They inserted a light-activated enzyme for cAMP synthesis into mouse sperm that lacked the endogenous enzyme. Sperm of these mice are usually non-motile, and the mice consequently infertile. After stimulation of these sperm with blue light, they produce cAMP, start to swim again, and are even able to fertilise eggs.
Fertilisation of egg cells is closely linked with cAMP synthesis. Sperm lacking the endogenous adenylate cyclase enzyme are unable to swim: the mice are infertile. However, if the bPAC sperm are stimulated with blue light, the cAMP concentration increases and the sperm swim faster, because the sperm tail beats faster.
This proves that it is possible to control something as fundamental as fertilisation with light. Optogenetics has thus conquered another field outside the neurosciences.