Bower‐building behaviour is associated with increased sperm longevity in Tanganyikan cichlids

We investigated the evolutionary relationship between spawning behaviour and sperm motility traits among Tanganyikan mouth‐brooding cichlid species that have developed diverse mating behaviours and male sexual traits. Mouth‐brooding behaviour is common among these fish, but different species demonstrate a range of spawning behaviours, bower construction, male sexual traits and timing of gamete release. We observed spawning behaviours and compared sperm motility traits of 28 Tanganyikan mouth‐brooding cichlids to elucidate the evolutionary correlations between these traits. Sperm longevity was considerably longer in bower‐building species that construct crater‐shaped spawning sites compared with species that do not build bowers. Male bower builders released sperm in the pit of the bower prior to spawning, and the time from ejaculation to fertilization was longer. Conversely, most mouth‐brooding cichlids deposited semen directly into the female buccal cavity, and spawned eggs were immediately picked up to be placed inside the cavity; thus, the time from ejaculation to fertilization was short. These observations suggest that increased sperm longevity is favoured in bower builders. Comparative phylogenetic analyses suggested that bower‐building behaviour and greater time from ejaculation to fertilization are associated with the extension of sperm longevity, whereas sperm competition rank does not play a major role. In addition, bower‐building behaviour preceded the emergence of increased sperm longevity. These results indicate that the extension of sperm longevity as a result of the emergence of bower builders may have acted as an evolutionary attractor for sperm longevity.     

Performance of non-motile male gametes in the sea: analysis of paternity and fertilization success in a natural population of a red seaweed,Gracilaria gracilis

In haploid–diploid red seaweeds, the dispersal of male gametes is presumed limited due to their lack of flagella. It has been suggested that this group suffers from sperm limitation and, consequently, that fertilization is relatively inefficient. Fertilization in most floridean rhodophytes results in the formation a cystocarp, a swelling on the haploid female thallus housing the diploid zygote and its thousands of diploid daughter spores. To study the performance of non-motile male gametes in the sea, we evaluated both female and male fertilization success in a natural population of the red marine alga Gracilaria gracilis. Female fertilization success, estimated by cystocarp yield per unit female thallus, was evaluated with respect to the availability of male gametes. Male fertilization success, estimated by the individual contribution of different males to zygotes, was assessed by paternity analyses on 350 cystocarps produced in one reproductive season using two microsatellite loci. The results show that cystocarp yield is not sperm limited and that the large variation in male fertilization success cannot be solely explained by the distance travelled by the male gamete to find a mate. Taken together, the results suggest that, not only is fertilization efficient, but that male–male competition and/or female choice may play a role in shaping population mating patterns.     

Gamete traits influence the variance in reproductive success, the intensity of sexual selection, and the outcome of sexual conflict among congeneric sea urchins

Sea-urchin species differ in susceptibility to sperm limitation and polyspermy, but the influences of gamete traits on reproductive variance, sexual selection, and sexual conflict are unknown. I compared male and female reproductive success of two congeners at natural densities in the sea. The eggs of the species occurring at higher densities, Strongylocentrotus purpuratus, require higher sperm concentrations for fertilization but are more resistant to polyspermy compared to S. franciscanus. Both species show high variance in male fertilization success at all densities and high variance in female success at low densities, but they differ in female variance at high densities, where only S. franciscanus shows high female variance. The intensity of sexual selection based on Bateman gradients is high in males of both species, variable in S. franciscanus females, and low in S. purpuratus females. Strongylocentrotus franciscanus females experience sexual selection at low densities and sexual conflict at high densities. Strongylocentrotus purpuratus may rarely experience sperm limitation and may have evolved to ameliorate sexual conflict. This reduces the variance in female fertilization, providing females with more control over fertilization. Sperm availability influences sexual selection directly by determining sperm-egg encounter probabilities and indirectly through selection on gamete traits that alter reproductive variances.     

