GEOGRAPHICAL PATTERNS OF GENETIC DIFFERENTIATION IN THE LANDSNAIL HELIX ASPERSA MuLLER (GASTROPODA: PULMONATA)

In order to investigate the initial processes responsible forthe geographical distribution of the Mediterranean landsnailHelix aspersa, populations from 64 locations in Maghreb andin Europe were studied at 17 enzyme loci by both starch andpolyacrylamide gel electrophoresis. Allele frequencies and genotypedistributions were analysed to provide information on populationstructure and genetic differentiation among populations. Valuesof Nei's unbiased (D) and Rogers (R) genetic distances led tothe formation of two main groups, namely Europe-west Algeriaand east Maghreb (average D = 0.182). Such splitting supportedthe pattern of anatomical variation recently described. Twosamples from Morocco were excluded from these groups becauseof an average D = 0.224. Within this Moroccan entity, geneticdivergences did not distinguish the ‘giant’ snailsH. a.maxima from the brownsnails H. a. aspersa. Such biochemicalsimilarities among morphological heterogeneity throws the subspecificstatus of H. a. maxima into question. Values of fixation indicesindicated that the greatest differentiation between samplesoccurred on the Maghreb territory ( = 0.393). The patterns of population differentiation are discussed inrelation to past geological and historical events. Thus, threescenarios, not mutually exclusive, are considered: the disjunctionof populations by plate tectonics during the Tertiary, the formationof geographically isolated populations during the Pleistoceneglacial/interglacial cycle and the possibility of human introductionas early as the Neolithic period. (Received 9 December 1992; accepted 11 November 1993)     

Selective Expression of PNA-Binding Glycoconjugates by Invasive Human Melanomas: A New Marker of Metastatic Potential

Alterations of cell-surface glycoconjugates have been associated with invasiveness and metastatic capacity in a number of experimental and human tumors (bladder and colon cancer). We have recently shown that human melanoma cells from variants selected for high metastatic potential in an animal model bind the lectin peanut agglutinin (PNA), and that human melanoma cell populations enriched for PNA binding cells generate a higher frequency of metastases when xenografted into immune suppressed neonatal rats. We have therefore sought cells binding PNA in biopsied human melanocytic tumors and compared frequencies of PNA binding by cells from benign nevi, early and late primary melanomas, and metastatic melanomas. Sections of conventionally processed tissues were deparaffinised and exposed to biotinylated PNA; PNA fixation was revealed by the avidine/peroxidase/AEC technique. In 51 specimens tested, PNA appears to react electively with invasive tumors, since only one of the 7 early primary melanomas (Clark III) reacted while 13/23 late primary melanomas (Clark III-V), and 4/21 melanoma metastases were reactive. In addition, only 1/17 benign nevi bound PNA. In primary tumors, the reactive cells were exclusively invasive tumors cells in the dermis. PNA reactive material was observed in the cytoplasm and plasma membrane of reactive cells. Hence, alterations in composition and cellular localisation of glycoconjugates detectable by lectin histochemistry in melanoma cells may be markers of metastatic potential that may be applicable on an individual patient basis.     

Exclusion of ground predators improves Northern Lapwing Vanellus vanellus chick survival

Many farmland‐breeding wader species have declined across Europe, probably due to reductions in reproductive output caused by high nest losses as a result of agriculture or predation, or low chick survival between hatching and fledging. Most studies have focused on nest failures, and the factors affecting post‐hatching survival of chicks are poorly known. In an experimental approach, we fenced parts of the arable foraging areas of Northern Lapwing Vanellus vanellus families to quantify chick survival simultaneously in the presence and absence of ground predators. Lapwing chicks were radiotagged to estimate survival probabilities by daily locations, applying multistate capture–recapture models. During the night, chick survival was considerably lower outside fenced plots than within. During the day, chick survival was higher than at night and did not differ between protected and unprotected plots. This suggests that nocturnal ground predators such as Red Foxes Vulpes vulpes were responsible for a significant proportion of chick mortality. Cumulative survival probability from hatching to fledging was 0.24 in chicks within fenced plots, but virtually zero in chicks outside fenced plots. In farmland, temporary electric fences can be effective in minimizing the impact of ground predators and offer a promising short‐term method to increase fledging success of precocial birds.     

Low genetic diversity and allozymic evidence for autopolyploidy in the tetraploid Pyrenean endemic larkspur Delphinium montanum (Ranunculaceae)

Allozyme electrophoresis was conducted to survey the levels and distribution of genetic diversity in the tetraploid perennial larkspur Delphinium montanum (Ranunculaceae), which is endemic to the eastern Pyrenees of Spain and France and considered to be endangered. Seven populations were sampled, resolving 14 putative enzymatic loci belonging to eight enzymes. Banding patterns stained in gels revealed several enzymatic duplications attributable to autotetraploidy, such as the presence of both balanced and unbalanced heterozygotes and the lack of fixed heterozygosity. However, variability in D. montanum ( P  = 23.8%, A  = 1.48, and H _e = 0.082) was lower than that expected for an autotetraploid species. This, in addition to the scarcity of loci showing three or four different alleles, could indicate that this species has suffered genetic erosion through population bottlenecks, or, alternatively, that it is undergoing diploidization.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 155 , 211–222.     

