Mechanisms of reproductive isolation among cryptic species in monogonont rotifers

The advent of molecular techniques has resulted in the discovery of previously unknown cryptic species in many organisms, including rotifers, allowing researchers to perform detailed tests on reproductive barriers between closely related taxa. Here, we review the available literature about molecular techniques implemented for detecting cryptic diversity in rotifers and delimiting species, and discuss the potential mechanisms of reproductive isolation among rotifer cryptic species. Half of the evaluated studies used quantitative statistical approaches for species delimitation. Species boundaries defined by molecular approaches were also examined by conducting mating experiments. Those mating experiments were used to test various reproductive barriers. Most (75%) studies that identified reproductive isolation mechanisms provided evidence of prezygotic barriers. The available literatures suggest that behavioral reproductive isolation may have a more important role than other prezygotic barriers in facilitating reproductive isolation. Postzygotic barriers such as hybrid unviability or sterility and female mortality also contribute to reproductive isolation among rotifer cryptic species. The prevalence of prezygotic barriers in our assessment may stem from the difficulty of studying postzygotic barriers, which can require long-term maintaining and monitoring of rotifer populations. Sequencing tools, including whole genome sequencing, could be implemented to investigate the molecular basis of reproductive isolation in rotifers.