From in vitro fertilization to early embryogenesis in maize

Summary The development of in vitro fertilization methods in plants, the characterization of developmental mutants, and the adaptation of molecular biology techniques to construct cDNA libraries from minute samples, all represent important recent technical break-throughs. They allow the study of fertilization and early embryogenesis at a molecular level and considerable improvement in the under-standing of higher plant reproduction can be predicted over the next few years. Important biological questions, such as polyspermy, gamete fusion physiology, asymmetrical cell division, embryo axis formation, can now be addressed experimentally in maize, which appears as a major study model in this area.Abbreviations PCR polymerase chain reaction - RFLP restriction fragment length polymorphism     

The evolution of gonad expenditure and gonadosomatic index (GSI) in male and female broadcast‐spawning invertebrates

Sedentary broadcast‐spawning marine invertebrates, which release both eggs and sperm into the water for fertilization, are of special interest for sexual selection studies. They provide unique insight into the early stages of the evolutionary succession leading to the often‐intense operation of both pre‐ and post‐mating sexual selection in mobile gonochorists. Since they are sessile or only weakly mobile, adults can interact only to a limited extent with other adults and with their own fertilized offspring. They are consequently subject mainly to selection on gamete production and gamete success, and so high gonad expenditure is expected in both sexes. We review literature on gonadosomatic index (GSI; the proportion of body tissue devoted to gamete production) of gonochoristic broadcast spawners, which we use as a proxy for gonad expenditure. We show that such taxa most often have a high GSI that is approximately equal in both sexes. When GSI is asymmetric, female GSI usually exceeds male GSI, at least in echinoderms (the majority of species recorded). Intriguingly, though, higher male GSI also occurs in some species and appears more common than female‐biased GSI in certain orders of gastropod molluscs. Our limited data also suggest that higher male GSI may be the prevalent pattern in sperm casters (where only males release gametes). We explore how selection might have shaped these patterns using game theoretic models for gonad expenditure that consider possible trade‐offs with (i) somatic maintenance or (ii) growth, while also considering sperm competition, sperm limitation, and polyspermy. Our models of the trade‐off between somatic tissue (which increases survival) and gonad (which increases reproductive success) predict that GSI should be equal for the two sexes when sperm competition is intense, as is probably common in broadcast spawners due to synchronous spawning in aggregations. Higher female GSI occurs under low sperm competition. Sperm limitation appears unlikely to alter these conclusions qualitatively, but can also act as a force to keep male GSI high, and close to that of females. Polyspermy can act to reduce male GSI. Higher male than female GSI is predicted to be less common (as observed in the data), but can occur when ova/ovaries are sufficiently more resource‐intensive to produce than sperm/testes, for which some evidence exists. We also show that sex‐specific trade‐offs between gonads and growth can generate different life‐history strategies for males and females, with males beginning reproduction earlier. This could lead to apparently higher male GSI in empirical studies if immature females are included in calculations of mean GSI. The existence of higher male GSI nonetheless remains somewhat problematic and requires further investigation. When sperm limitation is low, we suggest that the natural logarithm of the male/female GSI ratio may be a suitable index for sperm competition level in broadcast spawners, and that this may also be considered as an index for internally fertilizing taxa.     

Polyspermy barriers in plants: from preventing to promoting fertilization

Barriers to polyspermy (fertilization of a female gamete by more than one sperm) are essential to successful reproduction in a wide range of organisms including mammals, echinoderms, fish, molluscs, and algae. In animals and fucoid algae, polyspermy results in early death of the zygote due to transmission of extra centrioles from the sperm and consequent disruptions to the mitotic spindle. Accordingly, a variety of mechanisms have evolved to prevent penetration of an egg by more than one sperm, or more than one sperm nucleus from fusing with an egg nucleus. The evolution of internal fertilization has also provided an opportunity to limit the number of sperm that gain access to each egg, as occurs in the mammalian female reproductive tract. Polyspermy and polyspermy barriers in plants have received much less attention. Plants lack centrioles and therefore, polyspermy would not be expected to cause lethal aberrant spindle organization. However, we find evidence from cytological, genetic and in vitro fertilization studies for polyspermy barriers in plants. Angiosperms, like mammals, are internally fertilized, and exert a high level of control over the number of sperm that have access to each female gamete. In particular, regulation of pollen tube growth ensures that in general only two sperm enter each embryo sac, where one fertilizes the egg and the other the central cell. Despite this 1:1 ratio of sperm to gametes within the embryo sac, angiosperms still require a mechanism to ensure that each female gamete is fertilized by one and only one sperm. Here, we present evidence suggesting that a polyspermy block on the egg may be part of the mechanism that promotes faithful double fertilization.     

