ZO sex-determination system

The ZO sex-determination system is a special way that decides whether baby animals are male or female in certain insects, such as caddisflies and some moths.

In these species, there is one special kind of chromosome called Z. Males have two Z chromosomes, so they are called ZZ. Females, however, only have one Z chromosome, so they are called ZO.

This system comes from the ancestors of Lepidoptera, which includes butterflies and moths, and their close relatives, the Trichoptera. Originally, these ancestors used the ZO system to decide their sex. But in many later groups, like the large group called Ditrysia and another group called Tischerioidea, the system changed to something called the ZW system, where females have two different chromosomes, Z and W.

Interestingly, some of these ZW species later went back to using the ZO system again. This shows how living things can change and adapt their ways of deciding traits over time.