Primary Mode Of Reproduction Research Articles

Evidence of High Genetic Diversity and Differences in the Population Diversity of the Eucalyptus Leaf Blight Pathogen Calonectria pseudoreteaudii from Diseased Leaves and Soil in a Plantation in Guangxi, China.

Calonectria pseudoreteaudii is an important causal agent of Eucalyptus leaf blight in southern China. This pathogen causes Eucalyptus tree disease across numerous regions in southern China. In addition to diseased leaves, C. pseudoreteaudii has occasionally been isolated from soil in Eucalyptus plantations. The aim of this study was to clarify whether C. pseudoreteaudii causing Eucalyptus leaf blight in China is mainly clonally reproduced and to determine the potential spreading mechanism of C. pseudoreteaudii between diseased leaves and soil. To this end, 10 polymorphic microsatellite markers were analyzed to detect the genetic diversity of 97 C. pseudoreteaudii isolates from diseased leaves and soil in a Eucalyptus plantation in Guangxi Zhuang Autonomous Region, southern China. The analysis showed that the genetic diversity of the isolates from both the diseased leaves and soil was high. However, the gene and genotype diversity of the C. pseudoreteaudii isolates from diseased leaves were higher than those of the isolates from the soil. Moreover, all genotypes detected in the isolates from the soil were also found in the isolates from the diseased leaves. Structural analyses did not show clear population structures related to the population substrates of the diseased leaves or soil, and molecular variance analyses indicated that no significant genetic differentiation existed between the diseased leaf and soil populations. These results suggest that C. pseudoreteaudii in soil spreads from diseased leaves, and that an asexual cycle is the primary reproductive mode in both diseased leaf and soil populations. This is the first study on the genetic diversity and population structure of C. pseudoreteaudii. The high genetic diversity and spread pathways of this pathogen may pose challenges in controlling the disease. C. pseudoreteaudii from both diseased leaves and soils in Eucalyptus plantations needs to be carefully monitored for disease control and management.

Sperm-dependent asexual species and their role in ecology and evolution.

Sexual reproduction is the primary mode of reproduction in eukaryotes, but some organisms have evolved deviations from classical sex and switched to asexuality. These asexual lineages have sometimes been viewed as evolutionary dead ends, but recent research has revealed their importance in many areas of general biology. Our review explores the understudied, yet important mechanisms by which sperm-dependent asexuals that produce non-recombined gametes but rely on their fertilization, can have a significant impact on the evolution of coexisting sexual species and ecosystems. These impacts are concentrated around three major fields. Firstly, sperm-dependent asexuals can potentially impact the gene pool of coexisting sexual species by either restricting their population sizes or by providing bridges for interspecific gene flow whose type and consequences substantially differ from gene flow mechanisms expected under sexual reproduction. Secondly, they may impact on sexuals' diversification rates either directly, by serving as stepping-stones in speciation, or indirectly, by promoting the formation of pre- and postzygotic reproduction barriers among nascent species. Thirdly, they can potentially impact on spatial distribution of species, via direct or indirect (apparent) types of competition and Allee effects. For each such mechanism, we provide empirical examples of how natural sperm-dependent asexuals impact the evolution of their sexual counterparts. In particular, we highlight that these broad effects may last beyond the tenure of the individual asexual lineages causing them, which challenges the traditional perception that asexual lineages are short-lived evolutionary dead ends and minor sideshows. Our review also proposes new research directions to incorporate the aforementioned impacts of sperm-dependent asexuals. These research directions will ultimately enhance our understanding of the evolution of genomes and biological interactions in general.

Genome of the parasitoid wasp Dinocampus coccinellae reveals extensive duplications, accelerated evolution, and independent origins of thelytokous parthenogeny and solitary behavior.

