Population dynamics of the rare plant Kosteletzkya pentacarpos (Malvaceae): a nine-year study

Rare plant species have extremely narrow distributions that can be reduced to a single or few populations. The rare long-lived plant Kosteletzkya pentacarpos is one such species because only two extant localities are known in the western Mediterranean. In this study, we analyse the population dynamics over nine years of the only population known in north-east Spain, which is located at the Llobregat delta (Barcelona). We collected basic demographic data to build a transition matrix model. We computed population growth rates λ and their confidence intervals for each year of study. We conducted elasticity and variance decomposition analyses to determine the relative importance of vital rates to overall population dynamics. On average, the K. pentacarpos population exhibited an increasing dynamics. Survivorship of adult plants contributed the most to each λ, whereas temporal variance in fecundity and juvenile fate explained the observed variation in λ. Despite the increasing dynamics of K. pentacarpos , important reductions in fecundity resulting from biotic agents and recruitment owing to habitat limitations are constraints for population growth. We conclude that the knowledge generated in this long-term study should be used to create new K. pentacarpos populations at the Llobregat delta in order to minimize the risk of extinction following catastrophic events that are nearly impossible to predict.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 153 , 455–462.     

Drought stress affects response of phytopathogen vectors and their parasitoids to infection‐ and damage‐induced plant volatile cues

1. The response of a phytopathogen vector to pathogen‐induced plant volatiles was investigated, as well as the response of the phytopathogen vector's parasitoid to herbivore‐induced plant volatiles released from plants with and without drought stress. 2. These experiments were performed with Asian citrus psyllid (Diaphorina citri), vector of the plant pathogen Candidatus Liberibacter asiaticus (CLas) and its parasitoid Tamarixia radiata as models. Candidatus Liberibacter asiaticus is the presumed causal pathogen of huanglongbing (HLB), also called citrus greening disease. 3. Diaphorina citri vectors were attracted to headspace volatiles of CLas‐infected citrus plants at 95% of their water‐holding capacity (WHC); such attraction to infected plants was much lower under drought stress. Attraction of the vector to infected and non‐stressed plants was correlated with greater release of methyl salicylate (MeSA) as compared with uninfected and non‐stressed control citrus plants. Drought stress decreased MeSA release from CLas‐infected plants as compared with non‐stressed and infected plants. 4. Similarly, T. radiata was attracted to headspace volatiles released from D. citri‐infested citrus plants at 95% of their WHC. However, wasps did not show preference between headspace volatiles of psyllid‐infested and uninfested plants when they were at 35% WHC, suggesting that herbivore‐induced defences did not activate to recruit this natural enemy under drought stress. 5. Our results demonstrate that herbivore‐ and pathogen‐induced responses are environmentally dependent and do not occur systematically following damage. Drought stress affected both pathogen‐ and herbivore‐induced plant volatile release, resulting in concomitant decreases in behavioural response of both the pathogen's vector and the vector's primary parasitoid.     

Will European soil-monitoring networks be able to detect changes in topsoil organic carbon content?

Within the United Nations Framework Convention on Climate Change, articles 3.3 and 3.4 stipulate that some voluntary activities leading to an additional carbon (C) sequestration in soils could be accounted as C sinks in national greenhouse gas inventories. These additional C stocks should be verifiable. In this work, we assess the feasibility of verifying the effects of changes in land use or management practice on soil organic carbon (SOC), by comparing minimum detectable changes in SOC concentration for existing European networks suitable for soil monitoring. Among the tested scenarios, the minimum detectable changes differed considerably among the soil-monitoring networks (SMNs). Considerable effort would be necessary for some member states to reach acceptable levels of minimum detectable change for C sequestration accounting. For SOC, a time interval of about 10 years would enable the detection of some simulated large changes in most European countries. In almost all cases, the minimum detectable change in SOC stocks remains greater than annual greenhouse gases emissions. Therefore, it is unlikely that SMNs could be used for annual national C accounting. However, the importance of organic C in soil functions, and as an indicator of soil condition and trends, underlines the importance of establishing effective national SMNs.     

