MULTIPLE ROUTES OF SEXUALITY IN ALEXANDRIUM TAYLORI (DINOPHYCEAE) IN CULTURE1

Alexandrium taylori Balech is a cyst‐forming dinoflagellate species responsible for recurrent blooms in Mediterranean coastal waters. The nuclear development of the cells during the sexual cycle and the effect of different external nitrate and phosphate levels were studied. Nuclear fusion of gametes occurred 6–12 h after the complete cytoplasmic fusion. The U‐shaped nuclei fused through the end of one nucleus and the mid‐area of the other. The mobile and biflagellated zygote had a large, U‐shaped nucleus and may follow three different fates: direct division, short‐term encystment (ecdysal), and long‐term encystment (resting). Ecdysal cysts may divide in >24–96 h into two, four, six, or eight cells before germinating. Meiosis presumably occurred in three locations: in the planozygote, within the ecdysal cyst, and in the planomeiocyte (germling) liberated either from ecdysal or resting cysts. The effects of nutrients on these routes were studied in individually isolated sexual stages. (1) Direct divisions occurred mainly under replete conditions (L1), whereas no direct planozygote divisions were recorded in media with no phosphate added (L‐P). (2) Short‐term encystment was larger in media lacking phosphate (L‐P and L/30) than in medium with no nitrate added (L‐N) or under replete conditions (L1). (3) Long‐term encystment was only observed in medium with no nitrate added (L‐N). The long‐lived resting cyst, not previously described for this species, had a clear double wall, an irregular shape, a flat morphology, and a middle orange spot. No cysts germinated in 1–2 months, whereas 86% of the cysts germinated 2–3 months after being formed. A flow cytometry analysis showed that sexual induction and zygote formation were very fast and highly common processes, zygotes being nearly half of the population at days 3 and 5 after the induction of sexuality in the cultures.     

Economic damage by Limothrips denticornis in rye, triticale and winter barley

Abstract:  Limothrips denticornis is a common thrips infesting rye, triticale and winter barley in Sweden. Field experiments with insecticides were carried out 1983–1996 in farmers' fields to investigate the damage to the plant and to evaluate the relationship between thrips density and crop yield losses. The total number of experiments was 99 in rye, 41 in triticale and nine in winter barley. The mean number of thrips per leaf sheath was around 8 in all three crops. The highest number found was 27 thrips per leaf sheath. The population density in the leaf sheath was correlated with the damage observed on the leaf sheath as percentage damaged surface. The effect on the leaf sheath alone contributed to a significant proportion of the yield loss. Thrips in the ear and aphids also contributed to the damage but with multiple regression methods it was possible to estimate the damage done solely by L. denticornis . The yield loss caused by L. denticornis was about the same in all three crops, with a variation between 21 and 26 kg per thrips per leaf sheath and hectare. Compared with the yield loss for aphids, which is about 40 kg per aphid per tiller and hectare, this is considerably lower. The economic injury level was found to be about 14 thrips per straw. The economic threshold for L. denticornis has to be decided when the females are flying into the crop and it was found to be 0.5 females per straw at earing. Economic damage by L. denticornis alone was estimated to be 20% of the total number of experiments for all three crops but economic damage for the three pests together was estimated at 40% of the total number of experiments.     

Effects of fish farm effluents on egg-to-fry development and survival of brown trout in artificial redds

Egg-to-fry development and survival of brown trout Salmo trutta were compared in two rivers of the Pyrenean piedmont, the Nive d'Arnéguy, with few human activities, and the Nive des Aldudes, with many anthropogenic activities including 11 fish farms and two sewage treatment plants (STPs). Survival was estimated between spawning (early December) and emergence time (early March) by means of capsules, 7 cm³ in volume, filled with green eggs then inserted into the gravel of artificial redds at spawning sites. In the Nive des Aldudes, three sets of conditions were studied: a fish farm near the springs, a fish farm and an STP in a village, and a series of fish farms and an STP in a village. In each situation, two artificial redds were created upstream and two others downstream from the fish farms. In the Nive d'Arnéguy, four sites were equipped: each of the two downstream sites with two artificial redds, and the two upstream sites with one redd. Substratum characteristics (proportion of fine particles) and the quality of surface and interstitial water (oxygen content, ammonia and nitrite nitrogen) were periodically measured. There was no redd substratum difference between sites upstream and downstream of fish farms. Survival to fry emergence was higher in the Nive d'Arnéguy (63·5%) than in the Nive des Aldudes (47·7%). In this latter river, the nitrogen released from the STPs was 0·5% that from fish farms. Fish farming impaired survival close to fish farm effluents (31·6%), as compared to survival upstream (63·6%), and induced a development delay during the yolk-sac fry stage. These differences were mainly linked to a drop in the dissolved oxygen content in interstitial water induced by the nitrogen flow in surface and interstitial waters.     

Characteristics of brown trout males influence growth and survival of their offspring

Half-sib groups of juvenile brown trout Salmo trutta (0+ years) having fathers with different characteristics were compared in several respects. Two kinds of experiments were performed, aquarium observations in the laboratory and stocking in a semi-natural stream. It was found in the aquarium experiments that the offspring of fathers with larger adipose fins were more active and were probably better at defending territories. Offspring of males chosen by females had higher growth rate. The feeding rate was higher in juveniles having a less aggressive father or a father that was chosen by the mother. The stream experiments showed that offspring of dominant males had higher growth rates, and that offspring of less aggressive fathers had higher survival. Thus, characteristics of the males (fathers) influences the quality of the offspring, and there seem to be benefits for the females both to choose among potential mates and to spawn with the most dominant male. As these male characteristics are never regarded during the process of artificial breeding in hatcheries, the results indicate a basis for divergence between hatchery and wild populations.     