Dinocampus coccinellae (Hymenoptera: Braconidae) is a generalist parasitoid wasp that parasitizes >50 species of predatory lady beetles (Coleoptera: Coccinellidae), with thelytokous parthenogeny as its primary mode of reproduction. Here, we present the first high-quality genome of D. coccinellae using a combination of short- and long-read sequencing technologies, followed by assembly and scaffolding of chromosomal segments using Chicago + HiC technologies. We also present a first-pass ab initio and a reference-based genome annotation and resolve timings of divergence and evolution of (1) solitary behavior vs eusociality, (2) arrhenotokous vs thelytokous parthenogenesis, and (3) rates of gene loss and gain among Hymenopteran lineages. Our study finds (1) at least 2 independent origins of eusociality and solitary behavior among Hymenoptera, (2) 2 independent origins of thelytokous parthenogenesis from ancestral arrhenotoky, and (3) accelerated rates of gene duplications, loss, and gain along the lineages leading to D. coccinellae. Our work both affirms the ancient divergence of Braconid wasps from ancestral Hymenopterans and accelerated rates of evolution in response to adaptations to novel hosts, including polyDNA viral coevolution.

Sex, males, and hermaphrodites in the scale insect Icerya purchasi *

Androdioecy (the coexistence of males and hermaphrodites) is a rare mating system for which the evolutionary dynamics are poorly understood. Here, we investigate the cottony cushion scale, Icerya purchasi, one of only three reported cases of androdioecy in insects. In this species, female-like hermaphrodites have been shown to produce sperm and self-fertilize. However, males are ocassionally observed as well. In a large genetic analysis, we show for the first time that, although self-fertilization appears to be the primary mode of reproduction, rare outbreeding events do occur in natural populations, supporting the hypothesis that hermaphrodites mate with males and hence androdioecy is the mating system of I. purchasi. Thus, this globally invasive pest insect appears to enjoy the colonization advantages of a selfing organism while also benefitting from periodic reintroduction of genetic variation through outbreeding with males.

Worldwide Genetic Structure Elucidates the Eurasian Origin and Invasion Pathways of Dothistroma septosporum, Causal Agent of Dothistroma Needle Blight.

Dothistroma septosporum, the primary causal agent of Dothistroma needle blight, is one of the most significant foliar pathogens of pine worldwide. Its wide host and environmental ranges have led to its global success as a pathogen and severe economic damage to pine forests in many regions. This comprehensive global population study elucidated the historical migration pathways of the pathogen to reveal the Eurasian origin of the fungus. When over 3800 isolates were examined, three major population clusters were revealed: North America, Western Europe, and Eastern Europe, with distinct subclusters in the highly diverse Eastern European cluster. Modeling of historical scenarios using approximate Bayesian computation revealed the North American cluster was derived from an ancestral population in Eurasia. The Northeastern European subcluster was shown to be ancestral to all other European clusters and subclusters. The Turkish subcluster diverged first, followed by the Central European subcluster, then the Western European cluster, which has subsequently spread to much of the Southern Hemisphere. All clusters and subclusters contained both mating-types of the fungus, indicating the potential for sexual reproduction, although asexual reproduction remained the primary mode of reproduction. The study strongly suggests the native range of D. septosporum to be in Eastern Europe (i.e., the Baltic and Western Russia) and Western Asia.

Species Diversity, Mating Strategy and Pathogenicity of Calonectria Species from Diseased Leaves and Soils in the Eucalyptus Plantation in Southern China.

