Centenary coppicing maintains high levels of genetic diversity in a root resprouting oak (<Emphasis Type="Italic">Quercus pyrenaica</Emphasis> Willd.)

Studies disentangling the anthropogenic influences of traditional forest uses are crucial to assess the current conservation value of cultural landscapes. By promoting asexual regeneration, centenary coppicing in the predominantly root resprouter Quercus pyrenaica is assumed to have reduced genetic diversity levels contributing to the decline of abandoned coppices and the common lack of acorn production. This work aims to test the widespread assumption that historical coppicing in Q. pyrenaica has caused depleted levels of genetic diversity. Seven microsatellite markers were used to assess clonal structure and population genetic diversity levels in six abandoned coppices of Q. pyrenaica, which were compared to three open woodlands in national parks in Spain. Asexual regeneration was higher in coppices, leading to more frequent and larger clonal assemblies. Clonal diversity parameters (genotypic richness and proportion of unique genotypes) were significantly lower in coppices, although density of genotypes per surface area and levels of population genetic diversity were comparable to those observed in open parklands. Heterogenic clonal structures were found both within and among stands, hindering the inference of concrete anthropogenic disturbances. Despite promoting asexual reproduction, coppicing maintains high levels of genotypic and genetic diversity and allows the incorporation of new genotypes by seed recruitment. The natural resprouting capacity of Q. pyrenaica preserved the species in face of long-lasting anthropogenic disturbances, fostering ecosystem resilience and harbouring high conservation values.     

Persistence of the gypsophile <Emphasis Type="Italic">Lepidospartum burgessii</Emphasis> (Asteraceae) through clonal growth and limited gene flow

Lepidospartum burgessii is a rare gypsophilic shrub with limited distribution in New Mexico and Texas. Most of the known plants are restricted to two large populations, with a few small, isolated populations scattered in the surrounding area. The low recruitment observed in the two largest populations may be due to low seed set resulting from high inbreeding and/or self-incompatibility. We used eight microsatellite loci to quantify diversity, relatedness, inbreeding, population structure, and frequency of clonal reproduction. Seed collections were made to quantify seed set and germination rates. Overall, there was a moderate level of clonal diversity within patches of L. burgessii indicating asexual growth is important for population persistence. Inbreeding coefficients were high both between and within populations. Most patches showed a significant level of relatedness between individuals. At a fine scale, patches within populations were significantly different from each other, however when all patches were combined, the two populations of L. burgessii were not genetically distinct. Compared to a population of its common congener, Lepidospartum latisquamum, L. burgessii populations had similar measures of diversity, more clonal reproduction, and lower germination rates. High relatedness and inbreeding may explain the low seed set and recruitment in L. burgessii, however factors such as insect herbivory and precipitation changes may further depress recruitment.     

Microsatellite DNA analysis of spatial and temporal population structuring of <Emphasis Type="Italic">Phragmites australis</Emphasis> along the Hudson River Estuary

Phragmites australis is a perennial grass that has invaded wetlands of the northeastern United States over the past century. The Hudson River Estuary and surrounding watersheds are no exception in that populations of P. australis have spread dramatically along its shores and tributaries in the past 40 years. Recent studies have shown that genetically variable populations of P. australis can spread by seed dispersal in addition to clonal mechanisms. It is important to characterize the genetic variation of Hudson River populations as part of a management strategy for this species to determine the mechanisms by which its spreads and colonizes new habitats, particularly those with frequent anthropogenic disturbances. The goals of this study were to quantify levels of genetic variation and structuring in Hudson River populations of P. australis using microsatellite DNA analysis. A total of 354 culms of P. australis were collected from nine locations ranging from Albany, New York to Staten Island, New York in the summers of 2004 (N = 174) and 2011 (N = 180). Microsatellite data from eight loci indicated that the Hudson River Estuary has some of the highest levels of genetic variation of all U. S. Atlantic Coast regions containing P. australis. Gene diversity (Hs) across all loci in the 2004 collection was 0.45 (±0.02) and that of the 2011 collection was 0.47 (±0.07). Patches within sample sites were rarely monoclonal and had multiple genetic phenotypes. Moran’s Identity tests indicated that individuals within a patch were closely related, whereas little genetic relatedness was evident among individuals from sample sites >1 km apart. Spatial structuring was also not evident in autospatial correlation and principle coordinate analyses. These findings suggest that genetic diversity is maintained within stands by sexual reproduction and that seeds are important in dispersal of P. australis across the Hudson River Estuary. Ample habitats are available for establishment of new Phragmites stands due to high levels of anthropogenic disturbance from populations living along the Estuary. Wildlife managers should focus on monitoring habitats that provide seedbed for Phragmites and promote land use practices that prevent soil disturbance and establishment of new stands.     

