Comparison of geographically distant populations of Rhizobium isolated from root nodules of Phaseolus vulgaris

Seventy-two rhizobial strains were isolated from the root nodules of french beans ( Phaseolus vulgaris ). They were sampled from two geographically distant field populations and 18 additional different sites in France. They were characterized by a) plasmid profiles, (b) RFLP analysis of total cellular DNA using various chromosomal and symbiotic gene probes (including nif H from Rhizobium etli bv. phaseoli ) and c) their ability to nodulate a potential alternative host, L. leucocephala. Over half of the isolates were ascribed to Rhizobium leguminosarum bv. phaseoli on the basis of the hybridization analysis, the possession of multiple copies of nif H and their inability to nodulate L. leucocephala. The remaining isolates belonged to 2 groups which were shown to be genomically distinct from R. leguminosarum bv. phaseoli, R. etli bv. phaseoli and R. tropici. Most members of these two groups shared with R. tropici the ability to nodulate L. leucocephala and, for isolates of only one of these groups, the presence of one copy of nif H. Members of each of the 3 taxa were widely distributed in France and circumstantial evidence of pSym transfer between them was shown. R. leguminosarum bv. phaseoli and one of the two novel groups co-occurred within the two geographically distant populations. Individual genotypes were conserved between them. The finding of a third taxon at various other locations indicated additional diversity among rhizobia nodulating beans.     

LIFE-CYCLE COMPONENTS OF SELECTION IN ERIGERON ANNUUS: II. GENETIC VARIATION

Genetic variation for seedling and adult fitness components was measured under natural conditions to determine the relative importance of the seedling stage for lifetime fitness in Erigeron annuus. Variation in lifetime reproductive success can result from both the persistent effects of genetic variation expressed among seedlings and from variation in adult fitness components. Analysis of covariance was used to separate the stage specific from the cumulative effects of genetic variance expressed earlier in the life cycle. E. annuus produces seeds through apomixis, which allowed measurement of the fitness of replicate genotypes from germination through the entire life cycle. There were significant differences among genotypes for date of emergence, seedling size, survivorship and fecundity, but heritabilities were low, indicating slow response to selection. For all characters, environmental components of variance were one to two orders of magnitude larger than genetic variance components, resulting in broad sense heritabilities less than 0.1. For seedling size and fecundity, all of the genetic variance was in the form of genotype-environment interactions, often with large negative genetic correlations across environments. In contrast, genotypes differed in mean survivorship through one year, but there were no genotype-environment interactions for viability. Genetic differences in viability were primarily expressed as differences in overwinter survivorship. Genotype × environment interactions among sites and blocks were generated early in the life cycle while the genotype × environment interactions in response to competitive environment (open, annual cover, perennial cover) first appeared in adult fecundity. Genetic variation in lifetime fitness was not significant, despite a fourfold difference in mean fitness among genotypes.     

GENETIC AND SOCIAL INHERITANCE OF BODY AND EGG SIZE IN THE BARNACLE GOOSE (BRANTA LEUCOPSIS)

We present heritability estimates for final size of body traits and egg size as well as phenotypic and genetic correlations between body and egg traits in a recently established population of the barnacle goose (Branta leucopsis) in the Baltic area. Body traits as well as egg size were heritable and, hence, could respond evolutionarily to phenotypic selection. Genetic correlations between body size traits were significantly positive and of similar magnitude or higher than the corresponding phenotypic correlations. Heritability estimates for tarsus length obtained from full-sib analyses were higher than those obtained from midoffspring-midparent regressions, and this indicates common environment effects on siblings. Heritabilities for tarsus length obtained from midoffspring-mother regressions were significantly higher than estimates from midoffspring-father regressions. The results suggest that this discrepancy is not caused by maternal effects through egg size, nor by extra-pair fertilizations, but by a socially inherited foraging site fidelity in females.     