Many Calonectria species are causal agents of diseases on several forestry, agricultural and horticultural crops. Calonectria leaf blight is one of the most important diseases associated with Eucalyptus plantations and nurseries in Asia and South America. Recently, symptoms of leaf rot and leaf blight caused by Calonectria species were observed in a one-year-old Eucalyptus experimental plantation in GuangXi Province, southern China. To better understand the species diversity, mating strategy and pathogenicity of Calonectria species isolated from diseased tissues and soils, diseased leaves and soils under the trees from ten Eucalyptus urophylla hybrid genotypes were collected. Three hundred and sixty-eight Calonectria isolates were obtained from diseased Eucalyptus leaves and soils under these trees, and 245 representative isolates were selected based on the sampling substrates and Eucalyptus genotypes and identified by DNA sequence analyses based on the translation elongation factor 1-alpha (tef1), β-tubulin (tub2), calmodulin (cmdA) and histone H3 (his3) gene regions, as well as a combination of morphological characteristics. These isolates were identified as Calonectria hongkongensis (50.2%), C. pseudoreteaudii (47.4%), C. aconidialis (1.6%), C. reteaudii (0.4%) and C. auriculiformis (0.4%). This is the first report of C. reteaudii and C. auriculiformis occurrence in China. Calonectria pseudoreteaudii was isolated from both Eucalyptus diseased leaves and soils; the other four species were only obtained from soils. MAT1-1-1 and MAT1-2-1 gene amplification and mating type assignment results showed that C. pseudoreteaudii is heterothallic and an asexual cycle represents the primary reproductive mode, C. reteaudii and C. auriculiformis are likely to be heterothallic and C. hongkongensis and C. aconidialis are homothallic. Based on the genetic diversity comparisons for C. pseudoreteaudii isolates from diseased leaves and soils, we hypothesize that C. pseudoreteaudii in soils was spread from diseased leaves. Both the mycelia plug and conidia suspension inoculations indicated that all five Calonectria species were pathogenic to the two Eucalyptus genotypes tested and the tolerance of the two genotypes differed. It is necessary to understand the ecological niche and epidemiological characteristics of these Calonectria species and to select disease resistant Eucalyptus genotypes in southern China in the future.

Transgenerational effects of herbivory and soil nutrients transmitted via vegetative reproduction in the clonal plant Alternanthera philoxeroides

Epigenetic changes and maternal effects, collectively termed transgenerational effects, allow responses of organisms to environmental factors to be passed between generations. This is well-known in the case of sexual reproduction but little studied in asexual reproduction, which is often the primary mode of reproduction in clonal plants. To test for transgenerational effects via vegetative reproduction in the clonal herb Alternanthera philoxeroides, a first generation of clonal fragments were subjected to crossed nutrient and herbivory treatments, using the insect herbivore Planococcus minor. Stem and root fragments taken from these plants were then grown into a second vegetative generation and subjected to the same nutrient treatments. Adding nutrients increased total N concentration in first-generation fragments; herbivory increased total N slightly in shoots and decreased N slightly in roots. First-generation fragments given higher nutrients produced second-generation fragments with more final total dry mass and stem nodes. This effect was greater in second-generation fragments also given higher nutrients and greater in second-generation fragments derived from stems than in those derived from roots. Herbivory on first-generation fragments decreased growth of second-generation fragments slightly. This effect was greater if first-generation ramets had been given higher nutrients and greater in second-generation fragments derived from roots. Results strengthen evidence that transgenerational effects can be transmitted via vegetative reproduction in plants and show that such effects can be greater when resource availability is higher and can depend on the organ from which offsping are produced.

Asexual reproduction of a few genotypes favored the invasion of the cereal aphid Rhopalosiphum padi in Chile.

BackgroundAphids (Hemiptera: Aphididae) are insects with one of the highest potentials for invasion. Several aphid species are present globally due to introduction events; they represent important pests of agroecosystems. The bird cherry-oat aphid Rhopalosiphum padi (Linnaeus) is a major pest of cereals and pasture grasses worldwide. Here, we report the genetic features of populations of R. padi that colonize different cereal crops in central Chile.MethodsRhopalosiphum padi individuals were collected in central Chile and genotyped at six microsatellite loci. The most frequent multilocus genotype (MLG) was then studied further to assess its reproductive performance across cereal hosts under laboratory conditions.ResultsPopulations of R. padi in Chile are characterized by a low clonal diversity (G/N = 62/377 = 0.16) and the overrepresentation of a few widely distributed MLGs. One of the MLGs constituted roughly half of the sample and was observed in all sampled populations at high frequencies. Furthermore, this putative aphid “superclone” exhibited variations in its reproductive performance on cereals most commonly cultivated in Chile. The sampled populations also exhibited weak signs of genetic differentiation among hosts and localities. Our findings suggest that (1) obligate parthenogenesis is the primary reproductive mode of R. padi in Chile in the sampled range and (2) its introduction involved the arrival of a few genotypes that multiplied asexually.

Climate-woodland effects on population genetics for two congeneric lichens with contrasting reproductive strategies.

