The identification of d-tryptophan as a bioactive substance for postembryonic ovarian development in the planarian Dugesia ryukyuensis

Abstract

Many metazoans start germ cell development during embryogenesis, while some metazoans possessing pluripotent stem cells undergo postembryonic germ cell development. The latter reproduce asexually but develop germ cells from pluripotent stem cells or dormant primordial germ cells when they reproduce sexually. Sexual induction of the planarian Dugesia ryukyuensis is an important model for postembryonic germ cell development. In this experimental system, hermaphroditic reproductive organs are differentiated in presumptive gonadal regions by the administration of a crude extract from sexual planarians to asexual ones. However, the substances involved in the first event during postembryonic germ cell development, i.e., ovarian development, remain unknown. Here, we aimed to identify a bioactive compound associated with postembryonic ovarian development. Bioassay-guided fractionation identified ʟ-tryptophan (Trp) on the basis of electrospray ionization–mass spectrometry, circular dichroism, and nuclear magnetic resonance spectroscopy. Originally masked by a large amount of ʟ-Trp, ᴅ-Trp was detected by reverse-phase high-performance liquid chromatography. The ovary-inducing activity of ᴅ-Trp was 500 times more potent than that of ʟ-Trp. This is the first report describing a role for an intrinsic ᴅ-amino acid in postembryonic germ cell development. Our findings provide a novel insight into the mechanisms of germ cell development regulated by low-molecular weight bioactive compounds.

A comprehensive comparison of sex-inducing activity in asexual worms of the planarian Dugesia ryukyuensis: the crucial sex-inducing substance appears to be present in yolk glands in Tricladida

“…Their eggs are ectolecithal (cocoons), which have several fertilized eggs and numerous yolk gland cells [ 24 ]. In D. ryukyuensis (Tricladida), the existence of intact yolk gland cells in fresh cocoons collected within a day of deposition has been suggested by quantitative reverse transcription polymerase chain reaction analysis of a yolk gland marker gene [ 23 ].…”

“…In contrast, worms at stage 3 and beyond keep developing sexual organs even if feeding with the test food is stopped. Recently, we identified the ability of d-Trp to induce stage 2 ovaries in asexual worms of D. ryukyuensis [ 23 ]. However, d-Trp does not induce the other reproductive organs.…”

“…In the fractionation procedure, d-Trp is recovered primarily in Fr. M0 [ 23 ]. It may be reasoned that a few asexual worms of D. ryukyuensis fed with the test food containing the Fr.…”

“…3 ). Recently, we also found that a large amount of l-Trp is incorporated and pooled in the yolk glands, resulting in the accumulation of d-Trp that is involved in the ovarian development of asexual worms as a sex-inducing substance [ 23 ]. Motivated by these findings, we fed the asexual worms of D. ryukyuensis with fresh cocoons of D. ryukyuensis and Bd.…”

“…1a–c ) [ 21 , 22 ]. Recently, we found that d -Trp is involved in ovarian development of asexual worms as a sex-inducing substance [ 23 ]. However, d -Trp does not trigger complete sexual induction in asexual worms.…”