Two subunits of the Drosophila mediator complex act together to control cell affinity

The organizing centers for Drosophila imaginal disc development are created at straight boundaries between compartments; these are maintained by differences in cell affinity controlled by selector genes and intercellular signals. skuld and kohtalo encode homologs of TRAP240 and TRAP230, the two largest subunits of the Drosophila mediator complex; mutations in either gene cause identical phenotypes. We show here that both genes are required to establish normal cell affinity differences at the anterior-posterior and dorsal-ventral compartment boundaries of the wing disc. Mutant cells cross from the anterior to the posterior compartment, and can distort the dorsal-ventral boundary in either the dorsal or ventral direction. The Skuld and Kohtalo proteins physically interact in vivo and have synergistic effects when overexpressed, consistent with a skuld kohtalo double-mutant phenotype that is indistinguishable from either single mutant. We suggest that these two subunits do not participate in all of the activities of the mediator complex, but form a submodule that is required to regulate specific target genes, including those that control cell affinity.     

The evolution of sexual reproduction: A model which assumes individual selection

It is commonly thought that sexual reproduction evolved and is maintained because the more rapid production of recombinant genotypes is of advantage to the species, but this advantage is long-term and is maintained by group selection while the individual committed to sexual reproduction is at an immediate disadvantage. However, Williams & Mitton (1973) and Williams (1975) have recently put forward models for the evolution of sexuality which derive it from individual and not group selection. These are reviewed below and certain limitations pointed out. An alternative, more general model is described. This accounts for the evolution and maintenance of sexual reproduction by processes of individual selection, but does not require to assume enormous fecundity, hyperintense selection, or special life-history features. It is suggested that the present model is sufficiently general to apply to most or all cases of the evolution of sexual reproduction.