There is growing awareness of the importance of cooperative behaviours in microbial communities. Empirical support for this insight comes from experiments using mutant strains, termed ‘cheats’, which exploit the cooperative behaviour of wild‐type strains. However, little detailed work has gone into characterising the competitive dynamics of cooperative and cheating strains. We test three specific predictions about the fitness consequences of cheating to different extents by examining the production of the iron‐scavenging siderophore molecule, pyoverdin, in the bacterium Pseudomonas aeruginosa. We create a collection of mutants that differ in the amount of pyoverdin that they produce (from 1% to 96% of the production of paired wild types) and demonstrate that these production levels correlate with both gene activity and the ability to bind iron. Across these mutants, we found that (1) when grown in a mixed culture with a cooperative wild‐type strain, the relative fitness of a mutant is negatively correlated with the amount of pyoverdin that it produces; (2) the absolute and relative fitness of the wild‐type strain in the mixed culture is positively correlated with the amount of pyoverdin that the mutant produces; and (3) when grown in a monoculture, the absolute fitness of the mutant is positively correlated with the amount of pyoverdin that it produces. Overall, we demonstrate that cooperative pyoverdin production is exploitable and illustrate how variation in a social behaviour determines fitness differently, depending on the social environment.

Two mutations in Tetranychzts urticae and nine in Tetranychus paci/icus, all originating spontaneously, block the production of red and yellow carotenoid pigments in these spider mite species. Inter-mutant crosses were carried out to study complementation and recombination relationships between the mutations. In T. ,trticae, the two albino mutants complement one another completely, i.e., crosses between them produce wild-type hybrid females; while they recombine with a frequency of 0.05%. Of the nine mutants in T. pacificus, five p mutants in general are complementary to a high degree with four a mutants, p mutants fail to complement one another, while some a mutants are mutually complementary to a slight degree. Scoring the degree of complementation produced by all possible combinations of mutants permits the construction of a linear complemenration map. Certain combinations, however, are exceptional to such a representation. Moreover, marked reciprocal differences in complementation indicate that maternal effects are involved, implying that the albino locus may control more than one enzymatic step. Attempts to derive a genetic map were impeded by the absence of suitable linked markers, by a pronounced maternal effect (high pigmentation) in the haploid F2 males, and by the appearance of "pseudowild" type F2 males. The given genetic sequence, although comparable in a limited fashion to the complementation map, is considered tentative. "Pink" types appeared in crosses with certain p mutants. These were due to mutation at a separate locus, called "rose", and seem to involve the production of pink pigments in an alternative or substitute pathway. A scheme attempting to orientate the present state of understanding of pigmentation in spider mites is presented.

Analysis of a series of exceptional ry + half-tetrads, produced in mass matings involving rosy mutant heterozygous half-tetrads, provides rigorous demonstration of the occurrence of non-reciprocal as well as reciprocal recombination events within the rosy cistron of Drosophila melanogaster. Inferences about allele recombination drawn from this and other studies in Drosophila provide a strong argument that gene conversion occurs as a regular event in higher eukaryotes.

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