Plant neighbours mediate bird predation effects on arthropod abundance and herbivory

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The relative importance of dispersal and local processes in structuring phytoplankton communities in a set of highly interconnected ponds

1. The recognition that both local and regional processes act together in shaping local communities makes determining their relative roles in natural communities central to understanding patterns in community structure. 2. We investigated the relative influence of these processes on the phytoplankton communities of a highly interconnected pond system. We sampled the phytoplankton communities of 28 ponds concurrently with 20 local environmental variables. 3. We found that phytoplankton community variation, in terms of both phytoplankton community composition (PCC) and diversity, was only significantly explained by local environmental variables. These were mainly associated with the contrasting clear‐water and turbid ecological states of the shallow ponds studied. Clear‐water conditions favoured only a few taxa, resulting in a significantly lower taxon diversity and richness under these conditions. 4. The failure to explain variation in PCC by a dispersal model based on the water flow between ponds points at very effective species sorting. This is attributed to the high population turn‐over rates and sensitivity to environmental conditions of phytoplankton communities. Some evidence was found, however, that dispersal influences local communities through mass effects between neighbouring ponds. 5. Overall, our results emphasize both the strong selection pressure that components of the food web exert on phytoplankton communities and the high potential of these communities to respond to such environmental change, thereby effectively opposing the homogenizing effects of continuous dispersal.     

Invertebrate community patterns in Mediterranean temporary wetlands along hydroperiod and salinity gradients

1. Temporary aquatic habitats often are inhabited by a unique fauna and flora and contribute significantly to regional diversity. Temporary wetlands around the world are disappearing rapidly. The individual and interacting impacts of factors influencing community structure and dynamics in temporary wetlands are not always well known.
2. Camargue wetlands are mainly characterized by variable salinity and hydroperiod. The individual and combined impacts of these local factors, together with regional variables, on invertebrate communities remain unknown. We therefore characterized and sampled invertebrates in 30 temporary wetlands along salinity and hydroperiod gradients in the Camargue (Southern France) 3, 5 and 7 months after inundation.
3. Over the three sampling occasions, a total of 17 cladoceran species and 49 macroinvertebrate taxa were identified. Hydroperiod and salinity were the most important variables explaining variation in taxonomic composition and can be considered key factors shaping the invertebrate communities in Camargue wetlands. The impact on taxon richness was significantly positive for hydroperiod but significantly negative for salinity. Regional factors had no significant effect on the structure of the studied invertebrate communities, suggesting that dispersal was not limiting and that species sorting was the most important structuring process.
4. The results of this study suggest that the combined and interacting effects of salinization and hydrological modification of Mediterranean temporary wetlands (due to water management, climate change, etc.) can result in reduced diversity in large numbers of Mediterranean wetlands and induce a considerable decline in regional diversity of aquatic invertebrates.     

Growth,development, and life‐history strategies in an unpredictable environment: case study of a rare hoverfly Blera fallax (Diptera,Syrphidae)

1. Development in organisms can vary in response to fluctuating environments. In holometabolous insects, variation in adult phenotypic traits is strongly influenced by growth conditions experienced by larvae. The main aim of this study was to assess how much environmental insight can be gained from analysis of the phenotypic changes in an insect's life history parameters in response to realistic food limitations. 2. This investigation was motivated by a need for more information about the developmental requirements of the endangered pine hoverfly Blera fallax (Linnaeus) (Diptera, Syrphidae) in Scotland. Blera fallax depends on a scarce and often ephemeral habitat, rot holes of Scots pine Pinus sylvestris L. stumps. We studied how rearing conditions affected growth in captive larvae, and compared these responses with a wild population. 3. The growth curve observed in the field was similar to that in resource‐limited, lab conditions, suggesting that resources are limiting in nature. The effects of resource availability on development time and body size depended on sex. Adult females were larger but had more variable size at maturity compared with males. In contrast, males typically were not smaller in resource‐limited conditions, but rather continued to develop for another year. Between 2% and 20% of larvae extended development over 2 years regardless of growth conditions, perhaps indicating a semivoltine strategy to circumvent extinction during years with a low breeding success. 4. These results identify life history traits that may be important for other saproxylic Diptera in rot holes, and organisms that experience food restrictions during growth.     

Contribution of cyclic parthenogenesis and colonization history to population structure in Daphnia

Cyclic parthenogenesis, the alternation of parthenogenetic and sexual reproduction, can lead to a wide scope of population structures, ranging from almost monoclonal to genetically highly diverse populations. In addition, sexual reproduction in aquatic cyclic parthenogens is associated with the production of dormant stages, which both enhance potential gene flow among populations as well as impact local evolutionary rates through the formation of dormant egg banks. Members of the cladoceran genus Daphnia are widely distributed key organisms in freshwater habitats, which mostly exhibit this reproduction mode. We assessed patterns of genetic variation within and among populations in the eurytopic and morphologically variable species Daphnia longispina , using data from both nuclear (13 microsatellite loci) and mitochondrial (partial sequencing of the 12S rRNA gene) markers from a set of populations sampled across Europe. Most populations were characterized by very high clonal diversity, reflecting an important impact of sexual reproduction and low levels of clonal selection. Among-population genetic differentiation was very high for both nuclear and mitochondrial markers, and no strong pattern of isolation by distance was observed. We also did not observe any substantial genetic differentiation among traditionally recognized morphotypes of D. longispina . Our findings of high levels of within-population genetic variation combined with high among-population genetic differentiation are in line with predictions of the monopolization hypothesis, which suggests that in species with rapid population growth and potential for local adaptation, strong priority effects due to monopolization of resources lead to reduced levels of gene flow.