Genetic variation is expected to be influenced by the interaction between reproductive mode and dispersal traits on the one hand and environmental and habitat setting affecting establishment success on the other. We evaluated how environmental/habitat setting affects population genetic variation (i.e. variation in genetic diversity and structure) when regulated by contrasting dispersal traits. We used fungus-specific microsatellite markers to examine genetic diversity and structure of two closely related epiphytic lichen fungi that differ in their primary reproductive mode: Nephroma laevigatum (sexually reproducing, n = 191, 10 microsatellites) and N. parile (asexually reproducing, n = 182, 12 microsatellites), along a steep climatic gradient in Scotland. Despite their reproductive differences, we found a high proportion of clones in both species and a background pattern of genetic structure related to climatic gradients. We also demonstrated that woodland connectivity, rather than geographic distance, explained genetic diversity in both species. Environmental/habitat setting, modulated by the reproductive mode of the species, affects genetic diversity and structure, but the putative dissimilarity in their reproductive mode is less important than has been previously assumed. We reinforce the importance of protecting highly connected populations, positioned along a gradient capturing the segregation of gene pool differences in response to climatic variation.

Population Diversity of Grape Phylloxera in California and Evidence for Sexual Reproduction

Until recently the foliar forms of grape phylloxera have been absent or very rare in California, and nodosities have not been common on resistant rootstocks. Foliar phylloxera are now widely spread in the mothervine plantings of grape rootstock nurseries in Yolo and Solano counties. There have also been frequent observations of nodosities on resistant rootstocks. To determine the genetic relationships within and among these seemingly new types of phylloxera, collections were made across California from 2009 to 2011. Foliar-feeding phylloxera strains were collected from rootstock mothervines at the National Clonal Germplasm Repository in Winters, CA, the University of California, Davis (UCD) vineyards, and six commercial rootstock nursery plantings in Yolo and Solano Counties. Root-feeding samples were collected from a rootstock trial at the UCD Oakville research station vineyard in Napa County, which had previously been sampled in 2006 and 2007. Root-feeding samples were also collected from commercial vineyards in Napa and Sonoma Counties, and the UCD vineyard. All samples were tested and genotyped using simple sequence repeat (SSR) markers and the genetic structure of the populations was analyzed. The results identified four genetically distinct populations of phylloxera in California, which were named Davis, Foliar, Napa1 and Napa2. Davis, Napa1 and Napa2 were composed of root-feeding samples. Multilocus genotypes (MLG) with identical DNA fingerprint profiles were detected in the 2006–07 samples from the Oakville research station. More genetic divergence was observed in the Davis, Napa1 and Napa2 populations with evidence for sexual reproduction between members of Napa1 and Napa2. The Foliar population consisted of only foliar-feeding samples with MLGs that were not detected prior to 2009 and asexual reproduction was clearly the primary reproductive mode.

Seed dispersal at alpine treeline: an assessment of seed movement within the alpine treeline ecotone

AbstractAlpine treelines are expected to advance to higher elevations in conjunction with global warming. Nevertheless, the importance of treeline reproductive patterns and seed dispersal within the alpine treeline ecotone remains unresolved. In this study, we address two research questions at mountain hemlock treelines on the Kenai Peninsula, Alaska: (1) What is the primary mode of reproduction and (2) are seeds leading to recruitment derived from within the local treeline populations or are they arriving from more distant seed sources? To answer these questions, we exhaustively sampled mountain hemlock individuals along a single mountain slope, and genotyped single nucleotide polymorphisms using double‐digest restriction site‐associated DNA sequencing. First, we assessed mode of reproduction by determining the proportion of sampled individuals with identical multilocus genotypes that are the product of clonal reproduction. Second, we used a categorical parentage analysis to identify parent–offspring pairs, so that the proportion of treeline reproduction events could be spatially quantified and dispersal distance measured. We identified sexual reproduction as the primary mode of reproduction at our study site. Seedling establishment was characterized by extensive seed immigration and gene flow into the ecotone. The average dispersal distance was 73 m with long‐distance dispersal identified as dispersal occurring at distances greater than 450 m. We found that production of seeds within the alpine treeline ecotone is not necessarily a requirement for treelines to advance to higher elevations in response to climate change. The extensive seed dispersal and gene flow into the alpine treeline ecotone is likely sufficient to propel the ecotone higher under more favorable climate.

