An intraspecific crossbreeding experiment of Undaria pinnatifida was carried out by reciprocal crossing a natural strain (Kn) sampled from Kagoshima Prefecture (southern site) with an early harvesting cultivar (N1) from the Naruto area, Tokushima Prefecture. Seedlings of four experimental crosses, F1 hybrids (N1 ♀×Kn♂, Kn♀×N1♂) and inbreds (N 1♀×N1♂, Kn♀×Kn♂) by crossing free-living male/female gametophytes that respectively originated from single zoospores, were cultivated at Naruto. The hybrid N1♀×Kn♂ showed better growth for both blade length and blade weight than the two inbreds during the early harvesting period, suggesting heterosis effects on growth of the F1 hybrid. The hybrid N1♀×Kn♂ showed similar growth to the inbred Kn♀×Kn♂ compared to the other inbred N1♀×N1♂, indicating the great influence of male gametophytes on growth. In contrast, the other hybrid Kn♀×N1♂ showed the worst growth rates among all the seedlings. The “southern type”inbred Kn♀×Kn♂ grew much better in size compared to its parent Kn when cultured in the Naruto area. Morphologically, the hybrids showed intermediate characteristics of their parents, but rather closer to the female parent. These findings show that intraspecific crossbreeding of U. pinnatifida between a southern type strain and a cultivar can be an effective method for cultivar improvement for early harvesting.