THE QUICK AND THE DEAD? SPERM COMPETITION AND SEXUAL CONFLICT IN SEA

Our view of sperm competition is largely shaped by game-theoretic models based on external fertilizers. External fertilization is of particular interest as it is the ancestral mode of reproduction and as such, relevant to the evolution and maintenance of anisogamy (i.e., large eggs and tiny, numerous sperm). Current game-theoretic models have been invaluable in generating predictions of male responses to sperm competition in a range of internal fertilizers but these models are less relevant to marine broadcast spawners, the most common and archetypal external fertilizers. Broadcast spawners typically have incomplete fertilization due to sperm limitation and/or polyspermy (too many sperm), but the effects of incomplete (<100% fertilization rates) fertilization on game-theoretic predictions are unclear particular with regards to polyspermy. We show that incorporating the effects of sperm concentration on fertilization success changes the predictions of a classic game-theoretic model, dramatically reversing the relationship between sperm competition and the evolutionarily stable sperm release strategy. Furthermore, our results suggest that male and female broadcast spawners are likely to be in conflict at both ends of the sperm environment continuum rather than only in conditions of excess sperm as previously thought. Across the majority of the parameter space we explored, males release either too little to too much sperm for females to achieve complete fertilization. This conflict could result in a coevolutionary race that may have led to the evolution of internal fertilization in marine organisms.     

High fertilization success in a surface-brooding Caribbean gorgonian

Colonies of the Caribbean gorgonian Pseudopterogorgia elisabethae release eggs that are retained on the colony surface where they are fertilized and then develop. In December 2001, spawning on San Salvador Island, Bahamas, occurred over 6 d, with spawning by any one colony limited to 1-3 d. With the exception of the first and last days of the spawning period, fertilization success was high, often greater than 90%. Eggs collected in December 2001 had an overall fertilization success of more than 66%. At one site, the increase in fertilization after the first day of spawning correlated with male spawning, but male gonad index was a poor predictor of fertilization success. The number of male colonies close to a female was not correlated with fertilization success. Surface brooding is an efficient mechanism for "harvesting" sperm released upstream of female colonies. By maintaining their eggs at a single location, surface-brooding species can extend the period over which eggs are likely to encounter sperm. As a result, fertilization success is summed across the temporal variance in sperm availability, and the need for very high densities of sperm, with its concomitant risk of polyspermy, may be reduced.     

禾本科三倍体: 形成、鉴定与利用

禾本科三倍体的形成途径包括2n配子融合、倍性间杂交、多精受精和胚乳培养。其中, 2n配子融合和倍性间杂交分别为自然界和人工合成三倍体的主要途径。该文介绍了形态学观测、染色体分析、流式细胞术和分子标记等倍性鉴定方法在禾本科三倍体中的应用及其优缺点。目前, 三倍体在禾谷类作物中无直接应用价值, 但可作为通往多倍体、非整倍体和转移异源基因的遗传桥梁。多年生禾本科三倍体(特别是异源三倍体)在饲草或能源作物中已得到广泛应用, 在该类型禾本科作物中均可直接尝试三倍体育种。多倍体的三倍体育种和无融合生殖三倍体育种可作为未来禾本科三倍体的研究方向。三倍性胚乳培养可以一步合成三倍体, 多精受精可以实现遗传上3个不同基因组的一步融合, 在三倍体研究中应予以重视。鉴于2n配子融合、多精受精的稀有特性和倍性间杂交、胚乳培养频繁的染色体变异, 高通量三倍体鉴定技术的发展将是三倍体研究实现突破的关键。     