PhragNet: crowdsourcing to investigate ecology and management of invasive <Emphasis Type="Italic">Phragmites australis</Emphasis> (common reed) in North America

Invasion biology research, often performed by scientists at relatively small spatial scales, provides experimental precision but may be limited in generalizability. Conversely, large-scale invasive species management represents a largely untapped wealth of information on invasion ecology and management, but such data are difficult to capture and synthesize. We developed a network (“PhragNet”) of individuals managing wetlands occupied by native and non-native lineages of the invasive wetland grass Phragmites australis (common reed). This network collected environmental and genetic samples, habitat data, and management information to identify environmental and plant community associations of Phragmites invasion and patterns of management responses. Fifty managers overseeing 209 Phragmites stands in 16 US states and ON, Canada participated. Participants represented federal agencies (26%), municipalities (20%), NGOs (20%), academia (14%), state agencies (12%), and private landowners (8%). Relative to the native lineage, non-native Phragmites occurred in areas with higher nitrate/nitrite and ammonium than non-native Phragmites. Stand interiors had higher soil electrical conductivity than nearby uninvaded areas, consistent with use of road salt promoting spread of Phragmites. Non-native Phragmites co-occurred with fewer plant species than native Phragmites and was actively targeted for management. Herbicide was applied to 51% of non-native stands; surprisingly, 11% of native stands were also treated with herbicide. This project demonstrates the utility of crowdsourcing standardized data from resource managers. We conclude by describing how this approach could be expanded into an adaptive management framework, strengthening connections between wetland management and research.     

Multi-scale methods predict invasion speeds in variable landscapes

Spread rates of invasive plant species depend heavily on variable seed/seedling survivorships over various habitat types as well as on variability in seed dispersal induced by rapid transport of propagules in open areas and slow transport in vegetated areas. The ability to capture spatial variability in seed survivorship and dispersal is crucial to accurately predict the rate of spread of plants in real world landscapes. However, current analytic methods for predicting spread rates are not suited for arbitrary, spatially heterogeneous systems. Here, we analyze invasion rates of the invasive plant Phragmites australis (common reed) over variable wetland landscapes. Phragmites is one of the most pervasive perennial grasses, outcompeting native vegetation, providing poor wildlife habitat, and proving difficult to eradicate across its invasive range in North America. Phragmites spreads sexually via seeds and asexually via underground (rhizomes) and aboveground (stolons) stems. We construct a structured integrodifference equation model of the Phragmites life cycle capturing variable seed survivorship in a seed bank, sexual and asexual recruitment into a juvenile age class, and differential competition among all classes with adults. The demographic model is coupled with a homogenized ecological diffusion/settling seed dispersal model that allows for seed deposition that varies with habitat type. The dispersal kernel we develop does not require local normalization and can be implemented efficiently using standard computational techniques. The model generates a traveling wave of isolated patches, establishing only in suitable habitats. We use the method of multiple scales to predict invasion speed as a solvability condition at large scales and test the predictions numerically. Accurate predictions are generated for a wide range of landscape parameters, indicating that invasion speeds can be understood in landscapes of arbitrary structure using this approach.     

