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Species of planarians include both asexually reproducing individuals (reproduce through fission and regeneration) and sexually reproducing individuals (hermaphrodites that mate to produce cocoons). While some individuals can switch between the asexual and sexual modes of reproduction. In this study, we examined the reproductive modes and ploidy of Dugesia japonica and Dugesia ryukyuensis from three spring wells in Okinawa (Japan) during two consecutive years. D. japonica are mostly asexual and triploid. In contrast, only 40 % of D. ryukyuensis are asexual and triploid; the remaining are sexual, and diploid or triploid. The sexually reproductive season of D. ryukyuensis is winter. In July, the reproductive organs disappear, and the individuals start asexual reproduction through fission and regeneration. In January of the following year, the individuals develop ovaries and necessary reproductive organs and start sexual reproduction. When these species were lab-reared for a longer period, the reproductive cycles in three strains were repeated for three years. These results confirm that D. ryukyuensis population in Okinawa switches between reproductive modes on an annual cycle, even when kept under constant temperature and no light/dark cycle. • The reproductive cycles of the wild and lab-reared Dugesia ryukyuensis were examined. • The circannual reproductive patterns were repeated under constant temperature, without light/dark conditions. • The circannual reproductive cycles was continued over three years. • It switched from asexual to sexual mode without any external induction. [ABSTRACT FROM AUTHOR]

Although polyploids are common among plants and some animals, polyploidization often causes reproductive failure. Triploids, in particular, are characterized by the problems of chromosomal pairing and segregation during meiosis, which may cause aneuploid gametes and results in sterility. Thus, they are generally considered to reproduce only asexually. In the case of the Platyhelminthes Dugesia ryukyuensis, populations with triploid karyotypes are normally found in nature as both fissiparous and oviparous triploids. Fissiparous triploids can also be experimentally sexualized if they are fed sexual planarians, developing both gonads and other reproductive organs. Fully sexualized worms begin reproducing by copulation rather than fission. In this study, we examined the genotypes of the offspring obtained by breeding sexualized triploids and found that the offspring inherited genes from both parents, i.e., they reproduced truly bisexually. Furthermore, meiotic chromosome behavior in triploid sexualized planarians differed significantly between male and female germ lines, in that female germ line cells remained triploid until prophase I, whereas male germ line cells appeared to become diploid before entry into meiosis. Oocytes at the late diplotene stage contained not only paired bivalents but also unpaired univalents that were suggested to produce diploid eggs if they remained in subsequent processes. Triploid planarians may therefore form euploid gametes by different meiotic systems in female and male germ lines and thus are be able to reproduce sexually in contrast to many other triploid organisms. [ABSTRACT FROM AUTHOR]