Triploid planarian reproduces bisexually with euploid gametes

Polyploidization is common among plants and some animals, and is considered a major speciation and evolution mechanism for diversity. Triploids, however, confront problems of chromosomal pairing and segregation during meiosis, which may cause aneuploid gametes and result in sterility; therefore, triploids are believed to primarily reproduce asexually. Some planarians with triploid karyotypes, such as Dugesia ryukyuensis shown here -note the three 18S chromosome loci (green) in this in situ hybridization of a metaphase spread- are comprised of populations with different reproductive strategies, namely asexual, sexual, and seasonally switching. These populations contain various types of ploidy among individuals, and are normally found in nature as both fissiparous and oviparous. Fissiparous triploids, however, can be experimentally sexualized if they are fed sexual planarians. In these sexually reproducing individuals, female germ-line cells remain triploid until prophase I, while male germ-line cells appeared to become diploid before entry into meiosis. This unique reproductive system prevents an evolutionary dead end while sustaining the advantages of polyploidy that improve the fitness of the species.