Fine-scale clonal structure and diversity within patches of a clone-forming dioecious shrub, Ilex leucoclada (Aquifoliaceae)

BACKGROUND AND AIMS: The mode of reproduction (sexual vs. asexual) is likely to have important effects on genetic variation and its spatial distribution within plant populations. An investigation was undertaken of fine-scale clonal structure and diversity within patches of Ilex leucoclada (a clone-forming dioecious shrub). METHODS: Six patches were selected in a 1-ha plot previously established in an old-growth beech forest. Two of the selected patches were composed predominantly of stems with male flowers (male patch), and two contained stems with predominantly female flowers (female patch). The remaining two patches contained stems with male flowers and stems with female flowers in more or less equal proportions (mixed patch). Different genets were distinguished using random amplified polymorphic DNA (RAPD) markers. KEY RESULTS: One hundred and fifty-six genets with different RAPD phenotypes were identified among 1928 stems from the six patches. Among the six patches, the male patches had the lowest clonal diversity, and the mixed patches had the highest. Distribution maps of the genets showed that they extended downhill, reflecting natural layering that occurred when stems were pressed to the ground by heavy snow. In every patch, there were a few large genets with many stems and many small genets with a few stems. CONCLUSION: The differences in clonal diversity among patches may be due to differences in seedling recruitment frequencies. The skewed distribution of genet size (defined as the number of stems per genet) within patches may be due to differences in the timing of germination, or age (with early-establishing genets having clear advantages for acquiring resources) and/or intraspecific competition.     

Population demographics and trade-offs to reproduction of an invasive and noninvasive species of <Emphasis Type="Italic">Rubus</Emphasis>

Do trade-offs between growth and reproduction differ between an invasive and noninvasive plant species and how do such trade-offs relate to population demographics? To help address these questions, we compared demographics for an invasive plant species, Rubus discolor, with a noninvasive congener, R. ursinus, in several populations of varying density. Removal of floral buds from reproductive canes increased the size of juvenile canes that arose from clonal sprouting in R. ursinus, suggesting a trade-off between current reproduction and growth. Removal of floral buds had no effect on growth of R. discolor. R. ursinus displayed trade-offs between reproduction (sexual and vegetative) and future growth based on negative correlations between leaf area production and both clonal sprouting and seedling production during the previous year. R. discolor did not exhibit these trade-offs. Both species had high population growth rates in low-density populations, but exhibited little or no growth in high-density populations. A life table response experiment was used to determine the underlying cause for the effect of density on population growth. For R. ursinus, lack of population growth in high-density populations was due primarily to increased mortality of clonally sprouting canes, while for R. discolor, it was due to decreased clonal cane production. Elasticity analysis revealed that clonal growth was more important than sexual reproduction for population growth of both species. However, elasticity values for sexual reproduction in R. discolor were greater in high- than low-density populations. This suggests an increased reliance on sexual reproduction in populations that had reached stable sizes, which could increase the capacity of R. discolor to disperse to new sites. Elasticity analyses were also used to simulate the efficacy of various control strategies for R. discolor. Control of this species could be attained by reducing clonal production within existing populations while reducing seed production to limit establishment of new populations.     

Vegetative multiplication and patch colonisation of Asarum europaeum subsp. europaeum L. (Aristolochiaceae) inferred by a combined morphological and molecular study

Asarum europaeum subsp. europaeum (Aristolochiaceae) is a rhizomatous herb forming distinct patches in calcareous broadleaved forests. Within natural stands, patches were mapped. In two regions, at least four patches were dug out, and connections between leaf modules through rhizomatous spacers were checked for signs of clonal reproduction (decay, breaking). Modules were sampled for amplified fragment length polymorphism (AFLP) fingerprinting to test whether they represent unique genets or are merigenets of a larger genet (split up by clonal reproduction), respectively.Morphologically, merigenet-relationships were only revealed in few cases with disrupted spacers between modules. With the obtained AFLP profiles for two primer combinations, the samples could be assigned to genets; clonal descendants of the same genet were readily identified. In one patch analysed in detail, 18 samples from 17 unconnected “plants” belonged to only two genets, which were morphologically divided into two and 15 merigenets, respectively. These two genets probably belonged to different maternal lineages and came into contact after lateral spread from the established clones. They showed divergent affinities to samples from adjacent patches (which all represented unique genets).The findings support the suitability of the combined morpho-ecological and molecular approach: compared to either method alone, it allows a more detailed analysis and interpretation of the fine-scale clonal structure, patch colonisation and especially of vegetative multiplication (with morpho-ecological studies to discern clonal growth and clonal reproduction and AFLP fingerprinting for genet and merigenet identification, respectively).     

