Fig. 3

Five stages of sexual induction and estimation of stages by external observation. a Morphological changes during sexual induction allowed us to divide the process into five stages. The colored regions correspond to the reproductive organs: red, ovary; aqua blue, seminal receptacle; blue, testis; yellow, yolk gland; green, copulatory apparatus. Cell masses of female primordial germ cells (primordial ovaries) were histologically identified in the asexual worms (stage 0), although they were barely visible externally. The ovaries became sufficiently large to be externally apparent behind the head, although no oocytes or other reproductive organs were detectable at stage 1. Oocytes appeared in the ovaries, but other reproductive organs remained undetectable at stage 2. The primordial testes and yolk gland primordia emerged, and a copulatory apparatus became visible in the post-pharyngeal region at stage 3. The genital pore in the copulatory apparatus opened on the ventral side of the worm and spermatocytes appeared in the testes at stage 4. Mature yolk glands formed, and spermatids and spermatozoa were detectable in the testes at stage 5. b By external observation of the test worms, we can recognize a pair of ovaries and a copulatory apparatus (without or with a genital pore) in the ventral side of test worms. The colored regions correspond to these reproductive organs: red, ovary; green, copulatory apparatus. Three stages were identified by external observation: worms with only a pair of ovaries (stage 1–2), worms with a copulatory apparatus without a genital pore (stage 3), and worms with a copulatory apparatus with a genital pore (stage 4–5). This sexual induction has a point-of-no-return between stages 2 and 3. In the reversible phase, worms degenerate a pair of developing ovaries to return to asexual if feeding with sexual worms is stopped. In the irreversible phase, worms continue developing all the reproductive organs, even if feeding with sexual worms is stopped