Phenotypic responses of <Emphasis Type="Italic">Spartina anglica</Emphasis> to duration of tidal immersion

Although Spartina anglica C.E. Hubbard continues to be invasive in many countries, this species has experienced a drastic decline in coastal China over the last decade. We hypothesize that changes in the duration of tidal immersion were responsible for this decline because the elevation of the S. anglica-dominated area in coastal China has increased greatly over the last decade. We examined the effects of the duration of simulated tidal immersion and plant material provenance on growth, asexual reproduction, biomass accumulation, and allocation (percent of above-ground biomass to total biomass) of S. anglica in a greenhouse experiment. The provenance of S. anglica did not significantly affect any traits measured except for height, stalk diameter, and leaf area. However, all traits were affected by the duration of immersion. Plants grown under 6 h of immersion were taller and had more leaves, more roots, and larger leaf area than those under 2, 4, 8, and 10 h of immersion. Asexual traits and biomass of the plants grown under 6 h of immersion were significantly larger than those under other immersion durations. The results suggested that S. anglica benefits from tidal immersion and decreasing duration of tidal immersion may have resulted in the decline of the S. anglica populations in coastal China. Thus, controlling the duration of tidal immersion may be an effective way of controlling invasiveness of this species elsewhere in the world.     

Population size, pollination and reproductive success in two endangered Genista species

Habitat fragmentation often leads to small and isolated plant populations as well as decreased habitat quality. These processes can fundamentally disrupt the interactions between plants and pollinators and decrease reproductive success. This concerns especially self-incompatible, non-clonal species that depend on pollination for successful reproduction.In two rare and endangered heathland plant species, Genista anglica and G. pilosa, we examined pollination and reproduction in relation to population size. Eight populations of G. anglica and ten populations of G. pilosa were surveyed in the vicinity of Bremen, NW-Germany. We counted the visits of pollinators (honeybees, bumblebees, and other insects) and determined the reproductive output of the observed shoots.Contrary to our expectation to find increased pollinator visitation rates in larger populations of both Genista species, the number of flower-visiting insects was unrelated to the number of flowering shoots. Increasing shoot length had a positive and increasing temperature a negative impact on the number of visiting honeybees and bumblebees. Despite the general absence of population size effects on pollinator numbers, the number of fruits and seeds in G. anglica increased with increasing population size. Fruit and seed set in G. pilosa were negatively related to the number of ‘other insects’. Our field observations showed that larger populations of both Genista species flowered earlier than smaller populations and much earlier than reported in the literature. Flowering in large populations therefore tends to coincide less well with pollinator abundance, and this may cause a disruption of the temporal coincidence between flowering phenology and pollinator activity.     

Admixture between native and invasive populations may increase invasiveness of Mimulus guttatus

Self-fertilization and admixture of genotypes from different populations can have major fitness consequences in native species. However, few studies have addressed their potential roles in invasive species. Here, we used plants of Mimulus guttatus from seven native North American, three invasive Scottish and four invasive New Zealand populations to address this. We created seeds from self-fertilization, within-population outcrossing, between-population outcrossing within the same range, and outcrossing between the native and invasive ranges. A greenhouse experiment showed that native and invasive plants of M. guttatus suffered to similar degrees from inbreeding depression, in terms of asexual reproduction and biomass production. After outcrossing with plants from other populations, M. guttatus benefited from heterosis, in terms of asexual and sexual reproduction, and biomass production, particularly when plants from native and invasive populations were crossed. This suggests that, when novel genotypes of M. guttatus from the native North American range will be introduced to the invasive ranges, subsequent outcrossing with M. guttatus plants that are already there might further boost invasiveness of this species.     

Density,parasitism, and sexual reproduction are strongly correlated in lake Daphnia populations

Many organisms can reproduce both asexually and sexually. For cyclical parthenogens, periods of asexual reproduction are punctuated by bouts of sexual reproduction, and the shift from asexual to sexual reproduction has large impacts on fitness and population dynamics. We studied populations of Daphnia dentifera to determine the amount of investment in sexual reproduction as well as the factors associated with variation in investment in sex. To do so, we tracked host density, infections by nine different parasites, and sexual reproduction in 15 lake populations of D. dentifera for 3 years. Sexual reproduction was seasonal, with male and ephippial female production beginning as early as late September and generally increasing through November. However, there was substantial variation in the prevalence of sexual individuals across populations, with some populations remaining entirely asexual throughout the study period and others shifting almost entirely to sexual females and males. We found strong relationships between density, prevalence of infection, parasite species richness, and sexual reproduction in these populations. However, strong collinearity between density, parasitism, and sexual reproduction means that further work will be required to disentangle the causal mechanisms underlying these relationships.     

