Sexual reproduction in Daphnia: interspecific differences in a hybrid species complex

To investigate whether reproductive variation in the Daphnia galeata-cucullata-hyalina hybrid species complex can explain the occurrence of hybrids and backcrosses, a total of 43 clones from three north German lakes were tested for allocation to sexual reproduction under equal stress conditions. Six replicates per clone were followed until the seventh adult instar. I used the following cues to promote sexual reproduction: short photoperiod, water from a crowded Daphnia culture, fish-conditioned water and low food concentration. For each animal, clutch size and clutch sex were recorded. Ephippia, which were empty since the animals were cultured individually, were only produced by D. cucullata and D. cucullata x hyalina (26% and 6% of the broods, respectively), whereas almost all taxa produced males (the range was 2–15%). Intraspecific variation for male and ephippial production was also found. The fact that the taxa tested show different responses to the stimuli of-fered may indicate that there is reproductive isolation between them. This is supported by field data from the Tjeukemeer (The Netherlands) which shows that D. galeata mostly produces ephippia in spring and D. cucullata in autumn. The presence of hybrids with ephippia in both seasons, however, shows that backcrossing is also possible.     

Life-history variation in a hybrid species complex ofDaphnia

Summary Life-history variation was examined among members of theDaphnia longispina group, which consists ofD. galeata, D. hyalina, D. cucullata, and hybrids. Factorial experiments were conducted at two temperatures (14° and 20° C) and two food concentrations (0.2 and 1.0 mg Cl⁻¹). Differences in life-history features (size at maturity, age at first reproduction, size of first clutch, offspring size in first clutch) under the different environmental conditions were assessed among eightDaphnia clones, which represented members of this species complex. Significant differences between parentals and hybrids for most life-history features were observed under various treatments; generally, hybrid clones showed intermediate life-history traits when compared with parentals. When comparisons were made among clones within a given species (i.e.D. galeata, D. galeata xcucullata, D. cucullata), clonal differences were also noted for certain life-history traits. The data are discussed with reference to the formation and maintenance of hybrid species complexes in nature. This paper is dedicated to the memory of Hans Georg Wolf, who died suddenly in May 1990 at the age of 39 years.     

Intra-specific rDNA-ITS restriction site variation and an improved protocol to distinguish species and hybrids in the Daphnia longispina complex

A collaborative research effort was undertaken to evaluate the robustness of a recently developed genetic tool for species identification of members in the morphologically variable Daphnia longispina species complex. This genetic method, based on restriction fragment length polymorphism (RFLP) of the internal transcribed spacer region (ITS) of nuclear ribosomal DNA (rDNA) with restriction enzymes Mwo I and Sau96 I [Billiones et al., 2004. Hydrobiologia 526: 43–53], was applied to many different European populations. Results were compared with two or more independently obtained characters (morphology, allozymes, mitochondrial DNA (mtDNA), or cloned rDNA-ITS sequences). Individuals of most taxa were readily identified, but unexpected ITS-RFLP patterns were found in many individuals indicated by other markers to be D. galeata or one of its hybrids. Among 43 investigated D. galeata populations (902 specimen analysed by ITS-RFLP), deviant RFLP fragment patterns occurred in 26 (i.e., more than half) of the populations. The deviant patterns could be attributed to the loss of one single restriction site in the ITS2 region. This loss made the distinction of D. galeata from other species unreliable, and F1 hybrids could not be identified. Future users should be aware of this shortcoming of the Billions et al. [2004. Hydrobiologia 526: 43–53] protocol. As a solution to this problem, we present an improved genetic identification protocol based on a simple double digestion of the rDNA-ITS region with the restriction enzymes BsrB I and EagI. Sequence analyses of rDNA-ITS clones and preliminary testing indicate that the new protocol is unaffected by the rDNA variation which troubled the Mwo I/Sau96 I protocol. Further, the new protocol identifies all European species of the D. longispina complex, as well as their F1 hybrids. However, a wider screening is required to verify its general utility for all species, since yet unknown variation may occur. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Population structure and genetic diversity in two species of Hawaiian picture-winged Drosophila

