Genetic structure of natural populations of Nitrobacter in an aquatic environment

Nitrification has been essentially studied as a chemical process and the studies of the microbial population have been slightly worked out. This work constitutes a first attempt to study the diversity of Nitrobacter strains with the aim of working out the genetic structure of natural populations. The Nitrobacter population structure was compared between freshwater and sediments of the same lake. Nitrobacter isolates were identified and characterized by DNA/DNA hybridization, restriction pattern of rRNA genes, PCR/RFLP analysis of the ribosomal intergenic spacer and plasmid patterns. It was shown that this lake population was split into 3 subpopulations: a specific freshwater subpopulation, a specific sediment subpopulation and a nonspecific one. A 60 MDa plasmid was detected in 3,3% of freshwater isolates. A 37 MDa plasmid was detected in 59% of sediments isolates. This 37 MDa plasmid was not associated with a genomic group, but associated with a localization: the sediments. These results suggest that this 37 MDa plasmid could be implicated in the adaptation to sediment environment.     

Mechanisms of incomplete prezygotic reproductive isolation in an intertidal snail: testing behavioural models in wild populations

Two morphs (ecotypes) of the marine snail Littorina saxatilis coexist along Galician exposed rocky shores. They hybridize, but gene flow is impeded by a partial prezygotic reproductive barrier, and we have earlier suggested that this is a case of incipient sympatric speciation. To assess the mechanisms of prezygotic reproductive isolation, we estimated deviations from random mating (sexual selection and sexual isolation) of sympatric snails in 13 localities on the shore, and performed mate choice experiments in the laboratory. We also investigated the microdistribution of both morphs over patches of barnacles and blue mussels in the hybridization zone. We used computer simulations to separate the mechanisms contributing to reproductive isolation. On the shores sampled, male–female pairs were strongly assortative both with respect to morphs (mean Yule's V = 0.77) and size (mean Pearson's r = 0.47). In the laboratory, males of both morphs mounted other snails and mated other males and juveniles at random. However, mature females of equal sizes mated assortatively with respect to morph. The two morphs were nonrandomly distributed over barnacle and mussel patches in the hybridization zone. Monte Carlo simulations showed that this microdistribution could explain about half the morph and size relationships in male–female pairs, while a simple rejection mechanism, rejecting the first 1–3 mates if they were of contrasting morphs, accounted for the remaining part of the reproductive isolation, and for parts of the size relationships found between mates. A size discriminant mate choice mechanism may also, to a lesser extent, contribute to the sexual isolation. Sexual selection was observed for female size (larger ones being favoured) and among certain morphs, but distinct biological mechanisms may cause these processes.     

Niche relations in a small world: Epizoic algae on an intertidal gastropod

The patterns of microhabitat use were investigated in three coexisting epizoic algae on an intertidal gastropod, Lunella correensis in an attempt to enhance understanding of the niche relations among ecologically similar species in a space-limited environment. Our analysis of field-derived data showed clear spatiotemporal partitioning of shell microhabitats by epizoic algae with different degrees of resource specialization. As a substrate-specializing, obligate epizoic alga, Pseudocladophora conchopheria occupied wider micro-niches on the host of different sizes than facultative epizoic algae (encrusting rhodophytes, crustose Corallina and Gelidium). While Pseudocladophora demonstrated uniform use of all shell microhabitats on hosts of all sizes, crustose Corallina and Gelidium showed more varied microhabitat use with shifting positions on hosts of different sizes. Furthermore, in addition to slight differences in microhabitat use, crustose Corallina and Gelidium demonstrated differences in their interspecific relationship with Pseudocladophora. Our generalized linear models (GLM) analyses indicated that the occurrence/abundance of crustose Corallina was negatively affected by Pseudocladophora but that of Gelidium was positively affected, while Pseudocladophora appeared to be competitively inferior to both rhodophytes when they co-occurred on hosts of relatively large sizes. The present study, therefore, points to variable niche partitioning among algae even on a small spatial scale, which may result from complex interactions between spatio-temporal substrate heterogeneity on a live host and interspecific interactions.     

Positive abundance and negative distribution effects of a gastropod on an intertidal hermit crab

Summary Field experiments were used to determine the effect of a common intertidal snail (Nerita funiculata) on the use of space for foraging by the hermit crab Clibanarius digueti. Removals of Nerita resulted in an increased density of foraging Clibanarius, while additions of the gastropod had the opposite effect. The observed negative effect of the gastropod on individual hermit crabs appears to be food-related. Field surveys, however, suggested that the hermit crab population is limited by shell number, rather than food. Because Nerita contributes to the shell resource, its effect on the hermit crab population is positive. Nerita, therefore, has a negative effect on the distribution of foraging hermit crabs, but a positive effect on their abundance. Such decouplings of distribution and abundance effects are rare.     

