Sperm Morphology and Development in Two Acoel Turbellarians from the Philippines

Abstract

In this study we compare spermiogenesis and ultrastructure of the mature sperm in two species of acoel Turbellaria from the Philippines. Sperm development is divided into five stages: (1) the early undifferentiated state, with a large nucleus, sparse cytoplasm containing few organelles, and no inclusions; (2) spermiogenesis I, in which Golgi activity is prominent, dense bodies appear in the cytoplasm, and peripheral centrioles migrate toward the nucleus; (3) spermiogenesis II, in which a manchette of microrods forms around the nucleus, refractile bodies are produced by the Golgi, and free 9+0 flagella are seen between the cells; (4) spermiogenesis III, which is characterized by marked cell elongation, nuclear condensation, and flagellar elongation and incorporation into the developing spermatid shaft; and (5) the mature sperm, which has a proximal nucleus, a middle shaft region containing a central keel of microrods, laterally incorporated axonemes, and many inclusions such as refractile bodies, dense bodies, open vesicles, mitochondria, and a distal flagellar region containing the two 9+0 axonemes tapering to terminal basal bodies. We propose that the refractile bodies may function as acrosomes, that the central keel provides support, that the biflagellate condition is important in providing the motile force that moves the sperm through intercellular spaces, and that the 9+0 axonemes may contain some central structure. The microrods of the keel appear to be a previously undescribed cellular component. The peculiar morphology of these spermatozoa is probably an adaptation associated with locomotion through the interdigitated acoel parenchyma where an extremely elongate cell, propelled flagellar tip first by undulations, is particularly efficient.