Detection and distribution of six linear mitochondrial plasmids in the shiitake mushroom,Lentinula edodes

The presence of plasmids was surveyed in 90 wild isolates ofLentinula edodes collected from geographically different world regions. DNA plasmids of different sizes were found in about 80% of the isolates. The plasmids detected were of six kinds, designated as pLE1 (9.0 kb), pLE2 (11.1 kb,=pLLE1 described by other authors), pLE3A (9.8 kb), pLE3B (10.8 kb), pLE3C (12.1 kb), and pLE3D (12.3 kb). Hybridization analysis suggested that pLE1 and pLE2 were distinct plasmid types of different homology groups to each other, and the four other plasmids were variant types belonging to a third homology group. These plasmids had no homology with their host's and non-host's nuclear and mitochondrial genome DNAs. Restriction analysis and electron microscopy indicated that the plasmids are linear in form. Since all six plasmids were transmitted uniparentally in sexual crosses and were consistently associated with the DNA preparations from mitochondria fractionated from mycelia of representative isolates, they were suggested to be located in mitochondria, similar to many other known fungal DNA plasmids. Geographically, pLE1 and pLE2 were widely distributed in natural populations ofL. edodes, while the remaining four plasmids were uniquely present in delimited natural populations. Contribution No. 322 from the Tottori Mycological Institute.     

Distribution, biomass and primary production of an Ahnfeltia tobuchiensis (Ahnfeltiales, Rhodophyta) population in the Bay of Izmena, Kunashir Island

Regularities of distribution and primary production of an Ahnfeltia tobuchiensis (Kanno et Matsubara) Mak. population, an agar-containing red alga, were studied in the Bay ot Izmena. Experiments were conducted in a flow-through system under conditions similar to algal habitats. The population of A. tobuchiensis unattached to the ground may be from a few centimeters to as much as 1 m thick. It has been shown that only the upper part of a stratum 15–20 cm thick receives a sufficient amount ot light to realize its production potential. While 15–20% of photosynthetically active radiation (PAR) of that falling on the water surface reaches the stratum surface, only 0.1% of PAR from that falling on the water surface penetrates through stratum 15 cm thick. It has been shown for A. tobuchiensis that its photosynthetic rate curve during the daytime mainly follows the PAR intensity curve. The highest values of photosynthetic rate have been measured in the afternoon when PAR reaches its maximum. It is noted that a stratum 15–20 cm thick has peak values ot net primary production (NPP) which averages 3.2 g C m^?2 day^?1. The total area of A. tobuchiensis population was 23.4 km², and its biomass was 125 000 tons in this area. On average, the NPP of the A. tobuchiensis population made up in summer and in autumn was 46.8 and 25.0% of its biomass, respectively.     

Effects of grazing on plant species composition and spatial distribution in rangelands of the Sahel

The effects of grazing by livestock on plant species composition and spatial distribution have been studied at Sadoré, Niger. Herbaceous species were recorded in plots of increasing size from 1/64 to 1024 m² in ten fallow plots subjected to five different grazing treatments over the previous three years. Treatments consisted of three intensities of grazing, and of protection from grazing for either 3 or 14 years. For all treatments, the number of species fitted a normal distribution with the log (ln) of the area inventoried. However, the fit improved slightly when the model included two successive log-normal distributions respectively considered as species distribution within a patch and at the patch mosaic scale. Across treatments, the optimal sampling areas averaged 3.8 ± 1.1 m² for the within-patch and 725 ± 113 m² for the mosaic scale distributions. It is argued that similarity between treatments in the overall log-normal distribution resulted from compensations between the divergent trends that affected species distribution within and across patches depending on the grazing status. Long-term protection resulted in a regular spatial arrangement of highly contrasted, but internally homogeneous patches. Heavy grazing ensued the dominance of a few species in contagious patches but also left niches for scattered individuals of other species. Tests of the relative frequency of each plant species, together with the average area needed to record that species, were used to characterize specific response to grazing. A majority of species encountered in old Sahelian fallows were either fostered by grazing, indifferent or tolerant to grazing. However, more than a third of the species appeared sensitive to heavy grazing, and no relationships were found between species response to grazing and palatability.     

Distribution of resting spores of the Lymantria dispar pathogen Entomophaga maimaiga in soil and on bark

Cadavers of late instar Lymantria dispar (gypsy moth) larvae killed by the fungal pathogen Entomophaga maimaiga predominantly contain resting spores (azygospores). These cadavers frequently remain attached to tree trunks for several weeks before they detach and fall to the ground. Density gradient centrifugation was used to quantify resting spores in the soil and on tree bark. Titers of resting spores were extremely high at 0–10 cm from the base of the tree and the number decreased with distance from the trunk of the tree. Titers were also highest in the organic layer of the soil with numbers decreasing precipitously with increasing depth in the soil. While resting spores were obtained from tree bark, densities per unit area were much lower than those found in the organic soil layer at the base of the tree. Field bioassays were conducted with caged L. dispar larvae to compare infection levels with distance from the tree trunk as well as on the trunk. Highest infection levels were found at 50cm from the tree base with lowest infection on the tree trunk at 0.5 m height, although we expected the highest infection levels among larvae caged at the bases of trees, where highest spore titers occurred. Laboratory experiments demonstrated that L. dispar larvae exposed to resting spore- bearing soil at the soil surface became infected while larvae exposed to soil with resting spores buried at least 1 cm below the surface did not become infected.     

