Biometric sex discrimination is unreliable when sexual dimorphism varies within and between years: an example in Eurasian Oystercatchers Haematopus ostralegus

Molecular sexing of birds has been possible for over a decade, but for practical reasons many studies still use biometric data for sex discrimination. In some species, the sexes are easy to distinguish but sexual dimorphism is often more subtle, requiring the use of statistical analyses of biometric measurements to discriminate sexes. These models are usually parameterized and validated using data from a limited number of sites and years. However, the resulting discriminant functions are often applied to other populations and periods. A crucial, but usually untested, assumption is that sexual dimorphism does not vary in time and space. Here we illustrate the consequences of violation of this assumption in Eurasian Oystercatchers Haematopus ostralegus , a species for which most studies have relied on biometric sexing. Using biometric data from captures of known-sex birds, we show that sexual dimorphism varied substantially in time and even reversed in some months and years. Furthermore, some biometric traits used in sexing changed gradually over time, causing a reduction in sexual dimorphism. We show that the consequences of this variation on sex discrimination in Oystercatchers are subtle and easily overlooked, but can result in inaccurate and strongly male- or female-biased sex-ratio estimates. We recommend that biometric sexing should be avoided in Oystercatchers unless specific calibration for each month, year and area is carried out. This recommendation also applies to other species where biometric traits may depend on environmental conditions. We argue that this condition might apply to many bird species and therefore advise caution when interpreting results based on biometric sex discrimination.     

Herbivory in omnivorous fishes: effect of plant secondary metabolites and prey stoichiometry

1. Many animals that consume freshwater macrophytes are omnivorous (i.e., they include both plant and animal matter in their diet). For invertebrate omnivorous consumers, selection of macrophyte species depends partly on the presence of secondary metabolites in plants, plant carbon/nutrient balances and/or physical structure of plants. However, little is known about the mechanisms influencing consumption of macrophytes in aquatic vertebrates. 2. For two fish species, the omnivorous rudd (Scardinius erythrophthalmus) and herbivorous grass carp (Ctenopharyngodon idella), feeding preferences were determined in three choice experiments. We tested (i) whether the presence of secondary metabolites and macrophyte stoichiometry affects macrophyte species selection by fish, (ii) the importance of macrophyte stoichiometry by manipulating the macrophytes experimentally and (iii) the rate of herbivory when the most palatable macrophyte is offered simultaneously with a common animal prey. 3. In a choice experiment with five species of submerged macrophytes (Callitriche sp., Chara globularis, Elodea nuttallii, Myriophyllum spicatum and Potamogeton pectinatus), Myriophyllum was clearly consumed least by both fishes, which strongly correlated with the highest phenolic concentration of this macrophyte. Additionally, a significant negative relationship was found between consumption and C : N ratio of the five macrophytes. The two most consumed macrophytes also had the lowest dry matter concentration (DMC). 4. In a second choice experiment, the C : N ratio of the least (Myriophyllum) and most (Potamogeton) palatable plants was manipulated by growing the macrophytes under fertilised and unfertilised conditions and subsequently feeding them to rudd. The avoidance of consumption of the chemically defended Myriophyllum by rudd was partly alleviated by the lowered C : N ratio. 5. The third choice experiment showed that both fishes preferred animal prey (the amphipod Gammarus pulex) over the most palatable macrophyte (Potamogeton) when offered simultaneously. The C : N ratio of the amphipods was about half that of the lowest C : N ratio measured in the macrophytes. Consumption by the fishes could not clearly be related to C : P or N : P ratios of prey items in any of the experiments. 6. We conclude that omnivorous fish avoid macrophytes that are chemically defended. However, when these defences are only minor, stoichiometry (C : N ratio) in combination with DMC may be a determining factor for consumption by vertebrate facultative herbivores.     

The mixed xylem sap proteome of Fusarium oxysporum-infected tomato plants

Secreted proteins are known to play decisive roles in plant–fungus interactions. To study the molecular details of the interaction between the xylem-colonizing, plant-pathogenic fungus Fusarium oxysporum and tomato, the composition of the xylem sap proteome of infected tomato plants was investigated and compared with that of healthy plants. Two-dimensional gel separation and mass spectrometry yielded peptide masses and peptide sequences of 33 different proteins. Despite the absence of complete genome sequences of either tomato or F. oxysporum , 21 proteins were identified as tomato proteins and seven as fungal proteins. Thirteen of the tomato proteins were specific for infected plants. Sixteen tomato proteins were found in xylem sap for the first time, four of which were identified based on matches to expressed sequences only. Coding sequences for new proteins from F. oxysporum were identified through either direct matching to a database sequence, matching of peptide sequences to genome or expressed sequence tag databases of other Fusarium species, or PCR with degenerate primers on cDNA derived from infected plants followed by screening of a F. oxysporum BAC library. Together, these findings provide an excellent basis for further exploration of the interaction between xylem-colonizing pathogens and their hosts.     