The influence of population density on fission and growth of Holothuria atra in natural mesocosms

Investigation of the population dynamics, asexual reproduction (fission) and growth of holothuroids has been impeded by the difficulty of tagging individuals. We conducted the first tests on the interactions between population density, fission and growth of holothuroids in experimental populations placed in natural mesocosms (microatolls) at One Tree Reef (OTR), Great Barrier Reef. Similarly sized Holothuria atra were translocated to the microatolls in low (LDT) and high (HDT) density treatments. We hypothesised that holothuroids in lower density treatments would have more resources per individual and that this would promote higher frequencies of asexual reproduction and smaller individuals. The seasonal pattern of fission was similar in natural (unmanipulated) and experimental populations, with the maximum number of fission products occurring in winter and spring. The overall density of the LDT (0.19 ind. m^- 2) and HDT (0.59 ind. m^- 2) did not vary over time. This ‘steady state’ suggested that some fission products died and that asexual reproduction compensated for overall mortality and emigration. There was no difference in sediment chlorophyll pigments between treatments indicating that the different densities of H. atra did not affect benthic microalgal biomass. The percentage of fission products was greater in the LDT than the HDT but this difference was not statistically significant, providing some support for the hypothesis that H. atra in the LDT exhibit a higher fission rates. At the end of the experiment H. atra in LDT were significantly longer and heavier than in HDT. H. atra surpassed their initial deployment weight and length after 13 months in the LDT by 115.2% and 45.2% respectively and in the HDT 86.9% and 24.6% respectively, changing from the small to the large phenotype known for this species. This differential growth may be linked to habitat stability and high benthic productivity and demonstrates the phenotypic plasticity of holothuroids and potential to achieve ‘Optimum Individual Size’ with respect to environmental conditions. Our results will assist in finetuning conceptual models on asexual reproduction and future experimental studies on the phenomena of fission and plastic growth in holothuroids.     

Profound Effects of Population Density on Fitness-Related Traits in an Invasive Freshwater Snail

Population density can profoundly influence fitness-related traits and population dynamics, and density dependence plays a key role in many prominent ecological and evolutionary hypotheses. Here, we evaluated how individual-level changes in population density affect growth rate and embryo production early in reproductive maturity in two different asexual lineages of Potamopyrgus antipodarum, a New Zealand freshwater snail that is an important model system for ecotoxicology and the evolution of sexual reproduction as well as a potentially destructive worldwide invader. We showed that population density had a major influence on individual growth rate and early-maturity embryo production, effects that were often apparent even when comparing treatments that differed in population density by only one individual. While individual growth rate generally decreased as population density increased, we detected a hump-shaped relationship between embryo production and density, with females from intermediate-density treatments producing the most embryos and females from low- and high-density treatments producing the fewest embryos. The two lineages responded similarly to the treatments, indicating that these effects of population density might apply more broadly across P. antipodarum. These results indicate that there are profound and complex relationships between population density, growth rate, and early-maturity embryo production in at least two lineages of this important model system, with potential implications for the study of invasive populations, research on the maintenance of sex, and approaches used in ecotoxicology.     

初始克隆分株数对大米草表型可塑性及生物量分配的影响

克隆植物大米草 (Spartina anglica) 目前在我国出现了严重的自然衰退 (Dieback),为了阐明大米草衰退的机理,分析影响大米草形态可塑性的因素与自然衰退之间的相关性,以期为近缘植物互花米草 (S. alterniflora) 这一爆发种群的生物控制提供借鉴,对3种不同初始克隆分株数 (单克隆、三克隆和五克隆) 大米草的克隆生长、生物量累积与分配和异速生长特征进行了野外栽培试验。研究结果表明,初始克隆分株数对间隔子长度影响较弱;初始多克隆的分支强度高于初始单克隆;初始三克隆和五克隆在总生物量 (7.921 5~10.431 7 g 和 8.903 9~10.431 7 g)、地上生物量 (3.396 1~4.255 8 g 和3.618 4~4.338 9 g)、地下生物量 (4.286 9~5.206 6 g 和 5.298 8~6.079 3 g)和根状茎生物量 (1.318 6~1.767 7 g 和 1.499 1~2.038 7 g) 积累上均显著高于初始单克隆,不同初始克隆分株数条件下根生物量差异不显著;初始多克隆倾向于将资源更多地分配给根状茎,而初始单克隆倾向于将更多的资源分配给根系。由此推断,在不同初始克隆分株数条件下,大米草的形态可塑性和生物量分配格局的差异显示出在同样资源格局下,初始多克隆的克隆生殖能力较初始单克隆强。初始多克隆生长的大米草较初始单克隆生长的大米草更能占据优势生境,选择生境“觅养”的能力与克隆繁殖能力更强。     