Biogeography of <Emphasis Type="Italic">Phragmites</Emphasis><Emphasis Type="Italic">australis</Emphasis> lineages in the southwestern United States

The environmental and social impacts of Phragmites australis invasion have been extensively studied in the eastern United States. In the West where the invasion is relatively recent, a lack of information on distributions and spread has limited our ability to manage invasive populations or assess whether native populations will experience a decline similar to that in the East. Between 2006 and 2015, we evaluated the genetic status, distribution, and soil properties (pH, electrical conductivity, and soil texture) of Phragmites stands in wetlands and riparian systems throughout the Southwest. Native (subspecies americanus), Introduced (haplotype M), and Gulf Coast (subspecies berlandieri) Phragmites lineages were identified in the survey region, as well as watershed-scale hybridization between the Native and Introduced lineages in southern Nevada. Two Asian haplotypes (P and Q) that were previously not known to occur in North America were found in California. The Native lineage was the most frequent and widespread across the region, with four cpDNA haplotypes (A, B, H, and AR) occurring at low densities in all wetland types. Most Introduced Phragmites stands were in or near major urban centers and associated with anthropogenic disturbance in wetlands and rivers, and we document their spread in the region, which is likely facilitated by transportation and urban development. Soil pH of Native and hybrid stands was higher (averaging 8.3 and 8.6, respectively) than Introduced stands (pH of 7.5) and was the only soil property that differed among lineages. Continued monitoring of all Phragmites lineages in the Southwest will aid in assessing the conservation status of Native populations and developing management priorities for non-native stands.     

Multiple invasions in urbanized landscapes: interactions between the invasive garden ant <Emphasis Type="Italic">Lasius neglectus</Emphasis> and Japanese knotweeds (<Emphasis Type="Italic">Fallopia</Emphasis> spp.)

Urbanized landscapes are the theater of multiple simultaneous biological invasions likely to affect spread dynamics when co-occurring introduced species interact with each other. Interactions between widespread invaders call for particular attention because they are likely to be common and because non-additive outcomes of such associations might induce negative consequences (e.g., enhanced population growth increasing impacts or resistance to control). We explored the invasions of two widespread invasive taxa: the Japanese knotweed species complex Fallopia spp. and the invasive garden ant Lasius neglectus, in the urban area of Lyon (France). First, we investigated landscape habitat preferences as well as co-occurrence rates of the two species. We showed that Fallopia spp. and L. neglectus had broadly overlapping environmental preferences (measured by seven landscape variables), but their landscape co-occurrence pattern was random, indicating independent spread and non-obligatory association. Second, as Fallopia spp. produce extra-floral nectar, we estimated the amount of nectar L. neglectus used under field conditions without ant competitors. We estimated that L. neglectus collected 150–321 kg of nectar in the month of April (when nectar production is peaking) in a 1162 m² knotweed patch, an amount likely to boost ant population growth. Finally, at six patches of Fallopia spp. surveyed, herbivory levels were low (1–6% loss of leaf surface area) but no relationship between ant abundance (native and invasive) and loss of leaf surface was found. Co-occurrences of Fallopia spp. and L. neglectus are likely to become more common as both taxa colonize landscapes, which could favor the spread and invasion success of the invasive ant.     

Influences of the genetic neighborhood on ramet reproductive success in a clonal desert cactus