Changes in root development of Arabidopsis promoted by organic matter from oxisols

The presence of ramified and abundant lateral roots is fundamental to plant growth on highly weathered soils. In this work, the effects of humic acid (HA), fulvic acid and hexanic–methanolic (HM) extract, after alkaline extraction from topsoil of seven different oxisols, on the development of roots of Arabidopsis thaliana was evaluated. Furthermore, we used another emergent plant model, that is, micro‐tom (MT) tomatoes with size similar to that of Arabidopsis to test the effects of HA. It was observed that both humic fractions and the HM extract were able to change the root development, improving the number of lateral roots and their development in comparison to control plants. The promotion of root growth by the three organic matter fractions was higher than that observed by 10^?6 mol L^?1 indole acetic acid. The treatment of MT tomato mutant, less sensitive to auxin, with HA did not promote the emergence of lateral roots, being an additional indication of auxin‐like activities of HA. However, some organic matter fractions exhibited, besides promotion of lateral roots number, increase in the length of principal root, which is not a typical auxin effect, indicating that these substances could contain other physiologically active substances.     

Cascading costs: An economic nitrogen cycle

The chemical nitrogen cycle is becoming better characterized in terms of fluxes and reservoirs on a variety of scales. Galloway has demonstrated that reactive nitrogen can cascade through multiple ecosystems causing environmental damage at each stage before being denitrifled to N2. We propose to construct a parallel economic nitrogen cascade (ENC) in which economic impacts of nitrogen fluxes can be estimated by the costs associated with each stage of the chemical cascade. Using economic data for the benefits of damage avoided and costs of mitigation in the Chesapeake Bay basin, we have constructed an economic nitrogen cascade for the region. Since a single tonne of nitrogen can cascade through the system, the costs also cascade.Therefore evaluating the benefits of mitigating a tonne of reactive nitrogen released needs to consider the damage avoided in all of the ecosystems through which that tonne would cascade.The analysis reveals that it is most cost effective to remove a tonne of nitrogen coming from combustion since it has the greatest impact on human health and creates cascading damage through the atmospheric, terrestrial, aquatic and coastal ecosystems. We will discuss the implications of this analysis for determining the most cost effective policy option for achieving environmental quality goals.     

The influence of salt and nitrogen on herbivore abundance: direct and indirect effects

We report on the influence of experimentally increased interstitial salinity and plant nitrogen on the abundance of the delphacid planthopper, Prokelisia marginata (Van Duzee) (Homoptera: Delphacidae), which feeds on salt marsh cordgrass, Spartina alterniflora. We also report the effects of these treatments on parasitism of P. marginata eggs by the fairyfly parasitoid, Anagrus sophiae (Hymenoptera: Mymaridae). Soil salinity was significantly elevated following the addition of salt pellets broadcast over the ground and plant foliar nitrogen was significantly increased after the addition of fertilizer. The addition of fertilizer increased P. marginata densities on Spartina but addition of salt did not. Neither treatment significantly affected levels of egg parasitism by A. sophiae. In this system direct effects of plants on their herbivores via changes in plant chemistry appear more important than indirect effects of plants on herbivores via their natural enemies. Received: 1 August 1997 / Accepted: 29 September 1997     

Impact of a parasitic plant on the zonation of two salt marsh perennials

Animal, fungal, and bacterial consumers can have dramatic effects on the structure of plant communities, often by consuming dominant competitors and indirectly increasing the abundance of inferior competitors. We investigated the role of a consumer plant, the parasite Cuscuta salina, on plant zonation in a western salt marsh. Cuscuta had a strong host species preference in experiments, disproportionally infecting Salicornia virginica, the dominant competitor in most of the marsh. In plots with Cuscuta, which infected 18% of our study area over a 3-year period, Salicornia cover decreased and the cover of Arthrocnemum increased substantially in comparison to plots without Cuscuta. Deep in the Salicornia zone, the cover of Arthrocnemum in Cuscuta-infected plots increased by 558% in 1 year relative to uninfected plots. At the ecotone, the cover of Arthrocnemum in Cuscuta-infected plots increased by only 41% during the same time interval. These data suggest that the relative benefit of a consumer to a less-preferred, subordinate competitor may be strongest where competition is the most asymmetrical as predicted by recent theoretical models. By weakening the competitive dominant, which in the absence of the parasite can create virtual monocultures, Cuscuta enhanced community diversity and altered the ecotone between Salicornia and Arthrocnemum. Cuscuta patches were highly dynamic at the ecotone between Salicornia and Arthrocnemum, and thus the changes we measured in our sample plots were likely to be representative of large portions of the marsh. Our findings emphasize the importance of trophic interactions in salt marsh structure and zonation. Received: 23 April 1997 / Accepted: 10 October 1997     

Spore Dimorphism in a Microsporidan Isolate*

A microsporidan isolate currently considered to represent a mixed infection of Nosema necatrix Kramer, 1965 and Thelohania diazoma Kramer, 1965 was subjected to cultivation in hosts held at various temperatures. The ratio of the Nosema (monospore) to the Thelohania (octospore) forms at these temperatures was found to vary from 1:1 at 16 C to 1:0 at 32 C. Isolation technics using mechanical, temperature and temporal methods separated monosporous from octosporous forms for inoculation purposes. However, microscopic examination of hosts receiving these inocula revealed the presence of both monospores and octospores. Electrophoretic analysis of monospores and monospore-octospore mixtures indicated equivalent hydrophobic protein spectra. These observations suggest that this isolate has the ability to produce either single spores or spores in groups of eight. This microsporidan was not considered a member of the genus Stempellia since spores in groups of 2 or 4 were not observed. Retention of the name Nosema necatrix Kramer is suggested.