Cadmium hyperaccumulation and reproductive traits in natural Thlaspi caerulescens populations

During the last decade, the metal hyperaccumulating plants have attracted considerable attention because of their potential use in decontamination of heavy metal contaminated soils. However, in most species, little is known regarding the function, the ecological and the evolutionary significances of hyperaccumulation. In our study, we investigated the parameters influencing the Cd concentration in plants as well as the biological implications of Cd hyperaccumulation in nine natural populations of Thlaspi caerulescens. First, we showed that Cd concentration in the plant was positively correlated with plant Zn, Fe, and Cu concentrations. This suggested that the physiological and/or molecular mechanisms for uptake, transport and/or accumulation of these four heavy metals interact with each other. Second, we specified a measure of Cd hyperaccumulation capacity by populations and showed that T. caerulescens plants originating from populations with high Cd hyperaccumulation capacity had better growth, by developing more and bigger leaves, taller stems, and produced more fruits and heavier seeds. These results suggest a tolerance/disposal role of Cd hyperaccumulation in this species.     

Temperature-related diversity of shell colour in the intertidal gastropod Batillaria

The intertidal snail Batillaria exhibits remarkable variationin the shell colour within and among populations. Field studywas conducted to determine the factors in maintaining observedshell colour polymorphism. Geographical variations in shellcolour polymorphisms in B. attramentaria were significantlycorrelated with the temperature of the locality of the population.Darker morphs were predominant in colder regions, whereas lightermorphs increase their proportion in warmer regions. A consistentassociation was also found in B. multiformis that co-existedwith B. attramentaria. Strong predatory pressure imposed bydigenean trematode parasites was observed in B. attramentaria.However, it is unlikely to affect the colour variations, becauseno correlation exists between colour morphs and trematode parasitism.Although visual selection may also contribute to colour variationin Batillaria, no evidence is found for the existence of visualpredators that affect colour patterns of these snails. The deficitof variation in cold regions is possibly due to selection againstbrighter morphs, because bright colours reflect heat. Althoughdark shells absorb sunlight and may therefore be exposed tothe risks of overheating and drying up in a hot habitat, thedarkest morph was frequently observed in the warmer regions,suggesting that physical selection on the colour morphs canbe relaxed in the warmer environment. Our results suggest thatclimatic selection is one of the significant factors maintainingshell colour polymorphism in these intertidal snails. (Received 13 December 2006; accepted 14 April 2007)     

Ploidy levels and reproductive behaviour in natural populations of five Paspalum species

Knowledge of variation in ploidy levels and reproductive behaviour in natural populations is essential in order to understand the functioning of agamic complexes. The aim of this study was to analyse the ploidy level and mode of reproduction in several wild Paspalum populations. A total of 19 populations representing five different species (P. alcalinum, P. denticulatum, P. lividum, P. nicorae, and P. rufum) were collected. Ploidy level was determined in 1,187 individuals by using flow cytometry. Among these individuals, 2x, 3x, 4x, 5x, 6x, and 7x chromosome constitutions were observed. Diploid sexual cytotypes of P. denticulatum were detected for the first time; this will allow the development of future breeding strategies for this particular species. Flow cytometry seed screen (FCSS) in bulked and single seeds revealed the reproductive diversity of these species, ranging from complete sexuality in diploids and varying levels of facultative apomixis in most tetraploids, to obligate apomixis in pentaploids and hexaploids. A fully sexual tetraploid plant was never detected. Nevertheless, most tetraploid genotypes produced both maternal (by apomixis) and non-maternal (by sexuality) progeny. This residual sexuality is very interesting from an evolutionary point of view, since it would allow the creation of new genotypic combinations in natural populations. In addition, the residual sexuality found in some apomictic tetraploid populations can be used as a source of variability for genetic improvement.     

Sexual selection in complex communities: Integrating interspecific reproductive interference in structured populations

The social structure of populations plays a key role in shaping variation in sexual selection. In nature, sexual selection occurs in communities of interacting species; however, heterospecifics are rarely included in characterizations of social structure. Heterospecifics can influence the reproductive outcomes of intrasexual competition by interfering with intraspecific sexual interactions (interspecific reproductive interference [IRI]). We outline the need for studies of sexual selection to incorporate heterospecifics as part of the social environment. We use simulations to show that classic predictions for the effect of social structure on sexual selection are altered by an interaction between social structure and IRI. This interaction has wide‐ranging implications for patterns of sexual conflict and kin‐selected reproductive strategies in socially structured populations. Our work bridges the gap between sexual selection research on social structure and IRI, and highlights future directions to study sexual selection in interacting communities.     