Over the last several decades many picture-winged Drosophila have become less common in both geographical distribution and local population size (pers. obs., Foote pers. comm., Montgomerey pers. comm.). Here we report on a study of two Hawaiian Drosophila species, D. engyochracea, and D. hawaiiensis, to determine the impact that changes in population sizes over the past thirty years have had on the genetic diversity of these species. D. engyochracea is known from only two locations on the Island of Hawai'i (Kipuka Ki and Kipuka Pua'ulu), while D. hawaiiensis is currently more wide spread across Hawai'i Island. We collected 65 D. hawaiiensis and 66 D. engyochracea from two forest patches (kipuka) isolated by a 400 year old volcanic ash deposit. DNA sequence data for 515 bases of the mitochondrial gene COII was analyzed for both species to estimate relative total genetic diversity as well as inter-kipuka gene flow. The more wide spread species, D. hawaiiensis, has more genetic diversity (23 vs. 11 unique haplotypes) than the rarer species, D. engyochracea. The distribution of haplotypes in the kipuka is consistent with more gene flow in D. engyochracea than in D. hawaiiensis. Phylogenetic analysis indicates a small number of individuals morphologically identified as one species but have DNA sequence diagnostic for the other species. These results are consistent with these individuals being descendant from hybrids between species.     

Disturbance events affecting phytoplankton biomass,composition and species diversity in a shallow,eutrophic, temperate lake

The seasonal changes in phytoplankton biomass and species diversity in a shallow, eutrophic Danish lake are described and related to different disturbance events acting on the phytoplankton community.Both the spring diatom maximum and the summer bloom of the filamentous blue-green alga, Aphanizomenon flos-aquae (L.) Ralfs, coincided with low values of phytoplankton species diversity and equitability. Diatom collapse was mainly due to internal modifications as nutrient depletion (Si, P) caused by rapid growth of phytoplankton, and increased grazing activity from zooplankton. A large population of Daphnia longispina O.F. Müller in June effectively removed smaller algal competitors, thus favouring the development of a huge summer bloom (140 mm³ l^–1) of Aphanizomenon flos-aquae. Heavy rainfall and storms in late July increased the loss of Apahnizomenon by out-flow and disturbed the stratification of the lake. These events caused a marked decline in phytoplankton biomass but had no effect on species diversity. A second storm period in late August circulated the lake completely and was followed by a rapid increase in phytoplankton diversity, and a change in the phytoplankton community structure from dominance of large, slow-growing K-selected species (Aphanizomenon) to small, fast-growing r-selected species (cryptomonads).     

Experimental interspecific hybridization in Daphnia

Hybridization is a common phenomenon in Daphnia (Cladocera; Anomopoda); interspecific hybrids have been found between several species and hybrids are found in many European lakes. Although much information on the morphology, ecology and genetics of hybrids is available, little is known about the level of reproductive isolation among species or about the relative fitness of hybrids and parental species. In order to facilitate studies on differentiation and speciation processes and comparative experimental studies on hybrids and recombinant genotypes, we present the first successful laboratory crossing experiments of two different Daphnia species, D. galeata and D. cucullata. Males and sexual females from two D. galeata and two D. cucullata clones were reciprocally crossed, juveniles hatched from resting eggs and reared until maturity. Hatching and juvenile survival rates of hybrids were relatively low (12.1% and 24%, respectively). D. galeata and D. cucullata clones vary in their level of successful interspecific matings and in the number of subsequent offspring. In general, hybrid crosses between D. cucullata females and D. galeata males were more successful than reciprocal crosses.     

Intergeneric hybrids between Enarthrocarpus lyratus,a wild species,and crop brassicas

Summary Attempts were made to produce intergeneric hybrids between Enarthrocarpus lyratus, a wild species, and several species of crop brassicas: B. campestris, B. nigra, B. oleracea, B. juncea, B. napus and B. Carinata. Hybrids using E. lyratus as female parent were realized by means of embryo rescue in four combinations — E. lyratus x B. campestris, E. lyratus x B. oleracea, E. lyratus x B. napus and E. lyratus x B. carinata. Reciprocal crosses showed strong pre-fertilization barriers and yielded no hybrids except in one combination — B. Juncea x E. Lyratus — in which a single hybrid could be realized. All of the hybrids were multiplied in vitro through the multiplication of axillary shoots. Morphological and cytological studies confirmed hybridity. All hybrids were completely pollen sterile except for E. lyratus x B. carinata, which showed 2% pollen fertility. Attempts to double the chromosome number through the in vitro application of colchicine to axillary meristems of F₁ hybrids were successful in only one hybrid, E. lyratus x B. oleracea. Cytological studies of the hybrids indicated the presence of a partial homology between the genomes of E. lyratus and crop brassicas. Backcross progenies were raised from all of the five F₁ hybrids to develop malesterile alloplasmic lines.     

