with B. papayae in a 60-day observation period. Both sexes of B. papayae and males of B. carambolae were able to remate on the next day. However, B. carambolae females required a minimum refractory period of 8 days in order to be sexually receptive again.During the 60-day period, the mating propensity in B. papayae (100% success) was higher than B. carambolae (71–87% success) (P < 0.05). This observation was substantiated by a higher intraspecific mating success of B. papayae as compared to B. carambolae in a field cage (P < 0.001).     

Population dynamics of species with gynogenetic sibling species

Models are formulated for the population dynamics of a monoecious or dioecious species with an all-female parthenogenetic sibling species which is also gynogenetic. Continuous, deterministic reproduction and mortality, a stationary age distribution, random mating, and limited sexual competence for all individuals are posited. It is also supposed that in the dioecious case males do not distinguish between true and gynogenetic females. Similarly, hermaphrodites do not differentiate hermaphrodites and gynogens. The model implies that extinction is highly likely in the dioecious situation, but much less so in the monoecious one. Empirical evidence is reviewed and related to the assumptions and conclusions.     

Reduced fertilization constitutes an important prezygotic reproductive barrier between two sibling species of the hybridizing Brachionus calyciflorus species complex

Hydrobiologia - Brachionus calyciflorus is a species complex consisting of four recently described species. Although several lines of evidence support their species status, hybridization between...     

Ancestry analysis indicates two different sets of essential genes in eukaryotic model species

Essential genes are so-called because they are crucial for organism perpetuation. Those genes are usually related to essential functions to cellular metabolism or multicellular homeostasis. Deleterious alterations on essential genes produce a spectrum of phenotypes in multicellular organisms. The effects range from the impairment of the fertilization process, disruption of fetal development, to loss of reproductive capacity. Essential genes are described as more evolutionarily conserved than non-essential genes. However, there is no consensus about the relationship between gene essentiality and gene age. Here, we identified essential genes in five model eukaryotic species (Saccharomyces cerevisiae, Schizosaccharomyces pombe, Drosophila melanogaster, Caenorhabditis elegans, and Mus musculus) and estimate their evolutionary ancestry and their network properties. We observed that essential genes, on average, are older than other genes in all species investigated. The relationship of network properties and gene essentiality convey with previous findings, showing essential genes as important nodes in biological networks. As expected, we also observed that essential orthologs shared by the five species evaluated here are old. However, all the species evaluated here have a specific set of young essential genes not shared among them. Additionally, these two groups of essential genes are involved with distinct biological functions, suggesting two sets of essential genes: (i) a set of old essential genes common to all the evaluated species, regulating basic cellular functions, and (ii) a set of young essential genes exclusive to each species, which perform specific essential functions in each species.

     

Contrasting population makeup of two intertidal gastropod species that differ in dispersal opportunities

This study of population structure of two intertidal monodontine species: Diloma nigerrima and Diloma subrostrata, revealed the level of genetic connectedness among populations. Despite their markedly different geographic ranges (D. nigerrima is a geographically widespread species, inhabiting both Chile and New Zealand, including its subantarctic islands, whereas D. subrostrata is endemic to New Zealand), both species are believed to possess short-lived lecithotrophic larval stages. Polymorphic DNA microsatellite sequences were used to reveal the level of genetic connectedness among populations, thus inferring the two species' relative effective dispersal abilities. For each species F statistics, AMOVA values and the strength of the relationship between geographic and genetic distance were calculated. We observed a higher within-species level of genetic variation (Φ_ST = 0.099 vs. Φ_ST = 0.016) and a higher proportion of variance (11.15% vs. 0%) among populations of D. nigerrima than of D. subrostrata. A larger fraction of significant F_ST values was observed among D. nigerrima population pairs (65%) than among D. subrostrata population pairs (33%). Significant correlation between genetic and geographic distance was observed for D. nigerrima but not for D. subrostrata, but this relationship was not consistent among pairwise D. nigerrima population comparisons and PCA analysis confirms that, for each species, population structure does not follow a consistent pattern of increasing with geographic distance. The lack of population structure among D. subrostrata populations is probably due to its ubiquitous distribution, meaning little opportunity exists for genetic structure. D. nigerrima, by contrast has a patchier distribution, which allows for greater opportunities for genetic differentiation to occur. We argue that, despite the probable short larval stage in this species, the lack of geographical pattern in the genetic structure found in D. nigerrima is best explained by occasional dispersal over relatively short distances around the coast of New Zealand, over longer distances from New Zealand to the subantarctic islands and even across the Pacific Ocean from New Zealand to Chile.     

Regular patterns for proteome-wide distribution of protein abundance across species

A proteome of the bio-entity, including cell, tissue, organ, and organism, consists of proteins of diverse abundance. The principle that determines the abundance of different proteins in a proteome is of fundamental significance for an understanding of the building blocks of the bio-entity. Here, we report three regular patterns in the proteome-wide distribution of protein abundance across species such as human, mouse, fly, worm, yeast, and bacteria: in most cases, protein abundance is positively correlated with the protein's origination time or sequence conservation during evolution; it is negatively correlated with the protein's domain number and positively correlated with domain coverage in protein structure, and the correlations became stronger during the course of evolution; protein abundance can be further stratified by the function of the protein, whereby proteins that act on material conversion and transportation (mass category) are more abundant than those that act on information modulation (information category). Thus, protein abundance is intrinsically related to the protein's inherent characters of evolution, structure, and function.     

Bacterial communities of two parthenogenetic aphid species cocolonizing two host plants across the Hawaiian Islands

Aphids (Hemiptera: Aphididae) have been the focus of several studies with respect to their interactions with inherited symbionts, but bacterial communities of most aphid species are still poorly characterized. In this research, we used bar-coded pyrosequencing to characterize bacterial communities in aphids. Specifically, we examined the diversity of bacteria in two obligately parthenogenetic aphid species (the melon aphid, Aphis gossypii, and the cardamom aphid, Pentalonia caladii) cocolonizing two plant species (taro, Colocasia esculenta, and ginger, Alpinia purpurata) across four Hawaiian Islands (Hawaii, Kauai, Maui, and Oahu). Results from this study revealed that heritable symbionts dominated the bacterial communities for both aphid species. The bacterial communities differed significantly between the two species, and A. gossypii harbored a more diverse bacterial community than P. caladii. The bacterial communities also differed across aphid populations sampled from the different islands; however, communities did not differ between aphids collected from the two host plants.