New Data on Reproductive System and Spermatozoa Confirm Macrodasys as a Model in Comparative Reproductive Analysis in Macrodasyida (Gastrotricha)

Abstract

Gastrotricha Macrodasyida, aquatic worms, are primarily hermaphrodites with internal cross-fertilization and indirect sperm transfer. Insightful information on this matter dates to 1978 and derives from an illuminating work focused on two Macrodasys species. The role as a “model” of this taxon clashes, however, with the relative paucity of data concerning the ultrastructure of the accessory organs involved in the sperm transfer. Therefore, we have undertaken a new study on three additional species of Macrodasys focusing on the frontal and caudal organs and also on the spermatozoa. Our observations confirmed the structural plan of the reproductive system reported for the genus, but found some differences concerning the accessory reproductive organs. Most importantly, we found spermatozoa inside the caudal organ of several single specimens, suggestive of a more plausible hypothesis about sperm transfer modality in Macrodasys. In the spermatozoa, the position of the mitochondrion outside rather than inside the nucleus has been confirmed as a major difference between the spermatozoa of Macrodasys species and those of the other Macrodasyida. We suggest that the peculiar structure of the mature spermatozoon of Macrodasys could derive from an incomplete process of spermatogenesis and hence it would be useful to infer phylogenetic alliances.

Confocal Laser Scanning Microscopy Applied to a New Species Helps Understand the Functioning of the Reproductive Apparatus in Stylet-Bearing Urodasys (Gastrotricha: Macrodasyida)

“…Since the vas deferens are generally not in functional continuity with the caudal organ, the latter must first load itself with autosperm before transferring them into the partner's frontal organ. Therefore, it is a type of indirect fertilisation, and the frontal organ is female in function (e.g., [34]). To load the caudal organ with autosperm, the animal folds its caudal part ventrally until the opening of the copulatory organ encounters the male pore; a luminal continuity between the vas deferens and the caudal organ itself is therefore established and the passage of spermatozoa between the two structures may take place.…”

“…The description of such a reproductive behaviour is based on detailed observations carried out on species of the genus Macrodasys [34,35], and it is supposed to occur also in most other macrodasyidans, including the stylet-bearing Urodasys species (e.g., [20,21]).…”

“…As such, the reproductive behaviour of the stylet-bearing Urodasys and those described for Macrodasys are fundamentally different. In fact, Macrodasys representatives are equipped with a caudal organ blind-ended, and inoculation of several packets of sperm by a single partner is not possible (at least in the short run) given the time needed to reconstruct the copulatory tube [34]. Another difference between species of the two taxa is also related to where the entry of the allosperm takes place.…”