Climate change and disease: bleaching of a chemically defended seaweed

Disease is emerging as an important impact of global climate change, due to the effects of environmental change on host organisms and their pathogens. Climate‐mediated disease can have severe consequences in natural systems, particularly when ecosystem engineers, such as habitat‐formers or top predators are affected, as any impacts can cascade throughout entire food webs. In temperate marine ecosystems, seaweeds are the dominant habitat‐formers on rocky reefs. We investigated a putative bleaching disease affecting Delisea pulchra, a chemically defended seaweed that occurs within a global warming ‘hot‐spot’ and assessed how patterns of this phenomenon were influenced by ocean temperature, solar radiation, algal chemical defences and microbial pathogens. Warmer waters were consistently and positively correlated with higher frequencies of bleaching in seaweed populations, but patterns of bleaching were not consistently influenced by light levels. Bleached thalli had low levels of antibacterial chemical defences relative to healthy conspecifics and this was observed across entire thalli of partially bleached algae. Microbial communities associated with bleached algae were distinct from those on the surfaces of healthy seaweeds. Direct testing of the importance of algal chemical defences, done here for the first time in the field, demonstrated that they protected the seaweed from bleaching. Treatment of algal thalli with antibiotics reduced the severity of bleaching in experimental algae, especially at high water temperatures. These results indicate that bleaching in D. pulchra is the result of temperature‐mediated bacterial infections and highlight the potential for warming to influence disease dynamics by stressing hosts. Understanding the complex ways in which global change may affect important organisms such as habitat‐forming seaweeds, is essential for the management and conservation of natural resources.     

Exploring lake ecosystems: hierarchy responses to long‐term change?

Shifts in climate regime have provoked substantial trophic‐ and species‐dependent changes within ecosystems. With growing concerns of present global warming, we examined potential lake ecosystem responses, natural hierarchy responses (i.e. immediate responses at lower system levels as opposed to delayed responses at higher system levels), and possible shifts among abiotic (physics, nutrients) and biotic (phytoplankton, zooplankton) system components. Specifically, we analyzed decadal data collected from Müggelsee, a lake in Berlin, Germany, for climate‐induced abiotic and biotic changes, their timing and type, and classified them as abrupt permanent, gradual permanent, abrupt temporary, or monotonic. We further categorized variable changes as a function of system hierarchy, including lake physics (ice, temperature, stratification), nutrients (phosphorus, nitrogen, silicate), plankton, and levels of integration (i.e. species, taxonomic groups, and total plankton). Contrary to current theory, data suggest abrupt responses did not occur in a hierarchy‐dependent manner, nor was a clear pattern observed among functional system‐based categories. Abrupt permanent changes were the most prominent response pattern observed, suggesting they may be driven by large‐scale climatic oscillations and by surpassed thresholds, as noted in previous case studies. Gradual changes coincided with affected abiotic parameters spanning an expansive time range; for example, climatic effects in spring preceded changes in nutrient limitation. Variables displaying no long‐term changes pointed to compensation processes caused by, e.g., simultaneously acting forces of warming trends and climate‐independent changes in trophic state. Nevertheless, the complexity of response patterns at the single system level manifested clear chronological regime shifts in abiotic and biotic parameters in spring and, to a lesser extent, in summer. With regard to projected global warming, the majority of currently unaffected system levels may face impending thermal thresholds, achievement of which would result in an accelerated shift in ecosystem state.     

