Evolutionary trend for metamery reduction and gonad shortening in anurans revealed by comparison of gonad development

The gonads develop as the metameric mesodermal structures at the ventral surface of the mesonephroi. To study the evolutionary trends for anuran gonads, we performed comparative analysis of three species from the basal paraphyletic group – Archaeobatrachia (Bombina bombina, Xenopus laevis, Pelobates fuscus) and five species from more derived monophyletic Neobatrachia group (Hyla arborea, Bufotes viridis, Rana dalmatina, Rana arvalis, Rana temporaria). Light and scanning electron microscopy analysis showed that in Archaeobtrachia the undifferentiated gonads and ovaries, but not the testes, have pronounced external metamery. In contrast, Neobatrachia lacked external gonadal metamery and the internal metamery was well pronounced in the ovaries, but only rudimentary in the testes. There was also a difference in the site of genital ridge formation between these two groups. In Archaeobatrachia, the genital ridges developed along almost the entire length of mesonephroi, while in Neobatrachia they were limited to the anterior parts of the mesonephroi. A transient form was observed in P. fuscus, where the gonads formed along the anterior half of the mesonephroi. There was also an evolutionary trend for gradual postero-anterior shortening of the gonadal anlagen. In summary, our study indicates that among anurans there is an evolutionary trend for the gradual loss of metamery and shortening of the gonad.