Biogeography of sexual and asexual populations in Bostrychia moritziana (Rhodomelaceae, Rhodophyta)

Bostrychia moritziana (Sonder ex Kützing) J. Agardh is recorded from many regions around the world. Our laboratory culture investigations have verified a sexual life cycle in isolates from Australia, Venezuela, Colombia, South Africa, Fiji, New Zealand and Indonesia. By contrast, asexual isolates producing successive generations of tetrasporophytes in laboratory culture and, presumably, in the field, are known from Australia. New Caledonia and Japan. In Australia, asexual reproduction is absent only in Victoria. In Western Australia, Northern Territory and Queensland, 99% of the isolates have asexual reproduction. In New South Wales (NSW), asexual and sexual populations are often intermixed. Of the 176 worldwide field collections, 58% were vegetative, 39% were tetrasporic, 2% were female and 1% were male. After several years of observations on the asexual isolates in culture, at least 30 successive asexual tetrasporophytic generations have developed. Only two asexual isolates (3558 and 3575) from NSW have formed a single male and female gametophyte in culture. In a self-cross of 3568, the carpospores developed into tetrasporophytes that recycled asexually. All outcrosses done with normal sexual isolates produced normal carposporophytes and the carpospores developed into tetrasporophytes that also recycled asexually. Asexual populations may arise repeatedly by loss of meiosis in tetrasporangia of sexual populations. Asexual reproduction apparently does not diminish the overall dispersal and abundance in the field. Our present bio-geographic data show that sexually reproducing popuiations of B. moritziana occur worldwide, while asexuaily reproducing populations are confined to the western Pacific. Bostrychia bispora West et Zuccarello, initially described on the basis of its asexual reproduction to distinguish it from B, moritziana, is now reduced to synonymy with B. moritziana.     

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. VI. A GREENHOUSE TEST OF THE SIB-COMPETITION HYPOTHESES

This study tests the hypothesis that competition among groups of sexual and asexual siblings generates an advantage for sexual females. Individual tillers of Anthoxanthum odoratum were planted singly, among other siblings from the same family, and among groups of sexual and asexual siblings from different families in pots in an unheated greenhouse. Unlike previous field experiments, there was little difference between the performance of sexual and clonal tillers after two years, despite strong treatment effects and high mortality. The results demonstrate that sib competition does not generate an advantage for sexual reproduction in biotically simple environments.     

Disease of Red Mangrove (Rhizophora mangley) in Southwest Puerto Rico Caused by Cytospora rhizophorae1

A high frequency of dieback and mortality was found in Rhizophora mangle (red mangrove) in coastal mangrove forests of southwest Puerto Rico. Eight plots were established in a variety of mangrove environments to assess the extent and severity of the dieback and mortality. The imperfect fungus Cytospora rhizopborae was isolated consistently from stem dieback and associated canker tissues. No other pathogenic microorganisms were isolated. Pathogenicity was demonstrated in greenhouse and field inoculation experiments with C. rhizophorae using young R. mangle seedlings. In field studies of seedlings inoculated, mortality was 33 percent, and greenhouse mortality was as high as 50 percent. Cytospora rhizophorae was isolated consistently from diseased mangrove tissue; the fungus was grown in pure culture and then inoculated into healthy red mangroves, causing cankers. Koch's postulates were fulfilled by infecting R. mangle with C. rhizophorae, producing disease symptoms, and successfully re‐isolating C. rhizophorae from the diseased tissue. This is the first documentation of the pathogenicity of C. rhizophorae associated with mortality in red mangrove. In the coastal mangrove forests of southwest Puerto Rico, C. rhizophorae may play an important role in the frequency of dieback and mortality in R. mangle, producing slow stem diameter growth and causing frequent stem and root wounds.     