Self-Fertilization and the Role of Males in Populations of Tadpole Shrimp (Branchiopoda: Notostraca: Triops).

Self-fertilization has both negative and positive fitness effects on species evolution. Selfing can increase inbreeding depression, thereby decreasing genetic diversity. In contrast, self-fertilization can preserve beneficial gene combinations and facilitate colonization success. Within the class of crustaceans Branchiopoda, selfing is a primary reproductive mode. Some species of Triops, in the family Notostraca, are a few of the animal species thought to have a mixed mating system between hermaphrodites and males termed androdioecy. The objective of this study is to validate the reproductive mode utilized by Triops newberryi in southern New Mexico by the use of progeny arrays and population simulations. Individuals were reared in the lab from dried soil collected from temporary ponds inhabited by T. newberryi The adults reared and the encysted embryos contained within their brood pouches were genotyped using 7 T. newberryi specific microsatellite markers to determine the relatedness between parent and offspring. Overall microsatellite diversity was low with few heterozygous individuals and limited polymorphisms. Simulated populations and allele segregation analysis suggest hermaphroditism is the primary reproductive mode for T. newberryi In addition, based on the offspring's alleles, there was no direct evidence that a male (ovisacless) T. newberryi outcrossed with a female. Population simulations further suggest that the rate of successful outcrossing events must be low and could explain why outcrossing was not observed in the laboratory rearing trials.

Temporal patterns and behavioral characteristics of aggregation formation and spawning in the Bermuda chub (Kyphosus sectatrix)

Reef fish spawning aggregations are important life history events that occur at specific times and locations and represent the primary mode of reproduction for many species. This paper provides detailed descriptions of aggregation formation and mass spawning of the Bermuda chub (Kyphosus sectatrix). Spawning coloration and gamete release of K. sectatrix were observed and filmed at the Grammanik Bank, a deep spawning aggregation site used by many different species located on the southern edge of the Puerto Rican shelf 10 km south of St. Thomas, US Virgin Islands. Underwater visual surveys using technical Nitrox and closed circuit re-breathers were conducted from December 2002 to March 2013 and documented spatial and temporal patterns of movement and aggregation formation along 1.5 km of mesophotic reef. The largest aggregations of K. sectatrix (>200 fish) were observed on the Grammanik Bank January to March from 0 to 11 d after the full moon with peak abundance from 60 to 80 d after the winter solstice across all survey years. Aggregation formation of K. sectatrix coincided with the spawning season of Nassau (Epinephelus striatus) and yellowfin (Mycteroperca venenosa) groupers. These spatial and temporal patterns of aggregation formation and spawning suggest that K. sectatrix, an herbivore, may also be a transient aggregating species. On several occasions, chubs were observed both pair spawning and mass spawning. Color patterns and behaviors associated with aggregation and spawning are described and compared to spawning characteristics observed in other species, many of which are similar but others that appear unique to K. sectatrix. This represents the first report of a kyphosid species aggregating to spawn and illuminates a portion of the poorly understood life history of the Bermuda chub.

Seed germination and early development in seedlings of Noveloa coulteriana (Podostemaceae)

Unlike other aquatic plants, in Podostemaceae sexual reproduction is the primary mode of reproduction. Thus, germination and seedling establishment are essential processes in the life history of these plants, as they determine their distribution, survival, and abundance. Few studies have examined germination and early seedling development of the Mexican species of this family. In this paper, we experimentally evaluated the influence of light quality (white, red, far red, and darkness), storage time (three months, six months, and 10 years) and temperature (constant 25°C and alternate 15–25°C) on Noveloa coulteriana germination. Seeds were positive photoblastic, and showed high germination capacity in white and red light treatments (>80%). In contrast to previously studied Mexican podostemads, N. coulteriana seeds are recalcitrant. Seeds were also thermoblastic; germination rate was slower in alternate temperature compared to constant temperature. Additionally, using electron microscopy, we observed ex situ seedling development during one month (until its senescence). Micromorphologically, N. coulteriana seedlings showed the characteristic developmental pattern of the subfamily Podostemoideae, where the embryonic shoot and root meristems are lost. Our results indicate that environmental factors such as light quality or temperature fluctuation modify seed germination of N. coulteriana and may play an important role in the reestablishment of natural populations.

