Sperm morphology among the various members of the genusPseudomysshowed great interspecific variation. InP. apodemoides,P. desertor, P. fumeus, P. gracilicaudatus, P. hermannsburgensis, P. higginsiandP. nanusthe sperm head was bilaterally flattened and possessed three hooks, the length of which varied somewhat between species. The top hook contained nuclear, subacrosomal and acrosomal material, whereas the two lower ventral hooks displayed largely only an extension of the subacrosomal material. Four other species ofPseudomyshad no ventral hooks in their sperm head. InP. novaehollandiaethere was a single long top hook, whereas inP. pilligaensisit was almost nonexistent. Neither the sperm head ofP. delicatulus, nor that ofP. shortridgeipossessed any hooks; the former became narrower towards the tip, whereas the latter was spatulate in shape. The acrosome covered about two thirds of the nucleus and varied considerably in shape between species. Insertion of the connecting piece of the tail occurred on the lower ventral side of the sperm head in all species with a long top hook but was nearly basal inP. delicatulusand midbasal inP. shortridgei. The ultrastructure of mid and principal pieces was similar among species, but the number of gyres of mitochondria ranged from about 64 to 100.
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