Extensive induction of diploid aposporous gametophytes and their application in polyploid breeding of Undaria pinnatifida

Triploid breeding is a promising strategy for developing Undaria pinnatifida cultivars with higher yield, better quality, and stronger resistance to environmental stresses. We previously developed a triploid breeding method through crossing aposporous gametophytes derived from doubled haploid sporophytes with haploid gametophytes. However, only one male diploid aposporous gametophyte line was obtained. In the present study, we extensively induced three kinds of aposporous gametophytes, namely MoAG, PaAG, and CAG, from sporophytes derived, respectively, from selfing of monoicous gametophytes, parthenogenesis of female gametophytes, and cross-fertilization. We found that all the MoAGs were diploid and monoicous, the PaAGs all female and either diploid or haploid, and the CAGs all monoicous and either diploid or haploid. Parentage analysis revealed that cross-fertilization predominated when crossing the monoicous aposporous gametophytes with female haploid gametophytes. Triploid sporophyte lines were extensively generated by crossing the diploid gametophytes with haploid gametophytes. Six out of 17 triploid lines exhibited significantly superior comprehensive economic traits after cultivation in the sea and are therefore promising candidate cultivars for larger scale cultivation. Tetraploid sporophytes were obtained through selfing of diploid monoicous gametophytes. This study lays a solid foundation for polyploid breeding and development of superior cultivars in U. pinnatifida.