Poa annua shows inter-generational differences in response to elevated CO2

Inter-generational effects on the growth of Poa annua (L.) in ambient and elevated atmospheric CO₂ conditions (350 and 550 μl l^–1, respectively) were studied in two different experiments. Both experiments showed similar results. In a greenhouse experiment growth, measured as the numbers of tillers produced per week, was compared for plants grown from first and second generation seeds. Second generation seeds were obtained from plants grown for one whole generation in either ambient or elevated atmospheric CO₂ (‘ambient’ and ‘elevated’ seeds, respectively). First generation plants and second generation ‘ambient’ plants did not respond to elevated CO₂. Second generation ‘elevated’ plants produced significantly more tillers in elevated CO₂. In the second experiment model terrestrial ecosystems growing in the Ecotron and which included Poa annua were used. Above-ground biomass after one and two generations of growth were compared. At the end of Generation 1 no difference was found in biomass production while at the end of Generation 2 biomass increased in elevated CO₂ by 50%. The implications for climate change research are discussed.     

MHC‐dependent survival in a wild population: evidence for hidden genetic benefits gained through extra‐pair fertilizations

Females should prefer to be fertilized by males that increase the genetic quality of their offspring. In vertebrates, genes of the major histocompatibility complex (MHC) play a key role in the acquired immune response and have been shown to affect mating preferences. They are therefore important candidates for the link between mate choice and indirect genetic benefits. Higher MHC diversity may be advantageous because this allows a wider range of pathogens to be detected and combated. Furthermore, individuals harbouring rare MHC alleles might better resist pathogen variants that have evolved to evade common MHC alleles. In the Seychelles warbler, females paired with low MHC‐diversity males elevate the MHC diversity of their offspring to levels comparable to the population mean by gaining extra‐pair fertilizations. Here, we investigate whether increased MHC diversity results in higher life expectancy and whether there are any additional benefits of extra‐pair fertilizations. Our 10‐year study found a positive association between MHC diversity and juvenile survival, but no additional survival advantage of extra‐pair fertilizations. In addition, offspring with a specific allele (Ase‐ua4) had a fivefold longer life expectancy than offspring without this allele. Consequently, the interacting effects of sexual selection and pathogen‐mediated viability selection appear to be important for maintaining MHC variation in the Seychelles warbler. Our study supports the prediction that MHC‐dependent extra‐pair fertilizations result in genetic benefits for offspring in natural populations. However, such genetic benefits might be hidden and not necessarily apparent in the widely used fitness comparison of extra‐ and within‐pair offspring.     

Histochemical and genetic analysis of host and non-host interactions of Arabidopsis with three Botrytis species: an important role for cell death control

Susceptibility was evaluated of host and non-host plants to three pathogenic Botrytis species: the generalist B. cinerea and the specialists B. elliptica (lily) and B. tulipae (tulip). B. tulipae was, unexpectedly, able to infect plant species other than tulip, and to a similar extent as B. cinerea. To study host and non-host interactions in more detail, the three Botrytis species were inoculated on Arabidopsis wild-types and 23 mutant genotypes. Disease development was monitored macroscopically by quantifying the lesion area and microscopically by bright-field and fluorescence microscopy following histochemical staining. B. cinerea and B. tulipae were very similar in their ability to infect the tested Arabidopsis genotypes, whereas B. elliptica caused disease only on a few Arabidopsis mutant genotypes. Arabidopsis mutants with a delayed or reduced cell death response were generally more resistant to Botrytis infection, whereas mutants in which cell death was accelerated were more susceptible. Differences in susceptibility between genotypes were generally gradual. Only the camalexin-deficient mutant pad 3 was fully susceptible to all three Botrytis species. Cellular changes were monitored during compatible and incompatible interactions. The formation of papillae, the presence of lysosome-like vesicles and the intracellular accumulation of H₂O₂ and nitric oxide were visualized in the infection zones using fluorescent probes. Based on histology and responses of Arabidopsis mutants, a model is proposed in which resistance against Botrytis , besides the production of camalexin, depends on the balance between cell death and survival.