d-Tryptophan enhances the reproductive organ-specific expression of the amino acid transporter homolog Dr-SLC38A9 involved in the sexual induction of planarian Dugesia ryukyuensis

BACKGROUND: Many animals switch between asexual and sexual reproduction in nature. We previously established a system for the sexual induction of planarian Dugesia ryukyuensis by feeding asexual planarians with minced sexual planarians. We identified dl-tryptophan (Trp) as one of the sex-inducing substances. dl-Trp can induce ovarian development, the first and essential step of sexual induction. d-Trp must act as a principal bioactive compound in terms of ovarian development, because the ovary-inducing activity of d-Trp was 500 times more potent than that of l-Trp. However, how Trp controls sexual induction is still unknown. RESULTS: In this study, qRT-PCR analyses suggested that the putative amino acid transporter gene Dr-SLC38A9 is highly expressed in sexual worms, especially in the yolk glands. In situ hybridization analyses showed that Dr-SLC38A9 is expressed in the ovarian primordia of asexual worms and in the mature ovaries, testes, and yolk glands of sexual worms. In addition, Dr-SLC38A9 RNA interference during sexual induction resulted in the suppression of the development of reproductive organs. These results suggest that Dr-SLC38A9 is involved in the development of these organs. Moreover, we demonstrated that the reproductive organ-specific expression of Dr-SLC38A9 is enhanced by the addition of d-Trp. CONCLUSION: We propose that d-Trp activates the expression of Dr-SLC38A9 to promote sexual induction in the planarian D. ryukyuensis. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s40851-021-00173-z.