内蒙古典型草原建群种羊草基因型多样性抑制群落物种多样性的生态功能

植物群落的物种多样性以及群落建群种的基因型多样性对群落生态功能是否存在交互影响已成为群落生态学研究的热点内容。以内蒙古典型草原群落内常见物种为研究对象,研究了群落物种多样性与建群种羊草(Leymus chinensis)基因型多样性及其交互作用对群落生物量生态功能特性的影响。结果表明:(1)羊草基因型多样性、物种多样性及其交互作用对群落地上、地下和总生物量无显著影响(P0.05);(2)羊草基因型多样性、物种多样性及其交互作用对多样性效应(净多样性效应、互补效应和选择效应)有显著影响(P0.05)。羊草基因型多样性抑制多样性净效应的发挥,且主要抑制互补效应;而物种多样性则促进多样性净效应的发挥,主要表现为选择效应对地上生物量的正效应;(3)互补效应对群落生物量多样性净效应起主要贡献。实验所得结果不仅为探讨多样性效应在物种水平以及群落水平上对群落生物量的影响因素提供了重要启示,而且为内蒙古草原种质资源的保护及合理利用,乃至生态系统的恢复和重建提供理论指导。     

The ecological role of chemical stimuli for the zooplankton: predator-induced morphology in Daphnia

Summary Numerous adaptive predator-induced responses occurred when eight clones representing seven Daphnia (Crustacea: Cladocera) species were tested against three common predators: fourth instar larval phantom midge Chaoborus americanus, adult backswimmer Notonecta undulata, and small sunfish Lepomis macrochirus. The predators were confined within small mesh bags, suggesting that the signal for induction is chemical. The induced responses included longer tail spines, longer heads, smaller bodies, increased egg clutches, and decreased lipid reserves. Each Daphnia species responded to each of the three predators in a unique manner. Induced responses in the above characters showed no significant association. The induced morphological changes are generally consistent with current theories of what is an adaptive response for the various sizes of Daphnia exposed to tactile and visual predators. The abundance of induced responses in these experiments suggests that predator-induced responses are a widespread and ecologically important phenomenon of the freshwater zooplankton.     

Genetic diversity and genetic structure of Decalobanthus boisianus in Hainan Island,China

Decalobanthus boisianus is a native plant of Hainan Island, China, which has caused considerable damage to tropical forest ecosystems in recent decades. Understanding the genetic diversity and structure of this species can facilitate uncovering the molecular mechanism of its invasive ability. Here, we collected 77 individuals of D. boisianus spanning 8 distribution areas with a gradient of human disturbance intensity (i.e., low, moderate, and high disturbance intensity groups) to assess patterns of genetic diversity and structure using inter simple sequence repeat (ISSR) markers. We found that a total of 220 loci were scored with 13 primers using ISSR methods, and that 198 loci were polymorphic. The genetic diversity of D. boisianus among these eight forests decreased with increasing human disturbance intensity. Over 70% of the total genetic variation was present within populations, while less than 30% of variation was found among populations. There was a high gene flow (1.27) among them due to a lack of effective geographic barriers. The mean Nei's genetic distance of D. boisianus populations was found to be relatively small (i.e., 0.07), and the average genetic similarity of the eight populations was high (i.e., 0.93). Our findings indicate that the genetic diversity of D. boisianus correlated to human disturbance density, and that D. boisianus populations in Hainan Island have frequent gene exchange. We suggest that reduce deforestation to decrease human disturbance may be a good way to prevent the invasion of D. boisianus.     

Spatial structure and genetic diversity of three tropical tree species with different habitat preferences within a natural forest

