For examining whether or not a germ cell derived from a donor fish, which is transplanted into a heterogeneous recipient fish by surrogate fish technique, is grown and matured in a gonad of the recipient fish, it is required to utilize, as an indicator, a trait which is expressed specifically in the germ cell and can be used for distinguishing the recipient fish from the donor fish. Vasa gene, which is a gene specific to a germ cell, is specific to a primordial germ cell and/or a spermatogenous cell/oogonium, and the expression of Vasa gene is not observed in a somatic cell. The sequences of Vasa genes of a tuna, a chub mackerel, a spotted mackerel, an eastern little tuna and a drumfish are determined, and the expression of each of the genes is utilized as a maker for a germ cell. It is possible to detect only a tuna Vasa gene specifically from Vasa gene sequences which are highly conserved in fishes, without the need of sequencing. Therefore, it is possible to identify a tuna-derived germ cell in a gonad of a recipient fish in a simple and reliable manner. Thus, a tuna fish can be grown and bred with good efficiency. By utilizing the above-mentioned knowledge, it becomes possible to detect a germ cell derived from a donor fish from a germ cell derived from a heterogeneous recipient fish with good efficiency even when, besides a tuna fish, a fish belonging to the order Perciformes is used as a donor fish.
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