Abiotic stress tolerance and competition‐related traits underlie phylogenetic clustering in soil bacterial communities

Soil bacteria typically coexist with close relatives generating widespread phylogenetic clustering. This has been ascribed to the abiotic filtering of organisms with shared ecological tolerances. Recent theoretical developments suggest that competition can also explain the phylogenetic similarity of coexisting organisms by excluding large low‐competitive clades. We propose that combining the environmental patterns of traits associated with abiotic stress tolerances or competitive abilities with phylogeny and abundance data, can help discern between abiotic and biotic mechanisms underlying the coexistence of phylogenetically related bacteria. We applied this framework in a model system composed of interspersed habitats of highly contrasted productivity and comparatively dominated by biotic and abiotic processes, i.e. the plant patch‐gap mosaic typical of drylands. We examined the distribution of 15 traits and 3290 bacterial taxa in 28 plots. Communities showed a marked functional response to the environment. Conserved traits related to environmental stress tolerance (e.g. desiccation, formation of resistant structures) were differentially selected in either habitat, while competition related traits (e.g. organic C consumption, formation of nutrient‐scavenging structures) prevailed under high resource availability. Phylogenetic clustering was stronger in habitats dominated by biotic filtering, suggesting that competitive exclusion of large clades might underlie the ecological similarity of co‐occurring soil bacteria.     

Releasing small ejaculates slowly increases per‐gamete fertilization success in an external fertilizer: Galeolaria caespitosa (Polychaeta: Serpulidae)

The idea that male reproductive strategies evolve primarily in response to sperm competition is almost axiomatic in evolutionary biology. However, externally fertilizing species, especially broadcast spawners, represent a large and taxonomically diverse group that have long challenged predictions from sperm competition theory—broadcast spawning males often release sperm slowly, with weak resource‐dependent allocation to ejaculates despite massive investment in gonads. One possible explanation for these counter‐intuitive patterns is that male broadcast spawners experience strong natural selection from the external environment during sperm dispersal. Using a manipulative experiment, we examine how male reproductive success in the absence of sperm competition varies with ejaculate size and rate of sperm release, in the broadcast spawning marine invertebrate Galeolaria caespitosa (Polychaeta: Serpulidae). We find that the benefits of Fast or Slow sperm release depend strongly on ejaculate size, but also that the per‐gamete fertilization rate decreases precipitously with ejaculate size. Overall, these results suggest that, if males can facultatively adjust ejaculate size, they should slowly release small amounts of sperm. Recent theory for broadcast spawners predicts that sperm competition can also select for Slow release rates. Taken together, our results and theory suggest that selection often favours Slow ejaculate release rates whether males experience sperm competition or not.     

PATTERNS OF INTERSPECIFIC INTERACTIONS IN THE ULVA‐DOMINATED INTERTIDAL COMMUNITY IN A SOUTHERN COAST OF KOREA

The reproductive success of marine species with external fertilization depends on environmental conditions during gamete release. There is special interest presently in whether water motion causes sperm limitation under natural conditions. We investigated gamete release of Fucus vesiculosus from an exposed shore to ascertain: 1) when gametes are released during the tidal cycle, 2) when fertilization occurs, and 3) what the natural sperm:egg ratios are. Water samples were collected and concentrated over five minutes every half hour off Pemaquid Point, ME from three replicate sites within each of two locations using a pump‐filter device. Immunofluorescence microscopy revealed that gamete release occurred only on the two calmest spring tides. Sperm became present in the water column at the same time as oogonia (30 min^?1 h prior to high tide [HT]) and reached peak concentration at exactly HT. The sperm:egg ratio was 76:1 on 8 Oct 1999 and 21:1 on 8 Nov 1999 at exactly 30 min prior to HT and dropped sharply after HT. Gametes continued to be collected for several hours after HT but analysis of pronuclear position in aceto‐iron‐hematoxylin stained eggs revealed that all fertilization occurred at approximately HT. We modelled the total number of days when reproduction was possible using these results and wind and wave data from the National Data Buoy Center. Our research provides evidence that gamete release by F. vesiculosus occurs at slack HT on calm days and that sperm are not a limiting factor in fertilization for this species.     