Analyses of the spatial distribution pattern, spatial genetic structure and genetic diversity were carried out using a 33-ha plot in a hill dipterocarp forest for three dipterocarps with different habitat preferences, i.e. Shorea curtisii on the ridges, Shorea leprosula in the valleys and Shorea macroptera both on the ridges and in the valleys. The significant spatial aggregation in small-diameter trees of all the three species was explained by limited seed dispersal. At the large-diameter trees, only S. macroptera showed random distribution and this might further prove that S. macroptera is habitat generalist, whilst S. curtisii and S. leprosula are habitat specific. The levels of genetic diversity estimated based on five microsatellite loci were high and comparable in all the three studied species. As the three studied species reproduced mainly through outcrossing, the observed high levels of genetic diversity might support the fact that the plant mating system can be used as guideline to infer the levels of genetic diversity, regardless of whether the species is habitat specific or habitat generalist. The lack of spatial genetic structure but significant aggregation in the small-diameter trees of all the three species might indicate limited seed dispersal but extensive pollen flow. Hence, if seed dispersal is restricted but pollen flow is extensive, significant spatial aggregation but no spatial genetic structure will be observed at the small-diameter trees, regardless of whether the species is habitat specific or habitat generalist. The inferred extensive pollen flow might indicate that energetic pollinators are involved in the pollination of Shorea species in the hill dipterocarp forests.     

珍稀濒危树种陆均松天然种群结构与空间分布格局

为研究陆均松(Dacrydium pectinatum)种群生存现状及分布格局,对BWL、DLS、JFL三种不同分布区域的陆均松天然种群进行了调查,分析了其天然群落乔木层物种多样性及优势种组成、龄级结构及分布格局三个方面的空间异质性,结果表明:(1)三种陆均松天然群落乔木层物种丰富度及多样性指数均较高,基本表现为BWL>JFL>DLS,且远高于中高纬度温带及亚热带森林群落;群落内多为小密度种群,少有占绝对优势地位的树种,符合热带山地雨林物种组成的一般规律。(2)龄级结构有所差异,BWL和JFL种群呈衰退趋势,共同特点是幼年树数量较少,成年和老年树比例较高;DLS种群幼年树数量在所有个体中比例最高,属于增长型种群。(3)BWL和DLS种群整体呈聚集分布,幼年至老年阶段由聚集分布转变为均匀分布,JFL种群整体及不同发育阶段均呈均匀分布状态;聚集强度表现为DLS>BWL>JFL,幼年树>成年树>老年树。陆均松是热带山地雨林的关键种和建群种,对研究海南热带雨林的起源及其区系特征具有重要意义,针对其生存现状,可根据实际情况,利用人工辅助措施如开辟林窗、合理疏伐...     

Daphnia size structure,vertical migration,and phosphorus redistribution

The timing and magnitude of diel migration in two daphnid assemblages were determined from a series of vertical profiles of daphnid size distribution. Animals were collected concurrently for gut fullness determination. Only large daphnids (> 1.4 mm) migrated, but these animals could account for substantial vertical and diel differences in phosphorus excretion rate. Gut fullness measurements and time courses of diel vertical migration suggested that large Daphnia can cause a net downward flux of phosphorus during summer in thermally stratified lakes.     

Asexual genetic variability inAgavaceae determined with inverse sequence-tagged repeats and amplification fragment length polymorphism analysis

Agaves are succulent monocot plants rich in fibers, sugars and other important compounds. They are also valued as ornamental plants and for their ability to grow in poor soils. In the present study, inverse sequence-tagged repeats (ISTR) and amplified fragment length polymorphism (AFLP) analysis were used to study genetic diversity in differentAgavaceae plant samples. Comparison of the banding patterns between the mother plant and rhizome-derived daughter plants showed that genetic variability is generated during asexual reproduction in these species. Phylogenetic relationships amongAgave species were obtained using unweighted pair-group method, arithmetic average (UPGMA) analysis. Genetic diversity through asexual propagation allows for genetic selection and improvement within these asexually propagated plants.     

Initial genetic diversity enhances population establishment and alters genetic structuring of a newly established Daphnia metapopulation

When newly created habitats are initially colonized by genotypes with rapid population growth rates, later arriving colonists may be prevented from establishing. Although these priority effects have been documented in multiple systems, their duration may be influenced by the diversity of the founding population. We conducted a large‐scale field manipulation to investigate how initial clonal diversity influences temporal and landscape patterns of genetic structure in a developing metapopulation. Six genotypes of obligately asexual Daphnia pulex were stocked alone (no clonal diversity) or in combination (‘high’ clonal diversity) into newly created experimental woodland ponds. We also measured the population growth rate of all clones in the laboratory when raised on higher‐quality and lower‐quality resources. Our predictions were that in the 3 years following stocking, clonally diverse populations would be more likely to persist than nonclonally diverse populations and exhibit evidence for persistent founder effects. We expected that faster growing clones would be found in more pools and comprise a greater proportion of individuals genotyped from the landscape. Genetic composition, both locally and regionally, changed significantly following stocking. Six of 27 populations exhibited evidence for persistent founder effects, and populations stocked with ‘high’ clonal diversity were more likely to exhibit these effects than nonclonally diverse populations. Performance in the laboratory was not predictive of clonal persistence or overall dominance in the field. Hence, we conclude that although laboratory estimates of fitness did not fully explain metapopulation genetic structure, initial clonal diversity did enhance D. pulex population establishment and persistence in this system.     

