Novelties in the Evolution of the Urodasys reproductive system (Gastrotricha, Macrodasyida)

Macrodasyidan gastrotrichs are hermaphrodites with complex accessory reproductive organs that function in sperm transfer and receipt. Unfortunately, homology among the organs of members of different clades is largely undetermined, troubling a clear understanding of the evolutionary trends in the reproductive biology of these animals. The present study investigates the evolution of reproduction in species of the peculiar genus Urodasys, quite popular among researchers working on meiofauna in virtue of their very long tail. These gastrotrichs are extremely interesting also because they show a wide range of structures, modes, and reproductive strategies. It is likely that a phylogenetic study of these taxa may shed light on the evolutionary trends in the reproductive biology of the genus and possibly of the entire phylum. Urodasys presently consists of 15 species, one of which, U. viviparus, is parthenogenetic and ovoviviparous, while the others are hermaphroditic and oviparous. Four of the latter species show paired ovaries and testes, but no accessory sexual organ, whilst 10 possess two ovaries, a single testis and a sclerotic, copulatory organ called a stylet. Recently, a new species bearing a stylet but two testes was found at Lanzarote (Canary Islands). All this data was opportunely coded and cladistically analyzed. Meanwhile, phylogenetic analyses based on sequences of the 18S rDNA gene obtained from representative species were concurrently carried out. The results of the two analyses are widely overlapping and substantially separate the investigated species into two clades, with the new species that appears as an early divergent line along the evolutionary branch of the stylet-bearing taxa. However, U. viviparus appears to be allied with the stylus-lacking species in the morphological analyses, while the molecular analyses place it amongst the stylet-bearing species. In any case, regardless of the position of U. viviparus, the results indicate that the sequence of evolutionary transformations occurred in the reproductive system of the species of Urodasys is likely dissimilar from those previously proposed by other authors.