Male dwarfism (nannandry) is an extreme form of sexual dimorphism, that occurs in many different organism groups, but is unique to mosses among land plants. Mosses have haplodiploid life cycles with a dominant gametophyte generation and a diploid sporophyte generation that is attached to the maternal shoot. Fertilization is mediated by swimming sperm and therefore water dependent and limited to short distances. About 50% of the moss species are unisexual, having separate male and female individuals, a condition which potentially reduces fertilization success. Dwarf males (DM) are minute males, originating from spores that germinate on female plants and remain attached in proximity to the female archegonia. While nannandry might enhance mating success, it might also increase homozygosity in sporophytes if all the DM on a particular female are recruited from sporophytes carried by the same female. This project aims to explore the level of genetic diversity and inbreeding in a common moss species with facultative dwarfism. Next generation DNA sequencing (NGS) will be used to detect and score genetic markers in the form of single nucleotide polymorphisms (SNPs). The results will reveal the genetic consequences and driving forces underlying nannandry . The outcome of this project will contribute to our understanding of poorly understood nannandric reproductive strategies which have hitherto rarely been subject to genetic studies..