Extreme genetic diversity in asexual grass thrips populations

The continuous generation of genetic variation has been proposed as one of the main factors explaining the maintenance of sexual reproduction in nature. However, populations of asexual individuals may attain high levels of genetic diversity through within‐lineage diversification, replicate transitions to asexuality from sexual ancestors and migration. How these mechanisms affect genetic variation in populations of closely related sexual and asexual taxa can therefore provide insights into the role of genetic diversity for the maintenance of sexual reproduction. Here, we evaluate patterns of intra‐ and interpopulation genetic diversity in sexual and asexual populations of Aptinothrips rufus grass thrips. Asexual A. rufus populations are found throughout the world, whereas sexual populations appear to be confined to few locations in the Mediterranean region. We found that asexual A. rufus populations are characterized by extremely high levels of genetic diversity, both in comparison with their sexual relatives and in comparison with other asexual species. Migration is extensive among asexual populations over large geographic distances, whereas close sexual populations are strongly isolated from each other. The combination of extensive migration with replicate evolution of asexual lineages, and a past demographic expansion in at least one of them, generated high local clone diversities in A. rufus. These high clone diversities in asexual populations may mimic certain benefits conferred by sex via genetic diversity and could help explain the extreme success of asexual A. rufus populations.     

Pasteuria penetrans for Control of Meloidogyne incognita on Tomato and Cucumber,and M. arenaria on Snapdragon

Meloidogyne incognita and Meloidogyne arenaria are important parasitic nematodes of vegetable and ornamental crops. Microplot and greenhouse experiments were conducted to test commercial formulations of the biocontrol agent Pasteuria penetrans for control of M. incognita on tomato and cucumber and M. arenaria on snapdragon. Three methods of application for P. penetrans were assessed including seed, transplant, and post-plant treatments. Efficacy in controlling galling and reproduction of the two root-knot nematode species was evaluated. Seed treatment application was assessed only for M. incognita on cucumber. Pasteuria treatment rates of a granular transplant formulation ranged from 1.5 × 10⁵ endospores/cm³ to 3 × 10⁵ endospores/cm³ of transplant mix applied at seeding. Additional applications of 1.5 × 10⁵ endospores/cm³ of soil were applied as a liquid formulation to soil post-transplant for both greenhouse and microplot trials. In greenhouse cucumber trials, all Pasteuria treatments were equivalent to steamed soil for reducing M. incognita populations in roots and soil, and reducing nematode reproduction and galling. In cucumber microplot trials there were no differences among treatments for M. incognita populations in roots or soil, eggs/g root, or root condition ratings. Nematode reproduction on cucumber was low with Telone II and with the seed treatment plus post-plant application of Pasteuria, which had the lowest nematode reproduction. However, galling for all Pasteuria treatments was higher than galling with Telone II. Root-knot nematode control with Pasteuria in greenhouse and microplot trials varied on tomato and snapdragon. Positive results were achieved for control of M. incognita with the seed treatment application on cucumber.     

Aspects of sexual failure in the reproductive processes of a rare bulbiliferous plant,<Emphasis Type="Italic"> Titanotrichum oldhamii</Emphasis> (Gesneriaceae), in subtropical Asia

Titanotrichum oldhamii produces both flowers and asexual bulbils on its inflorescences. However, field observations and herbarium collections indicate that seed set is infrequent and that most reproduction is from vegetative bulbils. We have investigated the failure of sexual reproduction and identified four major causes: (1) in the wild, the seed:ovule ratio for open pollination was only 1.9%, in contrast to 10.1% for artificial cross-pollination, implying poor pollinator services. (2) The overall reproductive success was 5–10 times greater in glasshouse pollination treatments compared to field treatments. This suggests suboptimal environmental conditions for seed set in natural habitats, which may eliminate natural seed set completely. (3) Pollination experiments showed that outcrossed populations set significantly more seed and had higher germination rates than intra-populational and self crosses, in both field and glasshouse experiments. T. oldhamii thus appears to benefit from wide outcrossing. Pollen transfer between populations, however, seems to be infrequent because of the rarity and scattered distribution of Titanotrichum populations. (4) Flowers near the apex of the inflorescence are less likely to set seed, especially late in the season when inflorescences convert to bulbil production. In these late flowers, pollen tubes showed poor guidance as they approach the micropyle of the ovules. Even under optimal glasshouse conditions, the average outcrossing success was only 0.229. Almost one-half of the ovules remained undeveloped and 13.5% of ovules aborted after pollination, indicating a strong shift of resource allocation toward vegetative bulbils and rhizome development. Efficient reproduction from asexual bulbils may thus have released Titanotrichum from strong selection for efficient sexual reproduction. However, occasional seed set observed in the wild may be very important for maintaining some genetic diversity in populations, and promoting overall fitness.     