Clonal structure in clonal plants can affect sexual reproduction. Individual ramets can decrease reproduction if their neighbors are ramets of the same genet due to inbreeding depression or self-incompatibility. We assessed ramet reproductive success in the partial self-incompatible Ferocactus robustus (Cactaceae) as a function of floral display size in focal ramets and floral display size and clonal structure of their reproductive neighborhoods. Ramets were labeled, sized in number of stems, mapped and genetically identified through RAPD markers in one population. A pollen dispersal area of 15-m radius was established for each ramet to determine the clonal diversity in the neighborhoods. Flower production and fruit set were counted on a monthly basis during one reproductive season as a surrogate of ramet fitness. We expected a decrease in individual ramet reproductive success as a function of the number of reproductive ramets of the same genet in the neighborhood. A total of 272 sampled ramets revealed 116 multilocus genotypes, showing high clonal diversity in the population (G/N = 0.43, D = 0.98). Clonal diversity of neighborhoods ranged from 0.06 to 1 and fruit set varied from 0 to 76.9%. Individual ramet reproductive success was influenced by (1) mate availability, (2) floral display size of a genet within the reproductive neighborhood, and (3) the proportion of distinguishable genotypes. Floral display size of genets and ramets coupled with the genetic diversity within the reproductive neighborhood determines the low sexual reproduction in F. robustus.     

Contrasting patterns of clonality and fine-scale genetic structure in two rare sedges with differing geographic distributions

For plants with mixed reproductive capabilities, asexual reproduction is more frequent in rare species and is considered a strategy for persistence when sexual recruitment is limited. We investigate whether asexual reproduction contributes to the persistence of two co-occurring, rare sedges that both experience irregular seed set and if their differing geographic distributions have a role in the relative contribution of clonality. Genotypic richness was high (R=0.889±0.02) across the clustered populations of Lepidosperma sp. Mt Caudan and, where detected, clonal patches were small, both in ramet numbers (⩽3 ramets/genet) and physical size (1.3±0.1 m). In contrast, genotypic richness was lower in the isolated L. sp. Parker Range populations, albeit more variable (R=0.437±0.13), with genets as large as 17 ramets and up to 5.8 m in size. Aggregated clonal growth generated significant fine-scale genetic structure in both species but to a greater spatial extent and with additional genet-level structure in L. sp. Parker Range that is likely due to restricted seed dispersal. Despite both species being rare, asexual reproduction clearly has a more important role in the persistence of L. sp. Parker Range than L. sp. Mt Caudan. This is consistent with our prediction that limitations to sexual reproduction, via geographic isolation to effective gene exchange, can lead to greater contributions of asexual reproduction. These results demonstrate the role of population isolation in affecting the balance of alternate reproductive modes and the contextual nature of asexual reproduction in rare species.     

Genet structure and determinants of clonal structure in a temperate deciduous woodland herb, Uvularia perfoliata

1 We used isozyme variation to examine the genet structure of Uvularia perfoliata patches in gap and closed canopy habitats in a temperate deciduous forest in Maryland, USA.
2 A large patch in a gap habitat was composed of a small number of widely spread genets with many ramets, and a large number of genets with more restricted distribution and few ramets. Genets with many ramets were patchily distributed at a metre scale. Analysis of genet structure on a scale of square centimetres, however, revealed that the genets were highly intermingled with no clear boundaries between them. The presence at both scales of sampling of many genets with unique multilocus genotypes indicated continuing genet recruitment within the population.
3 In the closed canopy habitat, the patches examined were each composed of a single unique multilocus genotype, suggesting that each had developed by asexual propagation following the establishment of a single genet.
4 The clonal structure of U. perfoliata patches in both gap and closed canopy habitats therefore appears to depend on recruitment patterns of genets. Populations in closed canopy habitats are characterized by a 'waiting' strategy, in which asexual ramet production maintains populations until genet recruitment by seed production can occur under the more optimal conditions associated with canopy gaps. Extended sampling suggests that the genetic diversity of U. perfoliata populations is primarily controlled by the disturbance regime of the forest canopy.     

Host location and selection by the symbiotic sargassum crab <Emphasis Type="Italic">Portunus sayi</Emphasis>: the role of chemical,visual and tactile cues