Review Geographic differences in host specialization between the symbiotic water mites Unionicola formosa and Unionicola foili (Acari: Unionicolidae)

The host specificity and population genetic structure of the symbiotic water mites Unionicola foili from the host mussel Utterbackia imbecillis and Unionicola formosa from the mussels Pyganodon cataracta, Pyganodon grandis and Anodonta suborbiculata were examined over a broad geographical scale in order to determine the extent to which specialization by these water mites is structured geographically. The behavioural responses of U. foili and U. formosa were highly host-species specific, with adults of both species exhibiting negative phototaxis in the presence of a chemical factor from the species of mussel with which the mites had been associated in the field. The photobehaviour of these water mites in the presence of water from a non-host mussel varied depending on the species in question. Although U. foili from U. imbecillis exhibited negative phototaxis in water modified by A. suborbiculata, mites from this latter host did not exhibit a directional response in U. imbecillis water. Unionicola foili and U. formosa from A. suborbiculata were positively phototactic when they were exposed to water modified by either species of Pyganodon. The photoresponse of U. formosa from P. cataracta and P. grandis was positive in the presence of water modified by U. imbecillis and A. suborbiculta. However, these mussel-mites showed no directional response in water modified by their alternate species of Pyganodon. Unionicola foili and the host-associated populations of U. formosa were examined for allozyme variation at eight loci to determine the pattern and degree of genetic variation. There was a high degree of genetic differentiation when mite populations from the two species of Pyganodon were compared with U. foili or U. formosa from A. suborbiculata. These populational groupings were fixed for different alleles at two enzyme loci. The results of this study indicate that populations of U. formosa from P. cataracta and P. grandis are reproductively isolated from U. foili from U. imbecillis and from U. formosa from A. suborbiculata and contend that host specificity is an important mechanism in restricting gene flow among these populational groupings. Furthermore, this study indicates that specialization among unionicolid water mites can vary geographically, owing to differences in geographic distribution of available hosts and differences in host use.Exp Appl Acarol 22: 683697 © 1998 Kluwer Academic Publishers     

Distributions of the vectors of heartwater, Amblyomma hebraeum and Amblyomma variegatum (Acari: Ixodidae), in Zimbabwe

The tick vectors of heartwater (Cowdria ruminantium infection) in Zimbabwe, Amblyomma hebraeum and Amblyomma variegatum, historically were believed to be confined to the low-lying regions of the south and north-west of the country. However, country-wide surveys performed in 19751980 and 19881991 demonstrated that both species were also established in western parts of the highveld plateau and had started to encroach on the predominantly heartwater-free central and eastern highveld regions. To determine the current distributions of both the vectors and evaluate the potential threat of heartwater to animals in the highveld, a survey of ticks infesting cattle was performed in 1996 at 2994 locations in small-holder and large-scale commercial farming areas throughout Zimbabwe. Amblyomma hebraeum was collected at 1329 locations, A. variegatum at 72 locations and both A. hebraeum and A. variegatum at 13 locations. The results demonstrated that A. hebraeum was present, as previously recorded, throughout the southern half of the country and appeared to have undergone further limited spread into the central and eastern highveld regions. Only the northern-most region of the country appeared to be free of this species. Amblyomma variegatum was collected mainly in the north-west, as previously recorded, but was also found at isolated locations across the central highveld region and along the eastern border with Mozambique. This species was, however, still absent from the southern half and the northern-most regions of the country. An overlap of the distributions of the two species existed within a zone along the southern and eastern regions of the distribution of A. variegatum. These results suggest that the vectors of heartwater are spreading and threaten to introduce heartwater into intensive livestock-producing regions of the country.Exp Appl Acarol 22: 725740 © 1998 Kluwer Academic Publishers     

Optimal sampling and spatial distribution of Ixodes pacificus, Dermacentor occidentalis and Dermacentor variabilis ticks (Acari: Ixodidae)

A common method for sampling tick populations is flagging, which is a method of dragging a white cloth over a plant substrate for a fixed distance along a transect. Flagging over rough physical surfaces or using long subtransect lengths could lead to the underestimation of tick densities. Using estimates of the drop-off rates of adult Ixodes pacificus in flag sampling, optimal sampling schemes (the length and number of subtransects) were examined using the relationships between the tick drop-off rates (c), the tick density and distribution patterns and the roughness of the sampled plant substrate. It was found that the optimal number of subtransects and Lmax, the longest subtransect length which did not significantly underestimate the tick density from c, were affected by the tick density, substrate roughness and tick distribution pattern. This study also showed that the density and distribution of I. pacificus and Dermacentor occidentalis varied greatly over time in the populations sampled, while the Dermacentor variabilis densities were low and showed no significant changes over time. Both I. pacificus and D. occidentalis had clumped distributions along trails and these clumps were aggregated. However, the clump sizes (or individual clump areas) changed significantly over time because of density fluctuation or the movement of ticks. Finally, a positive association between the number of I. pacificus and D. occidentalis adults was observed from 2 m subtransect flag collections in March 1995; no relationship was found in 1994. © Rapid Science Ltd. 1998     

Causal Regularities in the Biological World of Contingent Distributions

Former discussions of biological generalizations have focused on the question of whether there are universal laws of biology. These discussions typically analyzed generalizations out of their investigative and explanatory contexts and concluded that whatever biological generalizations are, they are not universal laws. The aim of this paper is to explain what biological generalizations are by shifting attention towards the contexts in which they are drawn. I argue that within the context of any particular biological explanation or investigation, biologists employ two types of generations. One type identifies causal regularities exhibited by particular kinds of biological entities. The other type identifies how these entities are distributed in the biological world.     

烟蚜在烟株上的垂直分布及其分布型

在烤烟Ｋ－３２６团棵期烟株上,Ⅰ－Ⅲ龄若蚜,Ⅳ龄若蚜＋无翅成蚜在各叶位间的分布量具有显著差异,且这种差异与烟株上国蚜Ｍｙｚｕｓｎｉｃｏｔｉａｎａｅ（Ｂｌａｃｋｍａｎ）的密度有关。