Test of local adaptation to biotic interactions and soil abiotic conditions in the ant-tended <Emphasis Type="Italic">Chamaecrista fasciculata</Emphasis> (Fabaceae)

Few previous studies have assessed the role of herbivores and the third trophic level in the evolution of local adaptation in plants. The overall objectives of this study were to determine (1) whether local adaptation is present in the ant-defended plant, Chamaecrista fasciculata, and (2) the contribution of ant-plant-herbivore interactions and soil source to such adaptation. We used three C. fasciculata populations and performed both a field and a greenhouse experiment. The first involved reciprocally transplanting C. fasciculata seedlings from each population-source to each site, and subsequently applying one of three treatments to one-third of the seedlings of each population-source at each site: control, reduced ant density and reduced folivory. The greenhouse experiment involved reciprocal transplants of population-sources with soil sources to test for a soil-source effect on flower production and local adaptation to soil conditions. Field results showed that ant and herbivore treatments reduced ant density (increasing folivory) and herbivore damage relative to controls, respectively; however, these manipulations did not impact C. fasciculata reproduction or the likelihood of survival. In contrast, greenhouse results showed that soil source significantly affected flower production. Overall, plants in both experiments, regardless of population-source, always had higher reproductive output at one specific site. Native populations did not outperform nonnative ones, causing us to reject the hypothesis of local adaptation. The absence of treatment effects on plant reproduction and the likelihood of survival suggest a limited effect of ants and folivores on C. fasciculata fitness and local adaptation during the study year. Temporally inconsistent effects of biotic forces across years, coupled with the young age of populations, relative proximity of populations and possible counter effects of seed predators may reduce the likelihood of local adaptation in the populations studied.     

Incidence and Pathogenicity of Plant-Parasitic Nematodes Associated with Blueberry (Vaccinium spp.) Replant Disease in Georgia and North Carolina

Blueberry replant disease (BRD) is an emerging threat to continued blueberry (Vaccinium spp.) production in Georgia and North Carolina. Since high populations of ring nematode Mesocriconema ornatum were found to be associated with commercially grown blueberries in Georgia, we hypothesized that M. ornatum may be responsible for predisposing blueberry to BRD. We therefore tested the pathogenicity of M. ornatum on 10-wk-old Rabbiteye blueberries (Vaccinium virgatum) by inoculating with initial populations (Pi) of 0 (water control), 10, 100, 1,000. and 10,000 mixed stages of M. ornatum/pot under both greenhouse (25 ± 2°C) and field microplot conditions. Nematode soil population densities and reproduction rates were assessed 75, 150, 225, and 255, and 75, 150, 225, and 375 d after inoculation (DAI) in both the greenhouse and field experiments, respectively. Plant growth parameters were recorded in the greenhouse and field microplot experiments at 255 and 375 DAI, respectively. The highest M. ornatum population density occurred with the highest Pi level, at 75 and 150 DAI under both greenhouse (P < 0.01) and field (P < 0.01) conditions. However, M. ornatum rate of reproduction increased significantly in pots receiving the lowest Pi level of 10 nematodes/plant compared with the pots receiving Pi levels of 100, 1,000, and 10,000 nematodes 75 DAI. Plant-parasitic nematode populations were determined in commercial blueberry replant sites in Georgia and North Carolina during the 2010 growing season. Mesocriconema ornatum and Dolichodorus spp. were the predominant plant-parasitic nematodes in Georgia and North Carolina, respectively, with M. ornatum occurring in nearly half the blueberry fields sampled in Georgia. Other nematode genera detected in both states included Tylenchorhynchus spp., Hoplolaimus spp., Hemicycliophora spp., and Xiphinema spp. Paratrichodorus spp. was also found only in Georgia. In Georgia, our results indicate that blueberry is a host for M. ornatum and its relationship to BRD warrants further investigation.     

