Aquaculture of the hybrid cultivars of Saccharina japonica: Removing the obstacle of sori production by photoperiodic control

Abstract

In aquaculture of the kelp Saccharina japonica, hybrid cultivars could improve both the yield and quality of the farmed products. Hybrid cultivars refer to the diploid macroscopic offspring resulted from crossing a pair of haploid vegetative gametophyte cell lines. When these two cell lines are derived from individuals that are geographically isolated, the adult sporophytic offspring mostly remain immature in their second year of growth from June to September in the sea. The absence of sori in the adult sporophytes of the hybrid during this period prevents them from being used as parental ones for releasing spores to produce “summer seedlings” in hatchery. This is due to the seasonal restriction of hatchery operation which requires the availability of matured sori exclusively in August in the temperate zone in the northern hemisphere. The reasons for appearance of non-sporulating adult individuals has never been investigated although raising adult sporophyte in land-based tank filled with recirculated cold water at 10 °C has been a routine operation in hatchery enterprises in the subtropical zone in the south region of China for more than 20 years. While in the north region of China, parental sporophytes have been always raised in the sea until they are used for hatchery. From 2017 to 2019, we have conducted a series of culture experiments, both in beakers with isolated thallus in incubators and in large concrete tanks with intact individuals, trying to understand this phenomenon and look for a solution. The results showed that, (1) the hybrid cultivars demonstrated a feature of photoperiodic short-day plant and that it could form sorus within 25 days when exposed to SD at higher temperature (18 °C); (2) the hybrid cultivar maintains unique genetic structures different from the conventional ones as revealed by use of ten polymorphic microsatellite markers; (3) the sensitivity of the hybrids to the shortening of day-length differs in at least 10 days among individuals and could be inherited to its offspring; (4) sori induction in the intact adult sporophytes could be achieved within a month when supplied with SD, applicable to all current cultivars including the hybrids. The discovery and solution presented in this article has opened a gateway to utilization of hybrid cultivars in the giant kelp farming industry in China.