The development of a bdelloid egg: a contribution after 100 years

Rotifer development has received very little attention: studies date back to the 19th century and to the first half of 20th century, and very limited contributions have been added in recent times. All information we have on rotifer embryology is mostly based on in vivo observation of developing embryos by light microscopy, and only in a minor way by classical histology. The study of rotifer embryogenesis is approached here using in vivo observation and laser confocal microscopy. We revealed cytoskeletal components (filamentous actin and tubulin) and nuclear DNA of the embryos to draw the pattern of the early development of Macrotrachela quadricornifera. Our results were then compared to the literature data, to determine a development pattern that can be generalized to the whole rotifer group. On the whole, our results agree with the general description provided by previous authors, i.e. the holoblastic unequal segmentation, the transverse furrow of the first division, the typical 16-cell stage, and the early gastrulation by epiboly. A peculiar pattern could also be seen that was interpreted as the formation of the mastax; it seemed to start from a mould of actin, visible by confocal only. The present study provides a preliminary contribution to a too-long-neglected aspect of rotifer biology.