Effect of crop rotation and soil cover on alteration of the soil microflora generated by the culture of transgenic plants producing opines

The culture of transgenic Lotus corniculatus plants producing opines, which are bacterial growth substrates, leads to the selection of rhizospheric bacteria able to utilize these substrates. We have investigated the fate of the opine-utilizing community over time under different experimental conditions following elimination of selective pressure exerted by the transgenic plants. These plants were removed from the soil, which was either left unplanted or replanted with wild-type L. corniculatus or wheat plants. The density of opine-utilizing bacteria in the fallow soils remained essentially unchanged throughout the experiment, regardless of the soil of origin (soil planted with wild-type or transgenic plants). When wild-type Lotus plants were used to replace their transgenic counterparts, only the bacterial populations able to utilize the opines were affected. Long-term changes affecting the opine-utilizing bacterial community on Lotus roots was dependent upon the opine studied. The concentration of nopaline utilizers decreased, upon replacement of the transgenic plants, to a level similar to that of normal plants, while the concentration of mannopine utilizers decreased to levels intermediate between transgenic and normal plants. These data indicate that: (i) the opine-utilizing bacterial populations can be controlled in the rhizosphere via plant-exudate engineering; (ii) the interaction between the engineered plants and their root-associated micro-organisms is transgene specific; and (iii) alterations induced by the cultivation of transgenic plants may sometimes be persistent. Furthermore, opine-utilizing bacterial populations can be controlled by crop rotation. Therefore, favouring the growth of a rhizobacterium of agronomic interest via an opine-based strategy appears feasible.     

Evolution of virulence in a heterogeneous host population

Abstract.— There is a large body of theoretical studies that investigate factors that affect the evolution of virulence, that is parasite-induced host mortality. In these studies the host population is assumed to be genetically homogeneous. However, many parasites have a broad range of host types they infect, and trade-offs between the parasite virulence in different host types may exist. The aim of this paper is to study the effect of host heterogeneity on the evolution of parasite virulence. By analyzing a simple model that describes the replication of different parasite strains in a population of two different host types, we determine the optimal level of virulence in both host types and find the conditions under which strains that specialize in one host type dominate the parasite population. Furthermore, we show that intrahost evolution of the parasite during an infection may lead to stable polymorphisms and could introduce evolutionary branching in the parasite population.     

Phylogenetic relationships of functionally dioecious FICUS (Moraceae) based on ribosomal DNA sequences and morphology

Figs (Ficus, Moraceae) are either monoecious or gynodioecious depending on the arrangement of unisexual florets within the specialized inflorescence or syconium. The gynodioecious species are functionally dioecious due to the impact of pollinating fig wasps (Hymenoptera: Agaonidae) on the maturation of fig seeds. The evolutionary relationships of functionally dioecious figs (Ficus subg. Ficus) were examined through phylogenetic analyses based on the internal transcribed spacer (ITS) region of nuclear ribosomal DNA and morphology. Forty-six species representing each monoecious subgenus and each section of functionally dioecious subg. Ficus were included in parsimony analyses based on 180 molecular characters and 61 morphological characters that were potentially informative. Separate and combined analyses of molecular and morphological data sets suggested that functionally dioecious figs are not monophyletic and that monoecious subg. Sycomorus is derived within a dioecious clade. The combined analysis indicated one or two origins of functional dioecy in the genus and at least two reversals to monoecy within a functionally dioecious lineage. The exclusion of breeding system and related characters from the analysis also indicated two shifts from monoecy to functional dioecy and two reversals. The associations of pollinating fig wasps were congruent with host fig phylogeny and further supported a revised classification of Ficus.     

THE MAINTENANCE OF SEX BY PARASITISM AND MUTATION ACCUMULATION UNDER EPISTATIC FITNESS FUNCTIONS

The mutation accumulation hypothesis predicts that sex functions to reduce the population mutational load, while the Red Queen hypothesis holds that sex is adaptive as a defense against coevolving pathogens. We used computer simulations to examine the combined and separate effects of selection against deleterious mutations and host-parasite coevolution on the spread of a clone into an outcrossing sexual population. The results suggest that the two processes operating simultaneously may select for sex independent of the exact shape of the function that maps mutation number onto host fitness.     

