Symbiogenesis was a rare reproductive process. Instead of pollination or mating, symbiogenetic organisms merged with a second species to create a hybrid. lysosomal enzymes might be an indicator of symbiogenesis.
Andorian amoeba were a known example. They merged with other single-celled organisms to form an unique third species.
Symbiogenesis did not generally occur in a species as complex as a humanoid. However, the process occurred on the USS Voyager in 2372 when Lieutenant Commander Tuvok and Neelix found themselves combined to form a new individual they named Tuvix. It was theorized that this had been made possible by the fact that during transport they would have to be broken down to microcelluar level, their DNA and proteins all in a state of molecular flux. If an enzyme causing symbiogenesis interacted with DNA while in the matter stream, their patterns might then be merged.
Neelix and Tuvok had been collecting samples of a rare orchid when they were beamed up. The orchid enzymes reacted with the separated microcellular state the crewmen were in, combining their DNA. As a result, there was only one pattern, the combined one, to rematerialize.
The combination was a success, and Tuvix had no ill effects physically or mentally. Eventually, The Doctor found a way to separate them by using a radio-isotope which would attach itself to one of the original species' DNA patterns, but not the other, allowing the transporter to separate them, despite Tuvix's protests. (VOY: "Tuvix").