Chaoborus and fish-mediated influences on Daphnia longispina population structure,dynamics and life history strategies

Summary This study examined the long term effects of predation by larvae of the midge Chaoborus and simulated fish predation on experimental Daphnia longispina populations. Chaoborus predation, relative to fish predation, led to populations composed of larger individuals as a whole, larger egg-bearing individuals, and a larger primiparous instar. Daphnia retained helmets beyond the first instar in response to the presence of Chaoborus. Both types of predation, relative to predator-free controls, reduced prey population size and rates of increase, but increased population death rates. The reduction in population size due to predation led to increased resource availability for individuals remaining in the populations and increased individual fecundity in the predation treatments. The differences noted between the Chaoborus, fish, and control treatments increased with predation intensity.     

No Evidence for Adaptive Micro-Evolution to a Decrease in Phosphorus-Loading of a Daphnia Population Inhabiting A Pre-Alpine Lake

Many polluted lakes in Europe are being restored and phosphorus concentrations have dropped dramatically in these lakes. We studied the genetic structure of Daphnia galeata over the past 30 years in Lake Greifensee, Switzerland, a period during which the phosphorus concentration in the lake reduced dramatically. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from six different time horizons in dated sediments. We compared juvenile growth, size and egg production of D. galeata reared on high-P and low-P algae to investigate whether Daphnia have evolved to grow better on phosphorus-limited algae. Our results indicate that life histories of D. galeata differed significantly between both food types. We also found significant clone effects for size and egg number. But we found no significant interaction between the depth from which the clones were selected and food quality. This means that we found no evidence for adaptive micro-evolution in response to P reduction in the lake. We discuss our results in relation to other studies that found evidence for adaptive micro-evolution in comparable time frames.     

The relationship between temperature,competition and the potential for colonization of a subtropical pond by Daphnia magna

The role that temperature plays in excluding large daphnid species from subtropical and tropical ponds, and competitive relationships between Daphnia laevis and Daphnia magna, are explored. D. magna, a large temperate species is probably not excluded from subtropical or tropical systems by elevated temperatures. However, D. magna was a poorer competitor, under a restricted set of laboratory conditions, than D. laevis, the only pond dwelling daphnid in subtropical Florida. Competition is proposed as one mechanism that may limit the number of daphnid species in subtropical and tropical ponds and lakes. Reduced environmental fluctuations in subtropical and tropical systems (compared with temperate systems) may allow zooplankton populations to reach an environmental carrying capacity where competition limits the number of similar, coexisting species in a habitat.     

Genetic diversity and population structure in Malus sieversii, a wild progenitor species of domesticated apple

Malus sieversii (Lebed.) M. Roem. is a wild progenitor species of the domesticated apple. It is found across a mountainous region of central Asia and has been the focus of several collection expeditions by the USDA-ARS-National Plant Germplasm System. This study used microsatellite variation at seven loci to estimate diversity and differentiation within M. sieversii using several complimentary approaches. Multilocus genotypes were amplified from 949 individuals representing seedling trees from 88 half-sib families from eight M. sieversii populations collected in Kazakhstan. Apportioning of genetic variation was estimated at both the family and site level. Analyses using a hierarchical model to estimate F _st showed that differentiation among individual families is more than three times greater than differentiation among sites. In addition, average gene diversity and allelic richness varied significantly among sites. A rendering of a genetic network among all sites showed that differentiation is largely congruent with geographical location. In addition, nonhierarchical Bayesian assignment methods were used to infer genetic clusters across the collection area. We detected four genetic clusters in the data set. The quality of these assignments was evaluated over multiple Markov Chain Monte Carlo runs using both posterior likelihood and stability of the assignments. The spatial pattern of genetic assignments among the eight collection sites shows two broadly distributed and two narrowly distributed clusters. These data indicate that the southwestern collection sites are more admixed and more diverse than the northern sites.