The Sargassum community consists of a unique and diverse assemblage of symbiotic fauna critical to pelagic food chains. Associated symbionts presumably have adaptations to assist in finding Sargassum. In situ scattered Sargassum patches accumulate as they are pushed toward the shoreline (via wind, waves, currents or tides) and are frequently less than 1 m apart and in depths of 10 cm or less as the patches approach the shoreline Crabs, and other symbiotic fauna, must relocate to another patch that is seaward in direction or likely perish as their current patch will likely become beached. This study investigated sensory cues used for host location and selection by the Sargassum crab, Portunus sayi. Chemical detection trials were conducted with a two-chamber choice apparatus with Sargassum spp. and Thalassia testudinum as habitat source odors. Visual detection trials (devoid of chemical cues) and habitat selection trials were conducted in which crabs were given a choice between hosts. Results showed that P. sayi responded to chemicals from Sargassum spp. Crabs visually located host habitats but did not visually distinguish between different hosts. In host selection trials, crabs selected Sargassum spp. over artificial Sargassum and T. testudinum. These results suggest that crabs isolated from Sargassum likely use chemoreception; within visual proximity of a potential patch, crabs likely use both chemical and visual information.     

Developmental regulators in <Emphasis Type="Italic">Aspergillus fumigatus</Emphasis>

The filamentous fungus Aspergillus fumigatus is the most prevalent airborne fungal pathogen causing severe and usually fatal invasive aspergillosis in immunocompromised patients. This fungus produces a large number of small hydrophobic asexual spores called conidia as the primary means of reproduction, cell survival, propagation, and infectivity. The initiation, progression, and completion of asexual development (conidiation) is controlled by various regulators that govern expression of thousands of genes associated with formation of the asexual developmental structure conidiophore, and biogenesis of conidia. In this review, we summarize key regulators that directly or indirectly govern conidiation in this important pathogenic fungus. Better understanding these developmental regulators may provide insights into the improvement in controlling both beneficial and detrimental aspects of various Aspergillus species.     

Complex invader-ecosystem interactions and seasonality mediate the impact of non-native <Emphasis Type="Italic">Phragmites</Emphasis> on CH<Subscript>4</Subscript> emissions

Invasive plants can influence ecosystem processes such as greenhouse gas (GHG) emissions from wetland systems directly through plant-mediated transfer of GHGs to the atmosphere or through indirect modification of the environment. However, patterns of plant invasion often co-vary with other environmental gradients, so attributing ecosystem effects to invasion can be difficult in observational studies. Here, we assessed the impact of Phragmites australis invasion into native shortgrass communities on methane (CH₄) emissions by conducting field measurements of CH₄ emissions along transects of invasion by Phragmites in two neighboring brackish marsh sites and compared these findings to those from a field-based mesocosm experiment. We found remarkable differences in CH₄ emissions and the influence of Phragmites on CH₄ emissions between the two neighboring marsh sites. While Phragmites consistently increased CH₄ emissions dramatically by 10.4 ± 3.7 µmol m^?2 min^?1 (mean ± SE) in our high-porewater CH₄ site, increases in CH₄ emissions were much smaller (1.4 ± 0.5 µmol m^?2 min^?1) and rarely significant in our low-porewater CH₄ site. While CH₄ emissions in Phragmites-invaded zones of both marsh sites increased significantly, the presence of Phragmites did not alter emissions in a complementary mesocosm experiment. Seasonality and changes in temperature and light availability caused contrasting responses of CH₄ emissions from Phragmites- versus native zones. Our data suggest that Phragmites-mediated CH₄ emissions are particularly profound in soils with innately high rates of CH₄ production. We demonstrate that the effects of invasive species on ecosystem processes such as GHG emissions may be predictable qualitatively but highly variable quantitatively. Therefore, generalizations cannot be made with respect to invader-ecosystem processes, as interactions between the invader and local abiotic conditions that vary both spatially and temporally on the order of meters and hours, respectively, can have a stronger impact on GHG emissions than the invader itself.     

Comparison of phenotypic and genetic clone delineation in quaking aspen, <Emphasis Type="Italic">Populus tremuloides</Emphasis>

Key message

Clonal delineation at nuclear microsatellites and phenotypic traits showed high correspondence and revealed an important role of both sexual and clonal reproduction for stand genetic structure.