137 Gamete Recognition and Sexual Isolation in a Red Alga,Aglaothamnion Byssoides (Rhdophyta)

The binding of fluorescein isothiocyanate (FITC) conjugated lectins to gametes of Aglaothamnion byssoides Itono during the fertilization was studied by the use of confocal microscope. The physiological effects of lectins and carbohydrates on gamete binding were also examined. Three lectins, concanavalin A (ConA), Soybean agglutinin (SBA) and wheat germ agglutinin (WGA) bound to the surface of spermatia, but each lectin labeled different region of the spermatium. SBA bound only to the spermatial appendages but ConA bound to the whole spermatial surface except spermatial appendages. WGA labeled narrow region which connects spermatial body and appendages. During fertilization, ConA and WGA specific substances on the spermatial surface moved towards the area contacting with trichogyne and accumulated on the surface of fertilization canal. Spermatial binding to trichogynes was inhibited by pre‐incubation of spermatia with SBA, while trichogyne receptors were blocked by the complementary carbohydrate, N‐acetyl‐D‐galactosamine. WGA and its complementary carbohydrate had little effect on gamete binding. For searching the step of sexual isolation, crossing experiment was performed between Aglaothamnion byssoides and twelve other red algal species. Results showed that the gamete recognition was genus‐specific: the gametes bound freely with their partners of the same genus. When two species from same genus were crossed, sexual isolation occurs gradually during the fertilization process. Therefore, sexual isolation in red algae appears to be determined by multi‐step process and gamete binding is the initial step.     

REPRODUCTIVE SUCCESS OF FUCUS VESICULOSUS (PHAEOPHYCEAE) IN THE BALTIC SEA

Marine organisms colonizing brackish habitats such as the Baltic Sea must cope with the negative effects of low salinities on reproductive success because these may reduce gamete viability and/or increase polyspermy. Reproductive characteristics of the marine seaweed Fucus vesiculosus L. were studied in several brackish habitats, particularly in the northern Baltic Sea, to understand its ability to reproduce where few other marine species survive. Polyspermy and fertilization success were variable at the boundary of the continuous distribution of F. vesiculosus in the Baltic Sea, and polyspermy was high (10%–30%) when fertilization was successful. A strong female bias (80%–86%, ca. 5.5:1) was found at the northernmost limit of Baltic F. vesiculosus. Electrophysiological studies showed that many eggs have a high input resistance (519 ± 150 MΩ[mean ± SE, n = 14] at Drivan, 1995), which may be helpful in preventing polyspermy in this brackish habitat. The polyspermy block remains sodium-dependent in the northern Baltic. Sperm bound quickly to northern Baltic eggs in natural water, but fertilization was delayed compared to marine F. vesiculosus. A subset of northern Baltic eggs studied during an optimal reproductive period (7–11 July 1995) had a membrane potential (E_m) of ca. −100 mV and an effective fertilization potential (FP) of ca. 2 min with a plateau of −25 mV, but repolarized too rapidly for the FP to be protective. Pronuclear migration and cell wall secretion occurred more slowly in Baltic than in marine zygotes. The reproductive success of theseboundary populations may be dependent upon windows of opportunity when there are favorable combinations of the levels of salinity, water motion, population density, and sex ratio. These factors and the short duration of the reproductive season in the northern Baltic Sea may result in reproductive failure in some years.     

Eyespot behavior during the fertilization of gametes in Ulva arasakii Chihara (Ulvophyceae, Chlorophyta)

Behavior of the eyespots during the fertilization of Ulva arasakii Chihara was studied using field emission scanning electron microscopy (FE‐SEM). FE‐SEM enabled the visualization of the eyespot of biflagellate male and female gametes. The smaller male gamete has one protruded smaller (1.3 ± 0.15 μm× 1.0 ± 0.29 μm) eyespot and the larger female gamete has a larger (1.6 ± 0.2 μm× 1.1 ± 0.13 μm) one on a posterior position of the cell. The cell membrane over the eyespot region is relatively smooth compared to other parts of the cell body and exhibits hexagonal arranged lipid globules. Because the size of the cell and the morphology of the eyespot are different between male and female gametes, we could follow the fate of the eyespots during the fertilization. The initial cytoplasmic contact and fusion of the gametes takes place at their anterior end, slightly posterior to the flagellar base. The morphology of the fusing gametes followed two clearly distinguishable patterns. About half the gamete pairs lie side‐by‐side with their longitudinal axes nearly parallel, while the rest are oriented anti‐parallel to each other. In all cases, the larger female gamete fused along the same side as the eyespot, while the smaller male gamete fused along the side away from its eyespot. As fusion proceeds, the gamete pair is transformed into the quadriflagellate planozygote, in which the eyespots are positioned side‐by‐side on the region of cell fusion. These observations indicated that the opposite positioning of the eyespot relative to the cell fusion site in male and female gametes is important for the proper arrangement of the eyespots in the planozygote. The significance of this feature in advanced green algae is briefly discussed.     