Range‐wide population genetic structure of the Caribbean sea fan coral, Gorgonia ventalina

The population structure of benthic marine organisms is of central relevance to the conservation and management of these often threatened species, as well as to the accurate understanding of their ecological and evolutionary dynamics. A growing body of evidence suggests that marine populations can be structured over short distances despite theoretically high dispersal potential. Yet the proposed mechanisms governing this structure vary, and existing empirical population genetic evidence is of insufficient taxonomic and geographic scope to allow for strong general inferences. Here, we describe the range-wide population genetic structure of an ecologically important Caribbean octocoral, Gorgonia ventalina. Genetic differentiation was positively correlated with geographic distance and negatively correlated with oceanographically modelled dispersal probability throughout the range. Although we observed admixture across hundreds of kilometres, estimated dispersal was low, and populations were differentiated across distances <2 km. These results suggest that populations of G. ventalina may be evolutionarily coupled via gene flow but are largely demographically independent. Observed patterns of differentiation corroborate biogeographic breaks found in other taxa (e.g. an east/west divide near Puerto Rico), and also identify population divides not discussed in previous studies (e.g. the Yucatan Channel). High genotypic diversity and absence of clonemates indicate that sex is the primary reproductive mode for G. ventalina. A comparative analysis of the population structure of G. ventalina and its dinoflagellate symbiont, Symbiodinium, indicates that the dispersal of these symbiotic partners is not coupled, and symbiont transmission occurs horizontally.

Population Genetic Variation in the Tree Fern Alsophila spinulosa (Cyatheaceae): Effects of Reproductive Strategy

BackgroundEssentially all ferns can perform both sexual and asexual reproduction. Their populations represent suitable study objects to test the population genetic effects of different reproductive systems. Using the diploid homosporous fern Alsophila spinulosa as an example species, the main purpose of this study was to assess the relative impact of sexual and asexual reproduction on the level and structure of population genetic variation.Methodology/Principal FindingsInter-simple sequence repeats analysis was conducted on 140 individuals collected from seven populations (HSG, LCH, BPC, MPG, GX, LD, and ZHG) in China. Seventy-four polymorphic bands discriminated a total of 127 multilocus genotypes. Character compatibility analysis revealed that 50.0 to 70.0% of the genotypes had to be deleted in order to obtain a tree-like structure in the data set from populations HSG, LCH, MPG, BPC, GX, and LD; and there was a gradual decrease of conflict in the data set when genotypes with the highest incompatibility counts were successively deleted. In contrast, in population ZHG, only 33.3% of genotypes had to be removed to achieve complete compatibility in the data set, which showed a sharp decline in incompatibility upon the deletion of those genotypes. All populations examined possessed similar levels of genetic variation. Population ZHG was not found to be more differentiated than the other populations.Conclusions/SignificanceSexual recombination is the predominant source of genetic variation in most of the examined populations of A. spinulosa. However, somatic mutation contributes most to the genetic variation in population ZHG. This change of the primary mode of reproduction does not cause a significant difference in the population genetic composition. Character compatibility analysis represents an effective approach to separate the role of sexual and asexual components in shaping the genetic pattern of fern populations.

Reproduction of a Robinia pseudoacacia population in a coastal Pinus thunbergii windbreak along the Kujukurihama Coast, Japan

We investigated reproduction of the clonal tree Robinia pseudoacacia in a Pinus thunbergii windbreak. Microsatellite polymorphism analysis revealed that asexual reproduction through horizontal root elongation and ramet formation was the primary mode of reproduction in this population. Germination experiments indicated that the inability of established genets to produce viable seeds inhibited sexual reproduction. The boundaries between genet areas were relatively distinct, indicating that ramets within genets spatially excluded one another. Analyses of the number of annual rings and thickness of excavated horizontal roots provided new insight regarding photosynthate transfer through horizontal roots. The number of annual rings and thickness were highest for horizontal roots near the base of ramets and decreased along portions far from ramets. This result indicates that the new horizontal root is supplied with photosynthate from the mother ramet for a few years only, and that resource translocation between neighboring ramets within the same clone ceases after several years. Hypotheses about horizontal root expansion and cessation were considered to explain the exclusive distribution pattern of the horizontal root between genets.

