Clonality and its population genetic conseuqneces in seagrasses

Seagrasses are monocotyledonous species that are adapted to live and reproduce entirely in the marine environments, where they suffer from a variety of disturbances, such as tides, hurricanes. To adapt to these environments, seagrasses have evolved a series of characteristics, among which strong clonality is a prominent one. All the seagrasses have horizontal rhizomes, and many species have vertical rhizomes. In some seagrass species, clonal reproduction, via apomict, was also observed. We reviewed the parameters relating to clonal growth, such as internodal length, spacer length and branching angle as well as rhizome elongation and branching rates, of seagrass species. These parameters have determinant impacts on the expanding of seagrass patches. However, breeding system may also affect patch sizes. Strong clonality has essential influences on genetic variation of seagrasses. Generally, seagrass species have relatively lower within-population genetic variation than terrestrial plants or mangroves, another group of marine flowering plants, although DNA markers, such as microsatellites, have revealed much higher variation than allozymes did and higher than that of previous thought in seagrasses. High variation in clonal diversity was found in many seagrass populations, but single or few genotypes were also found in some populations due to founder effect and strong clonality. Although gene flow usually was thought to be restricted in clonal species, it was reported that clonal fragments may be transported to a long distance and therefore to promote gene flow among populations. Effects of clonality on within-population structuring and mating system were also reviewed.