3. Genetic conservation in seed banks

The safest and most economical and convenient way of preserving the genetic resources of the majority of crop plants is by long-term seed storage. The technology is well developed, but recent research resulting from a greater understanding of behaviour at very low water potentials is leading to further improvements     

ON THE LOW HERITABILITY OF LIFE-HISTORY TRAITS

Life-history traits such as longevity and fecundity often show low heritability. This is usually interpreted in terms of Fisher's fundamental theorem to mean that populations are near evolutionary equilibrium and genetic variance in total fitness is low. We develop the causal relationship between metric traits and life-history traits to show that a life-history trait is expected to have a low heritability whether or not the population is at equilibrium. This is because it is subject to all the environmental variation in the metric traits that affect it plus additional environmental variation. There is no simple prediction regarding levels of additive genetic variance in life-history traits, which may be high at equilibrium. Several other patterns in the inheritance of life-history traits are readily predicted from the causal model. These include the strength of genetic correlations between life-history traits, levels of nonadditive genetic variance, and the inevitability of genotype-environment interaction.     

The application of reproductive technology to endangered species breeding programmes

Captive breeding plays an increasingly important role in species conservation, but special problems are encountered in achieving the ideal of a demographically stable but genetically diverse population. Breeding programmes involving co-operation among a number of centres are now being developed which will overcome some of these difficulties by identifying individual animals, genetic lineages or age cohorts from which to breed. Application of techniques such as artificial insemination, embryo transfer and semen collection and storage, as well as the monitoring of reproductive status will contribute to the success of such programmes. The usefulness of these procedures for various population problems is discussed and criteria for their appropriate implementation within breeding programmes is outlined.     

What generates the diversity of Wolbachia—arthropod interactions?

Wolbachia are strictly endocellular, vertically transmitted bacteria associated with insects and crustaceans. This group of parasites modify their hosts' reproduction so as to increase their own fitness. This paper reviews the variability of these parasitic alterations and their consequences for host biology and populations. Wolbachia induce cytoplasmic incompatibility (a characteristic apparently specific to Wolbachia) in several insects and one isopod crustacean; parthenogenesis (thelytoky) in haplo-diploid insects; feminization in various isopods. The consequences of these phenomena on speciation, population dynamics and genetic polymorphism are discussed. The variability of the mechanisms of host sex determination is one important factor responsible for the diversity of Wolbachia-host interactions. However, parasite characteristics, such as the capacity to disturb host mitosis, and the ability to be horizontally transferred between hosts, also appear to play a role in this diversity.     

Population differentiation and evolution in the common guillemot Uria aalge

Common (Uria aalge) and Brünnich's guillemots (U. Iomvia) are colonial seabirds that nest in temperate to arctic oceans throughout the Northern hemisphere. They are very similar in the characteristics of ecology, demography and life history that are thought to determine the extent of differentiation among populations, yet geographic variation in morphology is notably greater in common guillemots. Despite evidence of strong natal philopatry, previous analyses of allozymes and the mitochondrial cytochrome b gene revealed little genetic differentiation among North Atlantic colonies of Brünnich's guillemots. To determine if the more extensive morphological variability in common guillemots reflects greater genetic variability, we sequenced part of the cytochrome b gene for 160 common guillemots from 10 colonies distributed throughout the Northern hemisphere. Genotype frequencies and phylogenetic relationships among genotypes both indicated that Atlantic and Pacific populations are genetically distinct. Genetic divergence among genotypes suggested that differentiation of these populations has resulted from separation by Pleistocene glaciers and the Bering Landbridge, as well as by currently unsuitable breeding habitat in the Arctic Ocean. Cytochrome b genotype frequencies also differed among Atlantic colonies, and appeared to define a cline similar to that described for morphological characters. Analyses of sequence variation suggested that this variation probably results from secondary contact between two refugial populations from the Pleistocene glaciations, rather than from isolation by distance or selection. In contrast, the Atlantic population of Brünnich's guillemots appears to have arisen through recent expansion of a single homogeneous refugial population.