Effects of myoinhibitory peptides on food intake in the German cockroach

Abstract.  Insect myoinhibitory peptides were discovered through their inhibitory activity on visceral muscle contraction. The present study tests the antimyotropic gut properties of three galanin-related myoinhibitory peptides (Mas-MIP II: GWQDLNSAW-NH2; Grb-AST-B1: GWQDLNGGW-NH₂; and Grb-AST-B3: AWRDLSGGW-NH2) in adult females of the cockroach Blattella germanica (L.) (Dictyoptera, Blattellidae). The three peptides elicit a strong inhibitory effect on both foregut and hindgut contractions, with ID₅₀ values in all the cases within the nanomolar range. In addition, the modulatory effects of these three peptides on food intake are studied on previously starved female cockroaches. The results show that Grb-AST-B3 is the most active peptide, inhibiting food intake by 60–80% at doses between 15 and 50 µg, followed by Grb-AST-B1 (45% inhibition of food intake at the 50 µg dose), whereas Mas-MIP II is inactive even at the 50 µg dose. The differences between the three peptides may be due to a differential effect of their structure on activity or to a differential degradation. These results show that myomodulatory gut activity in vitro and antifeeding effects do not always correlate.     

Analysis of microsatellite variation in the spider mite pest Tetranychus turkestani (Acari: Tetranychidae) reveals population genetic structure and raises questions about related ecological factors

The habitat fragmentation that characterizes agricultural systems made up of an array of sympatric crop and weed biotopes might be a major determinant of the population structure of plant‐feeding arthropods. Spider mites are interesting in this regard as several species are highly polyphagous, possibly generating homogenization by plant shifting, but also have the potential of forming host plant races which promotes specialization. This study analyses genetic diversity and structure in natural populations of a spider mite pest, Tetranychus turkestani, collected on both crops and weeds. Five microsatellite markers were used to genotype 283 individuals collected from 15 samples in four locations in southern France. The markers revealed considerable genetic variation, with an average heterozygosity of 0.68. Pairwise Φ_ST estimates calculated between localities showed differentiation in all comparisons. Although geographical distance appears to be a factor that influences T. turkestani population genetic structure at a regional scale, there was no clear evidence for differentiation between mites living on different host plants. A hierarchical analysis of the distribution of the genetic diversity within and between habitats showed that more than 97% of the observed genetic variation accounted for the differentiation between mites collected on the same host plant in a given locality. Significant heterozygote deficiency was found in 11 out of the 15 samples studied. Considering the biology of the mite, a Wahlund effect and inbreeding might explain such an excess of homozygosity. The data support the view that the plants in a given locality are colonized by mites that originate from diverse sources. They also support previous data suggesting that the demographic structure is made up of small demes of inbred individuals. The agricultural‐level implications of the data are discussed, notably the fact that mites may well be capable of moving between host plants, making weeds surrounding the crop fields potential reservoirs for the pest. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 69–78.     

Population genetics, phylogeography and speciation of Cystodytes (Ascidiacea) in the western Mediterranean Sea

Several morphotypes that so far have been attributed to the allegedly cosmopolitan ascidian Cystodytes dellechiajei occur in the Mediterranean Sea. Colour variation is the difference most frequently reported. In this study, we addressed the genetic structure of this ascidian in relation to geographical location and colour morph. Partial sequences of the gene cytochrome  c oxidase subunit 1 (COI) were obtained from seven populations of the western Mediterranean, encompassing eight colour varieties. All population genetic analyses (exact test, pairwise F _ST, hierarchical analysis of molecular variance, multidimensional scaling, nested clade analysis) indicated clearly that differences between colour morphs are large enough to obscure any geographical differentiation when colours are combined within localities. When variance due to colour divergence was removed, however, a significant geographical variability between localities remained. The genetic divergence between the colour morphs analysed was significant in comparisons of the brown and purple forms with the others, but not among the green, blue, and white morphs. Phylogeographic analyses suggest that population fragmentation and range expansions have shaped the present-day distribution of the haplotypes. Taken together with existing chemotype information, our results indicate that several species are present in the area, and that a thorough revision of the genus is necessary.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 203–214.     

