Evaluation of heterosis and high temperature tolerance in hybrid offspring of different local Saccharina japonica cultivars

As an economically and ecologically important seaweed, the genetic improvement of kelp Saccharina japonica has emerged as a significant research focus in the field of algal aquaculture. Crossbreeding, an essential method for kelp breeding, has yielded numerous elite varieties that have found practical applications. However, the mechanism underlying heterosis in kelp remains unclear, and the selection of parents for hybridization still lacks precision. In this study, we employed multiple kelp cultivars from both southern and northern kelp farms in China as parental cultivars for hybridization, and compared the gametophyte development and sexual reproduction during the hybridization process, as well as the growth rate and heat tolerance of the resulting hybrid progeny. Our findings revealed that the gametophyte development rate of all crosses is prone to exceed that of the parental selfing group, implying that the outbreeding is more successful than selfing in kelp wild population. Certain crossing combinations exhibited significant heterosis in terms of growth rate and high temperature tolerance in the hybrid progeny, while the hybrid weakness was also observed in other crosses. Through an analysis of the physiological (Fv/Fm) and biochemical (Total Antioxidant Capacity, H2O2, and malondialdehyde) responses of the hybrids to high-temperature treatment, we discovered that the initial antioxidant level and the subsequent increase in antioxidant capacity within 24 h are crucial for the ability to tolerate high temperature stress. This study systematically evaluated the heterosis of multiple hybrids and attempted to elucidate the mechanism of heat-tolerant heterosis in S. japonica from physiological and biochemical perspectives. The novel insights gained from this study could provide valuable ideas and a foundation for future researches of kelp crossbreeding efforts. •Outbreeding is more popular than selfing in Saccharina japonica sexual reproduction.•Kinship between male and female gametophyte affects the gametogenesis and reproduction of S. japonica.•The kindship of two parents decided the performance of hybrids, with only some crossing combinations showed heterosis.•Initial antioxidant level and 24 h antioxidant boost is crucial for the heat tolerance of S. japonica.