Newly unveiled meiosis elucidates the unreduced gamete frequency and its impact on the evolution of the Lemna minor complex.

Fusion of gametes possessing meiotically reduced (haploid) chromosome complements is the main pathway of propagation among eukaryotes. However, duckweeds, the smallest angiosperms, propagate mainly vegetatively, and meiosis has not yet been documented in detail for this plant family. More surprising was the recent evidence of rather frequent interspecific hybrids and triploid clonal accessions, which became obvious by genome size measurements, genomic in situ hybridization (GISH), and combined plastid and nuclear DNA markers. These observations indicated sexual propagation involving reduced as well as unreduced male and female gametes in Lemna minor and L. turionifera, leading to allodiploid and allotriploid hybrids (with genome compositions MT, MMT, MTT) and autotriploid L. minor (MMM) accessions. Here, we (i) document the meiotic stages of Lemna species for the first time photographically; (ii) provide evidence of unreduced male gametes through fluorescent in situ hybridization (FISH) with single locus probes; (iii) determine their abundance in different individuals; and (iv) discuss possible reasons for unreduced male gamete formation. These findings open new insights into the modes of sexual reproduction and evolution of duckweeds, which may be useful for future breeding efforts in this emerging crop.