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Abstract

Strikingly, during the sexual reproduction in free-living Strongyloides spp. only female progeny arise. Assuming "normal" male meiosis one would expect that 50% of the sperm formed lack an X chromosome and hence, upon fertilizing oozytes, give rise to male progeny. We had shown earlier by quantitative whole genome sequencing that in Strogyloides spp. nullo-X sperm is absent (in S. papillosus) or underrepresented (in S. ratti) among mature sperm. To study how and when male determining sperm is eliminated we used DIC microscopy, immunohistochemistry, fluorescent in situ hybridization (FISH) and electron microscopy to characterize spermatogenesis in S. ratti, S. papillosus and Parastrongyloides trichosuri. We found the meiotic divisions to occur "normally", as expected for an organism with XO males leading to four spermatocytes, two with and two without an X chromosome. All four products of meiosis are of approximately equal size and we did not observe residual bodies, as they are known from C. elegans. Further down in the gonad, where the sperm mature, we found most, if not all nuclei to contain an X chromosome and we observed structures that contained protein constituents of sperm, such as actin and major sperm protein (MSP) but no DNA. These structures are reminiscent of C. elegans residual bodies in appearance and may assume their function. We suggest that spermatocytes without an X-chromosome undergo some form of programmed cell death and transform into these residual body-like structures. Like in C. elegans, MSP is found in fibrous body-membranous organelles (FB-MOs). However, the FB-MOs in Strongyloides spp. differ from the ones in C. elegans in size, number and appearance. Knocking down MSP by RNAi showed that MSP is essential for viability.