Hybridity of mainly asexually propagating duckweeds in genus <i>Lemna</i> – dead end or breakthrough?

The cosmopolitan, mainly vegetatively propagating, organ-reduced monocotyledonous aquatic duckweeds are the smallest and fastest growing angiosperms, distributed world-wide and flower rarely in nature. Recently, we reported intra- and interspecific hybrids and ploidy variants in the genus Lemna. Thus, contrary to the expectation, sexual propagation may occasionally occur within and between Lemna species. Our main goal was to uncover whether the ecologically successful hybrids are evolutionary dead ends or initiate further speciation and novel sexual recombination. We investigated flower development, pollen viability, seed set, and seed germination in hybrids and their parental species and characterized genome size and genetic markers in the progenies. Intraspecific crosses yielded fertile progeny, but all diploid and triploid interspecific hybrids were male sterile. Only an established allotetraploid hybrid reproduced sexually, while colchicine-induced allotetraploids from allodiploids did not regain sexual competence so far. We concluded that only established allotetraploid hybrids represent an evolutionary breakthrough in duckweeds. Our results regarding sexual traits within the duckweed genus Lemna and the sexual competence of diverse hybrids pave the way for further investigation in this understudied field, provide fundamental data regarding the evolutionary potential of duckweed hybrids and are important for future breeding efforts on this emerging crop.