Depth limits and minimum light requirements of freshwater macrophytes

1. Data for maximum colonization depth (Z_c) of five groups of submerged macrophytes and light attenuation were collected for forty-five Danish lakes and 108 non-Danish lakes. The macrophyte groups were bryophytes, charophytes, caulescent angiosperms, rosette-type angiosperms and Isoetes spp. 2. The data showed systematic differences among the groups in the relationship of Z_c to water transparency. In lakes with low transparency (Secchi disc transparency (Z_s) less than 7 m) caulescent angiosperms and charophytes penetrated deepest followed by bryophytes and Isoetes spp. In more transparent lakes bryophytes grew deepest, followed by charophytes, caulescent angiosperms and Isoetes spp. Rosette-type angiosperms had the lowest Z_c in all types of lakes. Charophytes and caulescent angiosperms had similar depth limits in lakes with Z_s < 4 m but charophytes grew deeper in more transparent lakes. The depth limits of both groups were independent of light penetration in lakes with very low transparency (Z_s < 1 m). The annual light exposure for the deepest growing macrophytes (bryophytes) was 20–95 mol photons m^–2. 3. The relationship between Z_c, macrophyte type and lake transparency could be explained by three distinct processes regulating Z_c. In lakes with low transparency (Z_s < 1 m), tall macrophytes (caulescent angiosperms and charophytes) compensate for light limitation by shoot growth towards the water surface and Z_c is therefore independent of transparency. In lakes with medium transparency (1 m < Z_s < 4 m) Z_c for angiosperms, charophytes and Isoetes spp. is constrained by light attenuation in the water column, corresponding to a linear relationship between Z_c and Z_s. This pattern also applies to bryophytes, despite lake transparency. In transparent lakes, the minimum light requirement at Z_c increased with increasing transparency for angiosperms, charophytes and Isoetes spp. 4. The minimum light requirements among submersed macrophytes (including marine macroalgae) depend on their plant-specific carbon value (plant biomass per unit of light-absorbing surface area) for the species/group, indicating that the light requirements of submersed plants are tightly coupled to the plants’ possibility to harvest light and hence to the growth form. 5. The light requirements increased on average 0.04% surface irradiance per degree increase in latitude corresponding to an average decrease in Z_c of 0.12 m per degree latitude.     

Problems of isolating Borrelia burgdorferi from ticks collected in United Kingdom foci of Lyme disease

Abstract. Many isolates of Borrelia burgdorferi have been obtained from ticks and vertebrate tissues collected in North America and continental Europe but only one established culture of United Kingdom Borrelia burgdorferi has been recorded. In this paper we report the isolation of B.burgdorferi from one of 108 tick pools representing 733 ticks and eighty-four tissue samples from twenty-six rodents collected in the U.K., and the subsequent failure to establish the isolate (from ticks collected in Fordingbridge) in culture. In contrast, using identical techniques and culture medium, B.burgdorferi was isolated from one of seven tick pools collected in Switzerland, and from a single pool of ticks collected in Slovakia, and both isolates were successfully passaged. Analysis of questing I.ricinus collected from Fordingbridge by direct immunofluorescence showed 6/32 (19%) of adults and 8/108 (7%) of nymphs were positive for B. burgdorferi , although only one nymph contained ≥ 1000 spirochaetes. To examine further the problem of isolating U.K. B.burgdorferi , twelve Ixodes ricinus tick samples from Fordingbridge, a recognized focus of Lyme disease, were subjected to isolation and culturing techniques, and the procedures monitored by use of the polymerase chain reaction (PCR). Whereas 11/12 samples were PCR positive after 2 weeks in culture, only one was PCR positive after 4 weeks. Motile spirochaetes were not visible by dark-field microscopy in any of the cultures. The results indicate that the standard BSK II medium routinely used to isolate and culture B. burgdorferi does not readily support the replication of the Borrelia species endemic to the U.K.     

Ixodes (Pholeoixodes) hexagonus, an efficient vector of Borrelia burgdorferi in the laboratory

Borrelia burgdorferi Johnson et al. was first isolated from the midgut of Ixodes dammini Spielman et al. in the U.S.A. and from the midgut of I.ricinus (L.) in Europe. I.ricinus was considered to be the only tick vector of this borrelia, in Europe, until I.hexagonus Leach, the hedgehog tick, was found to harbour spirochaetes. This paper reports an evaluation of the vector competence of I.hexagonus for the spirochaete B.burgdorferi. Transovarial and trans-stadial survival were demonstrated and the spirochaete was transmitted to laboratory mice via the bites of trans-stadially infected I.hexagonus females.     

Sex and genetic structure across age groups in populations of the European marine invasive mollusc, Crepidula fornicata L. (Gastropoda)

In long-lived species, variance in allele frequencies over time may vary according to the number of generations contributing to progeny. Here, we investigate the temporal stability of genetic diversity and structure in relation to sex and age in introduced populations of Crepidula fornicata , an exotic gastropod that successfully invaded Europe . This protandrous species has the potential to change sex from male to female according not only to age, but also to local sex ratio (social environment). This mechanism may adjust the reproduction efficiency across different cohorts and thus decrease the likelihood of genetic drift in the following generations. Based on crude demographic structure analysis in two spatially closed introduced French populations, we demonstrate that recruitment is discontinuous. Although timing of sex change is different across populations, both populations have a similar age structure characterized by distributions of males and females changing across generations. Using five microsatellite loci, we show that both populations display a temporal genetic homogeneity and a stability in genetic diversity indices across age groups examined. Our results highlight that the social control of sex change in C. fornicata has strong implications to the maintenance of high genetic diversity by enhancing breeding across several generations at each reproductive season.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 365–374.     

Cinnamyl alcohol dehydrogenases-C and D, key enzymes in lignin biosynthesis, play an essential role in disease resistance in Arabidopsis

The deposition of lignin during plant–pathogen interactions is thought to play a role in plant defence. However, the function of lignification genes in plant disease resistance is poorly understood. In this article, we provide genetic evidence that the primary genes involved in lignin biosynthesis in Arabidopsis, CAD-C and CAD-D , act as essential components of defence to virulent and avirulent strains of the bacterial pathogen Pseudomonas syringae pv. tomato , possibly through the salicylic acid defence pathway. Thus, in contrast with cellulose synthesis, whose alteration leads to an increase in disease resistance, alteration of the cell wall lignin content leads directly or indirectly to defects in some defence components.