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Animals or plants with separate sexes are widespread in nature, and result from independent transitions from their hermaphroditic ancestor. The actual mechanism involved in the transitions between asexual and sexual reproductive modes, in other words, how is sex originated, remains an important and unanswered question. Excluding insects, about one third of the animal species, such as earthworms, snails, and some teleosts, are hermaphroditic. A comparison with their relatives who have evolved separate sexes, might reveal, how this particular trait originated and evolved in animals.

Blood flukes -- from hermaphrodite to separate sexes

Another important finding comes from the comparison of the sexually reproducing blood flukes to related hermaphroditic species. The transition into the state of separate sexes in the parasite occurred relatively recently, around 70 million years ago. The authors showed that during this transition, the gonad genes of schistosomes became less "feminized," in other words overall showed a lower ovary expression level compared to their counterpart in the hermaphroditic related species. They also identified a candidate gene, mag-1, whose disruption in schistosomes causes enlarged testis. Mutations in this gene might have played a critical role for the transition into the separate sexes of schistosomes.

Full open access paper: https://www.nature.com/articles/s41467-022-30578-z