Preventing polyspermy in mammalian eggs—Contributions of the membrane block and other mechanisms

The egg's blocks to polyspermy (fertilization of an egg by more than one sperm) were originally identified in marine and aquatic species with external fertilization, but polyspermy matters in mammalian reproduction too. Embryonic triploidy is a noteworthy event associated with pregnancy complications and loss. Polyspermy is a major cause of triploidy with up to 80% of triploid conceptuses being the result of dispermic fertilization. The mammalian female reproductive tract regulates the number of sperm that reach the site of fertilization, but mammals also utilize egg‐based blocks to polyspermy. The egg‐based blocks occur on the mammalian egg coat (the zona pellucida) and the egg plasma membrane, with apparent variation between different mammalian species regarding the extent to which one or both are used. The zona pellucida block to polyspermy has some similarities to the slow block in water‐dwelling species, but the mammalian membrane block to polyspermy differs substantially from the fast electrical block that has been characterized in marine and aquatic species. This review discusses what is known about the incidence of polyspermy in mammals and about the mammalian membrane block to polyspermy, as well as notes some lesser‐characterized potential mechanisms contributing to polyspermy prevention in mammals.     

Missing saiga on the taiga

Conservation biologists understand that linking demographic histories of species at risk with causal biotic and abiotic events should help us predict the effects of ongoing biotic and abiotic change. In parallel, researchers have started to use ancient genetic information (aDNA) to explore the demographic histories of a number of species present in the Pleistocene fossil record (see, e.g. Shapiro et al. 2004). However, aDNA studies have primarily focused on identifying long-term population trends, linked to climate variability and the role of early human activity. Population trends over more recent time, e.g. during the Holocene, have been poorly explored, partly owing to analytical limitations. In this issue, Campos et al. (2010a) highlight the potential of aDNA to investigate demographic patterns over such recent time periods for the compelling and endangered saiga antelope Saiga tatarica (Fig. 1). The time may come when past and current demography can be combined to produce a seamless record. [Figure: see text].     

Leaf litter species identity alters the structure of pond communities

The input of leaf litter resources is a major driver of ecosystem processes in terrestrial and freshwater habitats. Although variation exists in the quantity and composition of litter inputs due to natural and anthropogenic causes, few studies have examined how such variation influences the structure and composition of aquatic food webs. Using outdoor mesocosms, we examined the bottom–up effects of 10 chemically distinct tree litter species on microbial, algal, invertebrate and vertebrate fauna found in temperate ponds. We hypothesized that individual litter species, which differ in their traits, would differentially and predictably affect abiotic and biotic elements of pond communities. We further hypothesized that the presence of leaf litter, regardless of species, would elevate resource supply and increase the biomass of community members. Finally, we hypothesized that a mixture of litter species would have non‐additive effects on community responses. We followed the system for > 4 months and measured > 30 abiotic and biotic responses related to primary and secondary production. The different species of leaf litter had major effects on abiotic and biotic responses, including phytoplankton, periphyton, zooplankton, snails, amphipods and tadpoles. Most biological responses were negatively associated with soluble carbon content of litter, or litter decay rate. Other litter traits, including phenolic concentrations and litter C:N were of secondary importance but did exhibit both positive and negative associations with several responses. The absence of litter had pervasive effects on abiotic attributes, but did not promote substantial changes in organism biomass. Most responses to the litter mixture were additive. Our results suggest that changes in temperate forest composition can strongly affect pond communities.