Growth relations of lines selected for fast or slow larval feeding rate have been compared with those in the genetically heterogeneous control base population from which they were derived. Larvae of the slow strain have reduced growth rate and reach their critical weight for pupation later than unselected larvae. Larvae of the fast strain attain their critical weight at the same time as the unselected control larvae, suggesting that growth rate in the precritical period of development is already maximized in the base population and cannot be improved by increasing food intake. This constraint does not apply to the fixed period of post-critical growth however, since fast feeding larvae give rise to larger adult flies than the controls.Larval feeding rate is affected by genes located on all three major chromosomes. The small fourth chromosome has negligible effect. Selection for slow feeding rate has led to an increase in the frequency of recessive genes affecting the character. High scores of larvae selected for fast feeding rate depend upon interactions between non-homologous selected chromosomes which individually have little effect. Larval feeding rate in the control unselected population appears to be buffered, firstly by epistatic interactions against the effects of chromosomes tending to promote 'supra-optimal' feeding rate and, secondly, by dominance against chromosomes promoting a lowering of feeding rate.Under conditions of scramble type competition between the selected lines for limited resources, fast feeding larvae have a higher survival rate, and complete their period of larval development earlier to give larger adult flies than their slow feeding competitors. The contribution of larval feeding rate to competitive ability at different levels is discussed, and it is suggested that the effects of change in this behavioural character may be far reaching.

The genetic population structure and gene flow in the obligatory outbreeding plant species Plantago lanceolata L. were determined in the Westduinen (Wd) pasture population in the South-West of the Netherlands. Three experiments were performed: the measurement of genetic structure using allozyme variation, pollen flow in a wind-tunnel and the distribution of an allozyme marker in a garden experiment. The position and allozyme genotype of 524 plants from eight subpopulations in the Westduinen population were determined. The population exhibited local differentiation: genotype frequencies within subpopulations were mostly in Hardy-Weinberg equilibrium, and gene frequencies varied significantly between subpopulations. The wind-tunnel experiments showed that wind speed, source height and capture height of pollen influenced the pollen-mediated gene flow and therefore neighbourhood sizes. In the garden experiment a clear picture of actual pollen flow could be shown.Estimates of neighbourhood and gene transport parameters from the data of the three experiments indicated restricted gene flow: neighbourhood size (NJ was between 14 and 20 plants, mean gene transport per generation (M) between 02 and 14 metres. We argue that this combination of small effective number and low gene transport leads to the substructuring of the Westduinen population of P. lanceolata.

Four species of Drosophila (melanogaster, simulans, mauritiana and virilis) were tested for their ability to survive on erg strains of Saccharomyces cerevisiae each having a mutant block at a different reaction in the synthesis of ergosterol. Species capable of completing development on a given yeast mutant strain were tested for egg production and viability, and also for their ability to survive on the mutant yeast for five complete generations. Sterol analyses using gas-liquid chromatography show that the sterol composition of flies closely resembles that of the yeasts on which they are grown, confirming that the steric criteria for sterols capable of a structural role in lipoprotein membranes are relatively broad. Even so, different sterols are not equally efficient in this respect and there are differences between species in their tolerance to the sterols available from each of the yeast mutants. The range of sterols capable of satisfying the micronutrient, as distinct from the macronutrient, requirement is narrower. Growth on two of the yeast mutants (erg-2 and erg-6) leads to developmental arrest in the larval stage due, it is suggested, to a block in the synthesis of ecdysone. The four Drosophila species lack an effective level of A 8 -A 7 isomerase which is necessary for utilization of zymosterol and other A 8 sterol precursors of ergosterol. Ergosta-5,7-diene-3/?-ol appears to be capable of substituting for cholesterol in the metabolism of Drosophila.

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Inter-and intraspecific chioroplast DNA variation in four species of Plantago (P. lanceolata, P. major, P. media and P. coronopus) were analysed by comparing DNA fragment patterns produced by seven restriction endonucleases. Plant material was collected in seven European countries. Only 21.3 per cent of the 409 restriction sites were shared by all four species. Phylogenetic analysis, performed by constructing the most parsimonious trees, showed that genetic differentiation in cpDNA was very high among P. lanceolata, P. coronopus and the species pair P. media and P. major, which were more closely related.At the intraspecific level, four restricted site mutations were detected. Most of the variation was due to numerous small length mutations, one of which (70 bp) discriminated between the two subspecies of P. major (i.e. ssp. major and ssp. pleiosperma). This mutation was used successfully to show that, in Denmark, cpDNA introgression occurs from pleiosperma into major in the areas where the two subspecies grow together, whereas previous studies of the same subspecies in the Netherlands suggested introgression in the reverse direction. In P. lanceolata, five distinct types of cpDNA genome could be distinguished, one of these being widely distributed. Therefore, DNA variation was present both between and within populations. In P. media, three distinct cpDNA genomes were found, one being specific to diploid and tetraploid material from the Pyrenees, suggesting multiple origins of the autotetraploids in this species. Whereas cpDNA variation was observed in the two outcrossing species of Plantago (P. media and P. lanceolata) no variation was detected in the two autogamous subspecies of P. major. This suggests that chioroplast DNA variation may be related to nuclear genome diversity.