Many metazoans can reproduce sexually as well as asexually. When environmental factors are favourable, asexual reproduction is employed to exploit suitable conditions for survival. In general, they switch from an asexual to a sexual mode of reproduction, when individual survival is greatly hampered. Variations found in the offspring resulting from sexual reproduction allow some individuals to be better suited for survival and provide a mechanism for selective adaptation. Therefore, the reproductive strategy of switching between asexual and sexual reproduction may contribute to fitness. However, the mechanisms underlying the switch between the 2 modes remain unknown.

Some freshwater planarians (Platyhelminthes, Turbellaria, Seriata, Tricladida) reproduce asexually as well as sexually. The sexual worms are hermaphrodites, while the asexual worms reproduce by fission without forming any sexual organs. Certain worms develop sexual organs during the colder season, while they reproduce asexually with much degenerated gonads during the warmer season of the year. Asexual worms can switch to the sexual mode if they are fed sexual worms; this means that the sexual worms contain a sex-inducing substance that is not species-specific. The putative sex-inducing substance is likely to provide clues about the mechanisms underlying the switch from an asexual to a sexual mode of reproduction. However, very little information is available on the sex-inducing substance.

In order to isolate and identify this sex-inducing substance, we have established a bioassay system for sexual induction. We planned to use asexual worms exclusively as the test worms for isolating and identifying the sex-inducing substance. Other important issues regarding the animals were the supply (abundance, constancy, ease, and readiness), easy culture in laboratory, resistance to operations such as ablation, and adequate body size. Considering all these points, we chose the OH strain of Dugesia ryukyuensis as the test worms because spontaneous sexual induction has never been observed in this strain under laboratory conditions. No functional sexual organs were externally recognised in the test worms before or after they consumed conspecific asexual worms and chicken liver—their daily food. However, when they were fed the minced worms of B. brunnea, an oviparous species, they developed hermaphroditic sexual organs. We divided the process of sexual induction into 5 distinct stages by morphological changes [Figure]. In stage 1, the ovaries became sufficiently large to be externally apparent behind the head, yet no oocytes or other sexual organs were detectable. In stage 2, oocytes appeared in the ovaries, but other sexual organs remained undetectable. In stage 3, the primordial testes emerged and a copulatory apparatus became visible as a white speck in the post-pharyngeal region. In stage 4, yolk gland primordia developed, and spermatocytes appeared in the testes. In stage 5, matured yolk glands formed, and many matured spermatozoa were detectable in the testes.

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