Sexual determination is one of the most important developmental decisions, but comes down to one binary choice: becoming male or female. This decisions has huge consequences for the life history of the individual, especially in term of morphological, physiological and behavioural traits. Although the occurrence of only two sexes is universal in plants and animals, the molecular mechanisms for sex determination are extraordinary variable. One of the best studied groups are insects where male heterogamety (XX-XY), female heterogamety (ZW-ZZ), haplodiploidy and more remarkable mechanisms are present. The diversity of sexual phenotypes and the well studied life history traits in insects, makes them the ideal group to study variation in sex determination, the factors that shaped its evolution, and the relationship between life history and ecology of the species involved.
In insects, sex determination involves multiple steps, starting with a primary genetic signal that is different in males and females. This primary genetic signal is usually on one of the sex chromosomes and is transmitted through a cascade of genes to a master switch gene. This master switch gene then regulates all genes that are required for sex specific traits. The sex determination cascade in insects has a shape that resembles an hourglass. On top are all the different primary signal that sets the decision for the sexual development, in the centre is the axis of two important genes in sex determination: transformer and the master switch gene doublesex. At the bottom of the cascade are all the different sexual traits that are switched on during development.