Eco-physiological responses of the declining population Spartina anglica to N and P fertilizer addition

Nitrogen and phosphorus are both important life elements. N, P and combined N-P fertilizers were added to the declining population Spartina anglica Hubbard in coastal China. Some growth parameters and eco-physiological responses of S. anglica to different fertilizer treatments (N, P and combined N-P fertilizer addition with high, medium and low levels, respectively) were measured. The fertilizer addition had a highly significant effect on the dynamics of its height-growth, number of leaves, number of roots and total biomass. Only N addition had a significant effect on leaf area and leaf thickness in all fertilizer treatments. On the dynamics of its height-growth, the effect of N addition was the most apparent, and the effect of N-P addition was not greater than those of N and P addition separately. The photosynthesis rate was enhanced and the yield was the highest with the highest N, the highest N-P and the medium P addition. The rates were higher than those of CK by 19.08 μmol·m^?2·s^?1, 15.47 μmol·m^?2·s^?1 and 11.23 μmol·m^?2·s^?1, respectively. The activity of SOD and POD increased with the treatments after freshwater stress for 14 days. Effects of medium N and P addition were significant for SOD activity. However, POD activity was significantly higher with the treatment of higher N and higher N-P addition. In a word, fertilizer addition improved the growth of the declining population S. anglica. The results indicated that the decline of S. anglica was correlated with the nutriment deficiency in soil, especially with the lack of N.     

Agamospermous seed production of the invasive tussock grass Nardus stricta L. (Poaceae) in New Zealand – evidence from pollination experiments

The breeding system of the European tussock grass Nardus stricta L. (Poaceae) was investigated with pollination experiments. Plants were sampled from two populations at Lake Pukaki, Canterbury, New Zealand, where the species is recognised as an alien invader. Bagging of flowers with king-sized cigarette paper and hand-pollination were used to test for three modes of reproduction in the greenhouse: (1) agamospermy (apomixis), (2) autogamy and self-compatibility, and (3) allogamy (outcrossing). Two control groups without experimental treatments were further tested for seed set under (1) greenhouse and (2) field conditions. The success or failure of all experimental treatments was assessed with seed set and germination trials. All agamospermy treatments showed high seed set and germination proportions arguing for an apomictic mode of reproduction in Nardus stricta. Cross-pollination treatments were also successful making it difficult to estimate the degree of outcrossing, selfing, and agamospermous seed production in Nardus stricta. Fecundity in field populations was considerably reduced, possibly due to environmental factors acting upon seed development during maturation. The reproductive strategy of Nardus stricta might be particularly beneficial during invasion because single tussocks can form reproducing colonies and high reproductive output is ensured even in the absence of pollination. Genetic studies in combination with pollination experiments would be necessary to gain deeper insights into the breeding strategy of Nardus stricta.     

The relative importance of sexual and asexual reproduction in the spread of Spartina alterniflora using a spatially explicit individual-based model

This study investigates a spatially explicit, individual-based model for simulating the spread of invasive smooth cordgrass (Spartina alterniflora) in Yancheng coastal wetlands from 1995 to 2010. The model, which considers the landscape heterogeneity and changes detected by remote sensing, also reveals the relative importance of sexual and asexual reproduction in the spread by global sensitivity analysis. The model was verified as suitable for simulating the range expansion of S. alterniflora. The results show that: (1) although seedling recruitment is low, it significantly contributes to the range expansion of S. alterniflora. Removing sexual propagation greatly reduces the expansion rate. Rapid expansion requires both sexual and asexual reproduction; (2) in the global sensitivity analysis, the most significant affecters of S. alterniflora invasion were seed dispersal distance, adult survival rate and asexual recruitment survival rate. Sexual propagation contributes much more significantly to quick range expansion than asexual reproduction, but asexual reproduction is the main source of recruitment. Invasion control strategies should target a single reproduction mode. Here, limiting the germination and dispersal of seeds is suggested as a realistic strategy for controlling and managing invasion by this species.     

Population Growth of Pratylenchus penetrans on Winter Cover Crops Grown in the Pacific Northwest

Population growth of Pratylenchus penetrans on 13 fall and winter cover crops was studied in the greenhouse and field. All crops except oat cv. Saia supported population growth of P. penetrans in greenhouse experiments, although the response of P. penetrans to oat cv. Saia varied considerably between experiments. The mean ratio of the final population density/initial population density (Pf/Pi) after 16 weeks for P. penetrans added to a greenhouse soil mix was 0.09, whereas Pf/Pi values after 10 weeks for two experiments with naturally infested soil were 0.95 and 2.3. Although P. penetrans increased on sudangrass cv. Trudan 8 and sudangrass × sorghum hybrid cv. SS 222, subsequent incorporation of sudangrass vegetation into soil reduced P. penetrans populations to preplant levels. Field experiments were inconclusive but suggested that oat cv. Saia or rye cv. Wheeler may be better choices for winter cover than weed-contaminated fallow or other crops on P. penetrans-infested sites in the Pacific Northwest.     

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.