Abstract: Wolffia Horkel ex Schleid. (Lemnaceae) includes the world's smallest angiosperms. Morphologically, their bodies are extremely simplified, making classification difficult and long-disputed. No reports about the genetic structure of these clonal-dominant aquatic plants have been published until now. In this study, of 247 samples from 42 populations spanning representative locations in China, two chloroplast haplotypes (glo-cp and un-cp) and 66 amplified fragment length polymorphism genotypes were identified. Based on wide sampling, cpDNA haplotypes, and amplified fragment length polymorphism profiles, we found that there are two genetic lineages (glo- and un- lineages) of Wolffia species in China. Genotypic and genetic diversity of Wolffia species are high compared with other clonal plants (Simpson's index, D= 0.97; Nei's diversity, H= 0.1835). Different spatial structure patterns were detected between glo- and un- lineages. Positive autocorrelation at short distances (<400 km) and slightly negative autocorrelations at larger distances (>1500 km) were detected within the glo-lineage, but no significant spatial genetic structure was detected beyond 100 km within the un-lineage. Overall, spatial genetic analysis of W. globosa revealed significant autocorrelation within short distances, indicating that restricted gene flow might be one of the most important factors in shaping genetic structure.

Key words: AFLP, chloroplast DNA, genetic differentiation, spatial genetic structure, Wolffia.