Evolutionary history of Caenorhabditis elegans inferred from microsatellites: evidence for spatial and temporal genetic differentiation and the occurrence of outbreeding.

Although diverse biological disciplines employ the nematode Caenorhabditis elegans as a highly efficient laboratory model system, little is known about its natural history. We investigated its evolutionary past using 10 polymorphic trinucleotide and tetranucleotide microsatellites, derived from across the whole genome. These microsatellites were analyzed from the 35 previously available natural isolates from different parts of the world and also 23 new strains isolated from northwest Germany. Our results highlight that C. elegans lineages differentiate genetically with respect to geographic distance and, to a lesser extent, differences in the time of strain isolation. The latter indicates some turnover of strain genotypes at specific locations. Our data also demonstrate the coexistence of highly diverse genotypes in the population from northwest Germany, which is best explained by recent migration events. Furthermore, selfing is confirmed as the primary mode of reproduction for this hermaphroditic nematode in nature. Importantly, we also find evidence for the occurrence of occasional outbreeding. Taken together, these results support the previous notion that C. elegans is a colonizer, whereby selfing may permit rapid dispersal within new habitats even in the absence of potential mates, whereas occasional outcrossing may serve to compensate for the disadvantages of inbreeding. Such information about the natural history of C. elegans should be of great value for an in-depth understanding of the complexity of this organism, including its multifaceted developmental, neurological, or molecular genetic pathways.

DYNAMICS OF OAK (QUERCUS MARILANDICA AND Q. STELLATA) REPRODUCTION IN AN OLD-GROWTH CROSS TIMBERS FOREST

We present a case study from three topographically distinct stands in an old-growth cross timbers forest in north central Oklahoma. We determined the primary mode of reproduction, i.e., sprouting versus seedling, and population dynamics of small Quercus marilandica and Q. stellata reproduction (≤ 1 m height) in relation to previous disturbance events and changes in site conditions. Approximately 99 percent of the reproduction were of sprout origin and most were stump sprouts (root crown diameter > 5 cm). Surprisingly, root sprouts were an important component of reproduction across stands, comprising 24 and 30 percent of stems for Q. marilandica and Q. stellata, respectively. Mode of reproduction varied by stand, with seedling sprouts (root crown diameter ≤ 5 cm) occurring more frequently on the most xeric site. Stem growth rates increased from xeric to mesic stands and were highest for stump sprouts. Stem longevity was similar across stands and mode of reproduction and was probably limited by low available light. Despite the relatively short life span of aboveground stems (x̄ ∼6 years), tap-roots of seedling sprouts were capable of prolonged persistence (x̄ ∼20 years) through recurrent dieback and sprouting. Stem age structure indicated disturbance encouraged sprouting, as evidenced by increasing populations following fire and declining densities in the absence of perturbation. Seedling establishment for both species appeared to be episodic, as determined by age of tap-roots. Establishment by acorns probably provides a minimal input to oak populations on a yearly basis, but is an important component in the auto-accumulation of oak reproduction over time.

Factors affecting the observed sex ratio in parthenogenetic Schwiebea elongata (Acari: Acaridae) in Japan

The acarid mite, Schwiebea elongata, was demonstrated to reproduce via thelytokous parthenogenesis, but males were observed in laboratory populations. Experiments demonstrated that male sex ratio was not affected by different food substrates nor temperature variation from 10-30°C. Artificial selection of daughters and sisters of females which produced more male offspring increased the observed male ratio in laboratory colonies from 0 to 0.30 in five generations. Males appeared normal with respect to observed copulation and antagonistic behaviors, although actual sperm transfer or fertilization were not demonstrated. Although thelytoky appears to be the primary reproductive mode in this species, normal bisexual reproduction or deuterotoky cannot yet be ruled out.