Karyotype differentiation and reproductive isolation among natural populations of Drosophila lacertosa

Drosophila lacertosa is widely distributed from northern India to Far East of Russia throughout China. We have studied geographical distributions of three kinds of chromosomal karyotypes, type D (2n=10, 4 pairs of V-shaped metacentric chromosomes and a pair of micro-chromosomes), type L (2n=10, 5V with a pair of large-sized hetero-chromatic autosomes) and type M (2n=10, 5V with middle-sized ones). Type D was found exclusively in local populations of D. lacertosa distributed in Yun-Gui Plateau, southwestern China. Both type L and M have a wide range of distribution, and the former occurred in subtropical regions of China including Taiwan Island, whereas the latter in cool temperate regions of East Asia covering Far East of Russia, Korea and the Japan Islands. A strong premating isolation was detected between flies with type D and those with type L or M. These data demonstrate that genetic differentiation leading to cryptic speciation might have occurred in natural populations of Drosophila lacertosa.     

Developmental consequences of association with a photosynthetic substrate for encapsulated embryos of an intertidal gastropod

Aggregation of embryos in clutches that lack internal circulation can increase the risk of hypoxia by limiting gas exchange. As a result, limits on oxygen solubility and diffusion in water can constrain the size and embryo concentration of aquatic egg clutches. Hypoxia in egg masses can slow embryo development, increase mortality, and reduce size at hatching. The risk of hypoxia for embryos, however, can be reduced by association with photosynthetic organisms. We examined whether embryo development in egg ribbons of the cephalaspidean mollusk Haminoea vesicula is significantly influenced by oviposition on eelgrass (Zostera marina). Association with the photosynthetic substrate had marked effects on development relative to association with non-photosynthetic substrates, and the direction of these effects was mediated by light conditions. Under intermediate and high light levels, association with eelgrass accelerated embryo development, while under dim light, the presence of the macrophyte increased development rate and reduced hatchling shell size. Benefits of association with eelgrass at higher light levels likely result from oxygen production by eelgrass photosynthesis, while we attribute costs under low light to oxygen depletion by eelgrass respiration. Association with Z. marina also limited microphyte growth in egg ribbons of H. vesicula. In the field, measurements of light attenuation within an eelgrass bed showed that conditions under which benefits accrue to embryos are ecologically relevant and correspond to spatial patterns of oviposition on eelgrass in the field. The choice of a photosynthetic oviposition substrate under appropriate light conditions can improve embryo fitness by accelerating embryo development without compromising hatchling size and by reducing the potential for excessive and harmful fouling by microphytes.     

Embryogenesis and organogenesis in the intertidal gastropod Thais bufo (Lamarck)

The morphology of the egg capsule of Thais bufo is given. Embryogenesis is described from maturation divisions through the torsional stages. Organogenesis of organ systems has been followed through the first planktotrophic veliger stage.     

The charge-state model of protein polymorphism in natural populations

Routine electrophoretic surveys for genetic variation in natural populations depend primarily upon detecting differences in the net charge carried by a protein. We have calculated the proportion of base substitutions which would yield an electrophoretically detectable mutant protein, and the relative mutation rates among different chare classes, under a variety of simplifying assumptions. These calculations indicate that: (i) only 25 per cent of all single base mutations would lead to a charge change on a protein molecule. (ii) five distinct classes of electrophoretic variants can be generated from a specified protein by single base substitutions. (iii) the relative mutation rates differ markedly among the different charge classes which can be generated by single base substitutions. The estimates of the proportion of electrophoretically detectable mutant proteins and relative mutation rates among charge classes were relatively robust to changes in assumptions concerned with the kind and site of base substitutions and the amino acid composition of the protein.     

Habitat disturbance and the stability of freshwater gastropod populations

Summary The interaction of population stability and habitat permanence has a major influence on the microdistribution of freshwater snails. For two years (February 1980–January 1982), we monitored the abundance of macrophytes and the abundance and size structure of four species of macrophyte-associated freshwater snails in an English pond. Previous work (Lodge, in press) showed that two species, the pulmonate Lymnaea peregra (Mull.) and the prosobranch Valvata piscinalis (Mull.), were associated with cubmersed macrophytes, while two other species, the pulnonate Planorbis vortex (Linn.), and the prosobranch Bithynia tentaculata (Linn.), were associated with emergent macrophytes. A dramatic decline of submersed macrophytes provided a test of the hypotheses that the population stability of Lymnaea and Valvata was 1) high, and 2) an important cause of the association of those two species with submersed macrophytes.When the submersed macrophytes declined in August 1980, >99% of the Lymnaea and about 35% of the Valvata population died. The populations of Planorbis and Bithynia were not reduced. In 1980, Lymnaea and Valvata had simple annual life cycles, but with the regrowth of submersed macrophytes in spring 1981, the Lymnaea and Valvata populations responded with early, high, and repeated reproduction with some overlap of generations. In both years, Planorbis had an annual semelparous life cycle, while Bithynia lived up to 3 years and bred iteroparously.Following the terminology of Connell and Sousa (1983), Lymnaea exhibited low resistance to habitat disturbance but high adjustment following the disturbance. Valvata showed higher resistance than Lymnaea, and also high adjustment. Although the population stability of Planorbis and Bithynia could not be rigorously evaluated, published accounts of those species' life cycles suggest that stability, specially the adjustment component, was low. We suggest that the population stability of the four species is a major determinant of the association of Lymnaea and Valvata with the impermanent macrophyte habitat and that of Planorbis and Bithynia with the permanent macrophyte habitat.     

