Genetic structure of an aphid studied using microsatellites: cyclic parthenogenesis, differentiated lineages and host specialization

In a previous study, samples of the grain aphid Sitobion avenae (F.) were collected from wheat and adjacent cocksfoot hosts in a population thought to be primarily parthenogenetic, and DNA from individual aphids was analysed with a multilocus technique. Here we have applied single-locus microsatellites and a mitochondrial DNA marker to a subset of the same DNA extracts, and have made several additional inferences about important genetic and population processes in S. avenae . Microsatellite analysis indicated very high levels of genic and genotypic variation. S. avenae fell into three genotypic groups inferred to be almost noninterbreeding, while analysis of linkage and Hardy-Weinberg equilibria suggested high levels of sexual recombination within each genotypic group. Host specialization was evident: one lineage was found only on wheat, and one (bearing many alleles inferred to be introgressed from the blackberry-grass aphid S. fragariae (Walker)) was found only on cocksfoot. The third group of interrelated genotypes was found commonly on both hosts. Although most genotypes were found only once, some were much more numerous in the sample than expected from the frequency of the alleles they contained. This, and rapid temporal changes in genotypic composition of samples, indicates strong selective differences between genotypes and lineages. In the major genotypic group, the commonest genotypes were significantly more homozygous than were rare ones: thus these data may help to explain the frequent observation of homozygous excess in aphid allozymes. The genotype group showing S. avenae -like as well as S. fragariae -like alleles also carried S. fragariae -like mitochondrial DNA in at least 25/31 cases, indicating gender-asymmetrical hybridization.     

Microbial community responses to nitrogen- and phosphorus- deficient nutrient inputs: microplankton dynamics and biochemical characterization

Natural plankton communities from Masnou, a locality 20 km northof Barcelona (NW Mediterranean coast), were enclosed in 30 lmicrocosms to test the effect of different availability of nitrogen(N) and phosphorus (P) on the biomass of the main microplanktongroups, and the biochemical composition (DNA, protein and chlorophyllconcentration) of microbial communities. Immediately after enclosurein microcosms, three different nutrient enrichments were performed:N-deficient, P-deficient and nutrient-balanced. N and P deficienciesaffected the structure and the biochemical composition of themicrobial communities. Phytoplankton assemblages showed similartemporal patterns under the three nutrient treatments, althoughthe relative contribution of the different groups was notablyaffected. The lowest DNA concentration was measured in the P-deficienttreatment, suggesting that P availability imposes the limitson the DNA levels in the ecosystem. The availability of N inthe P-deficient microcosms allowed relatively high synthesisof chlorophyll and protein until the end of the experiment.Significantly high chlorophyll: DNA and protein: DNA ratioscharacterized the P-deficient treatment (where N was available)compared to the N-deficient microcosms. From the results obtained,we suggest that the protein: DNA ratios may constitute a biochemicalindicator of the P versus N availability in natural ecosystems.