Sexual System Evolution (Marine mussels)

"Sexual systems are one of the main drivers of evolution, as they promote genetic variability and thus provide a basis for natural selection" John Maynard Smith

Dioecy and hermaphroditism are the most frequent sexual systems in the natural world. Dioecy refers to organisms that are either female or male during their breeding life, while hermaphroditism refers to organisms that produce eggs and sperm during their reproductive life. These two systems are considered to be stable, as they have been successful in the reproduction and survival of many species (Avise & Mank 2009, Week 2012).

However, in the evolutionary process, intermediate reproductive systems may emerge as a transition from one system to another. One of these systems is androdioecy, where males and hermaphrodites co-exist within the same population. Another is gynodioecy, where females and hermaphrodites coexist. Lastly, there is trioecy, where hermaphrodites, females and males all inhabit the population (see Oyarzún et al. 2020).

The trioecy is a rare reproductive system, both in plants and animals. This system is considered to be a less stable reproductive system as compared to hermaphroditism and dioecy, as it is less likely for all three sexes to be present in the same population. This is because the reproductive success of a population may be hindered by the presence of the other sexes in the population (Oyarzún et al. 2020).