Phenotypic plasticity and sexual dimorphism in size at post-juvenile metamorphosis: common-garden rearing of an intertidal gastropod with determinate growth

Proximate factors of the intraspecific variation in molluscan shell morphology have long received attention in biology. The intertidal gastropod Monetaria annulus (Mollusca; Gastropoda; Cypraeidae) is particularly suitable for the study of variation in body size, because this species is a determinate grower in the sense that soft-body size shows no further increase after the juvenile stage. Cross-sectional field surveys on post-juvenile individuals have indicated that the mean body size varies widely among populations and is larger in females than in males within populations. To examine whether these patterns are due to genetic differences, we conducted a common-garden rearing experiment with juvenile individuals collected from two populations on Okinawa Island. After adjusting for among-individual differences in initial degree of development, statistical analyses revealed that this species exhibits female-biased sexual size dimorphism mediated by a longer development time rather than by faster growth rates in females. Although wild individuals show a remarkable size difference between populations, no size difference was found between the populations in the individuals reared in a common-garden condition. This result suggests that the among-population size difference does not have a genetic basis and is caused by phenotypic plasticity based on environmental heterogeneity among habitats.     

Reproductive interference between Rana dalmatina and Rana temporaria affects reproductive success in natural populations

Experimental evidence suggests that reproductive interference between heterospecifics can seriously affect individual fitness; support from field studies for such an effect has, however, remained scarce. We studied reproductive interference in 25 natural breeding ponds in an area where two ranid frogs, Rana dalmatina and Rana temporaria, co-occur. The breeding seasons of the two species usually overlap and males of both species are often found in amplexus with heterospecific females, even though matings between heterospecifics produce no viable offspring. We estimated species abundance ratios based on the number of clutches laid and evaluated fertilization success. In ponds with low spatial complexity and a species abundance ratio biased towards R. temporaria, the average fertilization success of R. dalmatina eggs decreased, while this relationship was not detectable in spatially more complex ponds. Fertilization success of R. temporaria did not decrease with increasing relative numbers of heterospecifics. This asymmetry in fitness effects of reproductive interference may be attributed to R. temporaria males being more competitive in scramble competition for females than R. dalmatina males. Our study is among the first to demonstrate that in natural breeding populations of vertebrates interference among heterospecifics has the potential to substantially lower reproductive success at the population level, which may in turn affect population dynamics.     

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.     

Environmental and behavioural factors affecting activity in the intertidal gastropod Hydrobia ulvae

Laboratory microcosms were used to investigate the mud snail Hydrobia ulvae (Pennant) bioturbation activities and behavioural changes in response to snail density, algal food, sediment moisture content, light regime and water cover conditions. Density-dependent kinetics of bioturbated muddy areas were described by von Bertalanffy equations, which provided reliable estimates of mud surface covering rates by snail tracks (m² h⁻¹ snail⁻¹). Snails need a wet habitat to be active either covered by seawater or by moving in fluid layers for low-tide conditions. Light and microphytobenthic biomass, which are less potent to affect snail activity, are positively interrelated to increase covering rates in the tested chl a concentrations within the range of 1-15 μg g⁻¹. Experimental results suggested us the relevance of microphytobenthos migration processes in affecting crawling activities of H. ulvae that appeared to adjust their foraging efforts in response to benthic algal biomass. Behavioural processes of H. ulvae, in terms of floating, crawling, burrowing and inactive snails, were described using a Markov model. Finally, an empirical model based on von Bertalanffy equations was proposed to describe kinetics of sediment covering by snail tracks under the influences of snail density, sediment moisture content, chl a concentrations and the four combinations of presence/absence of light and seawater. This model should provide a base for further development of a hydrosedimentary model to simulate the effects of H. ulvae bioturbation activities on the resuspension of the intertidal cohesive sediment-water interface for various in situ conditions.