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Premise of research.If sexual reproduction is necessary for maintaining genotypic diversity, then plant populations lacking sex might be expected to exhibit less genotypic diversity than sexually reproducing populations. This pattern could be particularly pronounced in mosses of harsh environments, where haploid gametophytes persist in the apparent absence of sex, presumably through vegetative cloning. Here we compare genetic diversity in four Mojave Desert populations of the moss Syntrichia caninervis to assess the degree to which sexual reproduction is associated with genotypic diversity. This study helps define the role of reproductive modes in producing and maintaining genetic diversity.Methodology.Eight microsatellite loci were used to genotype four Mojave Desert populations of S. caninervis, which consisted of two sites with lower environmental stress and sexual reproduction and two sites with higher stress and no apparent sexual reproduction.Pivotal results.Of 341 ramets that amplified successfully across at least seven of the eight loci, 191 unique genotypes were identified; of these, 87 and 131 differed by one and two stepwise mutations, respectively. Genetic diversity was high and did not differ significantly between sexual and asexual populations. Estimates of mutation-scaled migration rates between sites ranged from four to 48 individuals per generation.Conclusions.Despite different levels of sex expression and sporophyte production, genetic and clonal diversity did not differ significantly between higher- and lower-stress populations. The relatively low population structure among sites, high number of clonal lineages, and predominance of genotypes differing by only one or two mutation steps suggests that migration and somatic mutation are responsible for the observed genotypic diversity. These results support the idea that established populations can persist through cloning and that migration can occur over large physical distances. However, retention of genotypic diversity from a time when the environment was more permissive to sexual reproduction cannot be ruled out as an influence on population structure.