The male germ cells must adopt the correct morphology at each differentiation stage for proper spermatogenesis. The spermatogonia regulates its differentiation state by its own migration. The male germ cells differentiate and mature with the formation of syncytia, failure of forming the appropriate syncytia results in the arrest at the spermatocyte stage. However, the detailed molecular mechanisms of male germ cell morphological regulation are unknown. Here, we found that EXOC1, a member of the Exocyst complex, is important for the pseudopod formation of spermatogonia and spermatocyte syncytia in mice. EXOC1 contributes to the pseudopod formation of spermatogonia by inactivating the Rho family small GTPase Rac1 and also functions in the spermatocyte syncytia with the SNARE proteins STX2 and SNAP23. Since EXOC1 is known to bind to several cell morphogenesis factors, this study is expected to be the starting point for the discovery of many morphological regulators of male germ cells.

31Spermatogenesis requires high regulation of germ cell morphology. The 32 spermatogonia regulates its differentiation state by its own migration. The male 33 germ cells differentiate and mature with the formation of syncytia, failure of 34 forming the appropriate syncytia results in the arrest of spermatogenesis at the 35 spermatocyte stage. However, the detailed molecular mechanisms of male germ 36 cell morphological regulation are unknown. Here, we found that EXOC1 is 37 important for the pseudopod formation of spermatogonia and spermatocyte 38 syncytia in mice. We found that while EXOC1 contributes to the inactivation of 39Rac1 in the pseudopod formation of spermatogonia, in spermatocyte syncytium 40 formation, EXOC1 and SNAP23 cooperate with STX2. Our results showed that 41 EXOC1 functions in concert with various cell morphology regulators in 42 spermatogenesis. Since EXOC1 is known to bind to several cell morphogenesis 43 factors, this study is expected to be the starting point for the discovery of many 44 morphological regulators of male germ cells.

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