Abstract

Quaking aspen (Populus tremuloides Michx.) grows throughout the northern and central portions of North America. Reproduction occurs both sexually via seeds and clonally from root suckers. Clonal delineation using morphological/phenological traits, and more recently, highly variable nuclear microsatellites have shown considerable variation in the size of clonal assemblies, and the relative importance of sexual versus clonal reproduction across the species range. In order to provide reliable estimates of genet size (N/G; ramets per sampled genet) and genotypic diversity (G/N; genets/ramets), and to compare genetic and phenotypic clone delineation, we characterized 181 sampled stems (ramets) at seven nuclear microsatellites, and morphological and phenological traits from six clones (genet size ≥11). Genotypic diversity was moderate (G/N = 0.18) and within the range reported in other studies across North America. Multivariate statistics revealed a high correspondence between genetic and phenotypic clone delineation, both with and without predefined genetic groups (94.2 %, 81.7 %). Moderate average genet size (5.6 ramets per genet) and the occurrence of genetically distinct single-ramet genets surrounded by larger genets suggested intermediate levels of sexual reproduction contributing to the genetic structure of this stand. Significant differences among genets were found for phenological and morphological traits such as bark thickness and leaf shape. However, most clones showed no significant differences in diameter growth which was likely caused by poor drainage in this high clay soil that inhibited the expression of genetic differences in growth.

     

Ecomorphology of a generalist freshwater gastropod: complex relations of shell morphology,habitat, and fecundity

Evolutionary and ecological situations in a species’ native and invasive ranges can be drastically different. This is the case for Potamopyrgus antipodarum Gray (1843) a morphologically highly variable freshwater snail native to New Zealand, where sexual and asexual individuals coexist and experience selective pressure by sterilizing endoparasites. By contrast, only a few asexual lineages have been established in invaded regions around the globe, where parasite infection is extremely rare. We analyzed the ecomorphology of 996 native P. antipodarum in a geometric morphometric framework, using brood size as proxy for fecundity, and mtDNA and nuclear SNPs to account for relatedness and identify reproductive mode. As expected, we found genetic and morphological diversity to be higher in native than in invasive snails investigated previously, but surprisingly no higher morphological diversity in sexual versus asexual individuals. The relationships between shell morphology, habitat, and fecundity were complex. Shape variation was primarily linked to genetic relatedness but specific environmental factors including flow rate induced similar shell shapes. By contrast, shell size was largely explained by environmental factors. Fecundity was correlated with size but showed trade-offs with shape in increasingly extreme conditions. With increasing flow and toward small springs, the trend of shell shape becoming wider was reversed, i.e., snails with narrower shells were brooding more embryos. We concluded that both genetic and environmental contributions to variation in shell morphology in P. antipodarum likely play an important role in the ability of this species to adapt to a wide spectrum of habitats.     

Phenotypic influences on the reproductive strategy of the facultative sexual rotifer <Emphasis Type="Italic">Brachionus rubens</Emphasis> (Monogononta)

Facultative sexual species employ a dual reproductive strategy (heterogony) comprising primarily asexual reproduction with intermittent sexual reproduction. Given the higher relative costs of sexual reproduction, elucidating the triggers underlying these transitions might help our understanding of the evolution of (obligate) sex in general. Existing hypotheses into how and when facultative sexuals invest into sex focus largely either on environmental (habitat-deterioration and resource-demanding hypotheses) or genetic factors (condition-dependent hypothesis), but tend to lack experimental evidence, especially with respect to within-population variation. To address this deficit, we examined the influence of several variables that potentially affect fitness (food quality, water temperature, physiological acclimation, and all combinations thereof) on both the lifetime reproduction (total number of offspring) and investment into sexual offspring per female in a clonal population of the monogonont rotifer Brachionus rubens. Investment into sex, both absolutely and relative to lifetime reproduction, was tied most closely to and positively correlated with individual fitness (i.e., lifetime reproduction): individuals with higher fitness invested more into sexual reproduction. These results run contra to the condition-dependent hypothesis and indicate an energy-budget analogue of the resource-demanding hypothesis. Furthermore, investment into sex increased after a period of physiological acclimation to the new conditions, probably because of the amelioration of short-term stress effects or clonal selection. Our results underscore that life history and general phenotypic considerations—here, energetic provisioning of offspring, the presence of a sexual resting stage, and the relative timing of sexual versus asexual reproduction—can modify existing hypotheses based either on environmental or genetic factors alone.