Spermiogenic histone transitions and chromatin decondensation in Decapoda

Decapoda are among of the most diverse groups of Crustacea with an important economic value, and have thus been the focus of various reproductive biology studies. Although spermatozoa are morphologically diverse, decapod spermatozoa possess common features, such as being non-motile and having uncondensed nuclear chromatin. Many scholars have studied uncondensed chromatin in decapod spermatozoa; however, the role of biologically regulated decondensation in spermatozoa remains unclear. In this study, histone changes in the spermatozoa of five commercially relevant aquatic crustacean species (Eriocheir sinensis, Scylla paramamosain, Procambarus clarkii, Fenneropenaeus chinensis, and Macrobrachium nipponense) were studied via liquid chromatography-tandem mass spectrometry (LC-MS/MS) and immunofluorescence. The LC-MS/MS results confirmed that all four core histones were present in the sperm nuclei of the five Decapoda species. Positive fluorescent signals from histones H2A, H2B, H3, and H4 were detected in the spermatozoa nuclei of E. sinensis, S. paramamosain and M. nipponense via immunofluorescence. Histone H2A was first identified in the membrane sheets or cytoplasm of mature sperm in P. clarkii and F. chinensis, whereas H3 and H4 were generally distributed in the nucleus of the spermatozoa. Histone H2B gradually disappeared during spermiogenesis and was not found in the sperm of P. clarkii and F. chinensis eventually. Our data suggest that core histones are instructive and necessary for chromatin decondensation in decapods spermatozoa. Thus, our results may help resolve the complex sperm histone code and provide a reference for the study of spermatozoa evolution in Decapoda.