Spatio-temporal variation in the strength and mode of selection acting on major histocompatibility complex diversity in water vole (Arvicola terrestris) metapopulations

Patterns of spatio-temporal genetic variation at a class II major histocompatibility complex (MHC) locus and multiple microsatellite loci were analysed within and between three water vole metapopulations in Scotland, UK. Comparisons of MHC and microsatellite spatial genetic differentiation, based on standardised tests between two demographically asynchronous zones within a metapopulation, suggested that spatial MHC variation was affected by balancing selection, directional selection and random genetic drift, but that the relative effects of these microevolutionary forces vary temporally. At the metapopulation level, between-year differentiation for MHC loci was significantly correlated with that of microsatellites, signifying that neutral factors such as migration and drift were primarily responsible for overall temporal genetic change at the metapopulation scale. Between metapopulations, patterns of genetic differentiation implied that, at large spatial scales, MHC variation was primarily affected by directional selection and drift. Levels of MHC heterozygosity in excess of Hardy–Weinberg expectations were consistent with overdominant balancing selection operating on MHC variation within metapopulations. However, this effect was not constant among all samples, indicating temporal variation in the strength of selection relative to other factors. The results highlight the benefit of contrasting variation at MHC with neutral markers to separate the effects of stochastic and deterministic microevolutionary forces, and add to a growing body of evidence showing that the mode and relative strength of selection acting on MHC diversity varies both spatially and temporally.     

When can ecological speciation be detected with neutral loci?

It is not yet clear under what conditions empirical studies can reliably detect progress toward ecological speciation through the analysis of allelic variation at neutral loci. We use a simulation approach to investigate the range of parameter space under which such detection is, and is not, likely. We specifically test for the conditions under which divergent natural selection can cause a ‘generalized barrier to gene flow’ that is present across the genome. Our individual‐based numerical simulations focus on how population divergence at neutral loci varies in relation to recombination rate with a selected locus, divergent selection on that locus, migration rate and population size. We specifically test whether genetic differences at neutral markers are greater between populations in different environments than between populations in similar environments. We find that this expected signature of ecological speciation can be detected under part of the parameter space, most consistently when divergent selection is strong and migration is intermediate. By contrast, the expected signature of ecological speciation is not reliably detected when divergent selection is weak or migration is low or high. These findings provide insights into the strengths and weaknesses of using neutral markers to infer ecological speciation in natural systems.     

Clutch size manipulation, hatching success and offspring phenotype in the ball python (Python regius)

In a diverse array of avian and mammalian species, experimental manipulations of clutch size have tested the hypothesis that natural selection should adjust numbers of neonates produced so as to maximize the number of viable offspring at the end of the period of parental care. Reptiles have not been studied in this respect, probably because they rarely display parental care. However, females of all python species brood their eggs until hatching, but they do not care for their neonates. This feature provides a straightforward way to experimentally increase or reduce clutch size to see whether the mean clutch size observed in nature does indeed maximize hatching success and/or optimize offspring phenotypes. Eggs were removed or added to newly laid clutches of Ball Pythons ( Python regius ) in tropical Africa (nine control clutches, eight with 50% more eggs added, six with 42% of eggs removed). All clutches were brooded by females throughout the 2-month incubation period. Experimental manipulation of clutch-size did not significantly affect the phenotypes (morphology, locomotor ability) of hatchlings, but eggs in 'enlarged' clutches hatched later, and embryos were more likely to die before hatching. This mortality was due to desiccation of the eggs, with females being unable to cover 'enlarged' clutches sufficiently to retard water loss. Our results support the notion of an optimal clutch size, driven by limitations on parental ability to care for the offspring. However, the proximate mechanisms that generate this optimum value differ from those previously described in other kinds of animals. © 2003 The Linnean Society of London . Biological Journal of the Linnean Society 2003, 78 , 263–272.