Involvement of Hydrogen Peroxide, Peroxidase and Superoxide Dismutase in Response of Medicago truncatula Lines Differing in Susceptibility to Phoma medicaginis Infection

This study was undertaken to assess the involvement of hydrogen peroxide (H₂O₂), peroxidase (POX; EC 1.11.1.7) and superoxide dismutase (SOD; EC 1.15.1.1) in Medicago truncatula in relation with susceptibility to Phoma medicaginis infection. Several M. truncatula lines were studied in terms of their response to P. medicaginis infection. Fifteen days after inoculation (dai), differences in susceptibility were found. DZA45.5 was the least susceptible line and F83005.5 was the most susceptible line. Microscopic analysis of fungal development was performed in inoculated detached leaves of the DZA45.5 and F83005.5 lines. No significant difference was observed in events from conidia germination to penetration. Differences became apparent during the colonization process as the pathogen was able to sporulate rapidly increasing its concentration on the tissue of F83005.5 in comparison with DZA45.5. To characterize the susceptibility of the two lines, histochemical detection of H₂O₂ was made in detached leaves. H₂O₂ detection showed an early accumulation of this component in cells of DZA45.5 at 1 dai. However, H₂O₂ was detected in few, if any, cells in the tissues of the most susceptible line, F83005.5. The activity of POX and SOD were determined spectrophotometrically in leaves of intact inoculated plants of both lines. Phoma medicaginis inoculation of DZA45.5 and F83005.5 did not affect POX activity level in leaves when compared with control uninoculated plants. SOD activity showed a significant decrease in F83005.5 and DZA45.5 leaves at 4 dai and 9 dai, respectively, in comparison with control plants. In control plants POX activity was significantly higher in the least susceptible line DZA45.5 in comparison with F83005.5. Early and higher production of H₂O₂ and elevated basal POX activity in cells of the least susceptible line, DZA45.5 could explain its ability to be less favourable to the colonization and reproduction of P. medicaginis in comparison with the most susceptible line, F83005.5.     

Morphology,microhabitats and geographical variation of Kuhnia spp. (Monogenea: Polyopisthocotylea)

Kuhnia scombri and K. sprostonae are the only species of the genus recognized as valid. Hosts are scombrid fishes and possibly the genus Scomber only. Examination of many populations from localities in the Atlantic and Pacific showed much geographical variation. Variation is greatest in K. sprostonae, with a distinctly different population on S. scombrus in the North Atlantic. There are intermediate stages in all characters between different populations of both species (except possibly K. sprostonae in the North Atlantic). Some populations within each species have different microhabitats on the gills and pseudobranchs. Differences between parasite populations correspond to some degree to those of host populations, but at least some of the differences are non-adaptive.     

A model for the origin of group reproduction during the evolutionary transition to multicellularity

During the evolution of multicellular organisms, the unit of selection and adaptation, the individual, changes from the single cell to the multicellular group. To become individuals, groups must evolve a group life cycle in which groups reproduce other groups. Investigations into the origin of group reproduction have faced a chicken-and-egg problem: traits related to reproduction at the group level often appear both to be a result of and a prerequisite for natural selection at the group level. With a focus on volvocine algae, we model the basic elements of the cell cycle and show how group reproduction can emerge through the coevolution of a life-history trait with a trait underpinning cell cycle change. Our model explains how events in the cell cycle become reordered to create a group life cycle through continuous change in the cell cycle trait, but only if the cell cycle trait can coevolve with the life-history trait. Explaining the origin of group reproduction helps us understand one of life''s most familiar, yet fundamental, aspects—its hierarchical structure.     

Adaptive laboratory evolution of microbial co‐cultures for improved metabolite secretion

Adaptive laboratory evolution has proven highly effective for obtaining microorganisms with enhanced capabilities. Yet, this method is inherently restricted to the traits that are positively linked to cell fitness, such as nutrient utilization. Here, we introduce coevolution of obligatory mutualistic communities for improving secretion of fitness‐costly metabolites through natural selection. In this strategy, metabolic cross‐feeding connects secretion of the target metabolite, despite its cost to the secretor, to the survival and proliferation of the entire community. We thus co‐evolved wild‐type lactic acid bacteria and engineered auxotrophic Saccharomyces cerevisiae in a synthetic growth medium leading to bacterial isolates with enhanced secretion of two B‐group vitamins, viz., riboflavin and folate. The increased production was specific to the targeted vitamin, and evident also in milk, a more complex nutrient environment that naturally contains vitamins. Genomic, proteomic and metabolomic analyses of the evolved lactic acid bacteria, in combination with flux balance analysis, showed altered metabolic regulation towards increased supply of the vitamin precursors. Together, our findings demonstrate how microbial metabolism adapts to mutualistic lifestyle through enhanced metabolite exchange.