Validation of mating type-related markers in Ulva prolifera (Ulvophyceae, Chlorophyta) and their detection during various reproductive modes

Ulva prolifera dominates the world's largest green tide in the Yellow Sea. In addition to the typical sexual life cycle, which is diplohaplontic and isomorphic, U. prolifera can reproduce in a variety of asexual ways, and its reproductive strategy contributes significantly to the blooms. However, survey statistics on the life cycle dynamics have been seriously restricted owing to difficulties in morphologically determining the ploidy or sex of thalli, as well as the lack of reliable molecular markers. Here, two mating type (MT)-specific markers were modified based on the published MT-locus gene of U. prolifera, and validated in a sexual life cycle. The two markers paired in diploid sporophytes and segregated in haploid gametophytes, with mating occurring only between those with different markers, which confirmed that these markers were allelic and MT-related in U. prolifera. Furthermore, the ploidy variations in three types of reproduction, parthenogenesis, apomeiosis, and so-called somatic cell germination were determined in U. prolifera. No ploidy change was detected using flow cytometry between parents and progeny, and this was confirmed using these markers. In addition, the somatic cell germination process was characterized in detail. It is essentially a process of vegetative reproduction—in situ somatic embryogenesis, and it may occur in both sporophytes and gametophytes of U. prolifera. In U. prolifera, these MT-related markers can determine the ploidy or sex of thalli in one step, and the rules determining marker transmission along during the reported reproductive pathways were elucidated. This marker pair is an important molecular tool that can be used in future studies on the reproductive strategy of U. prolifera during green tides.