Sperm morphology of the Australasian hydromyine rodents and the interactions between the spermatozoon and oocyte at the time of fertilisation

Context This paper explores the morphology of spermatozoa in Australian hydromyine rodents, specifically focusing on the plains mouse (Pseudomys australis ), and examines the interactions between sperm and eggs at time of fertilisation. Aims The aim of this study is to provide an overview of sperm morphology of hydromyine rodents, comparing its morphology across the different species and to investigate the interactions between the gametes at fertilisation in the plains mouse. Methods We summarise the sperm head morphology of the hydromyine rodents across the six divisions, with emphasis on the structure in the plains mouse and its interactions with the zona pellucida during fertilisation. Key results Most hydromyine rodents, including the plains mouse, exhibit a highly complex sperm head morphology with two prominent ventral processes in addition to the apical hook. These processes primarily contain filamentous actin with some species of the New Guinea Pogonomys Division having a nuclear extension into the lower process. Nevertheless three species in the Pogonomys Division and a few in the Pseudomys Division have derived sperm heads which lack the ventral processes which in the plains mouse bind the sperm to the zona pellucida around the ovulated oocyte. This may stabilise the sperm head at this time and facilitate zona pellucida penetration and fusion with the oolemma at this time. Conclusion The complex sperm head morphology in most of the hydromyine rodents is likely to date back over one million years with, in the plains mouse, interaction between sperm and egg during fertilisation involving sperm head stabilisation and zona pellucida attachment. Implications These findings suggest in hydromyine rodents valuable insights into the evolutionary development of sperm morphology and sperm-egg interactions during fertilisation, and in particular that the role of the ventral processes may be critical for successful fertilisation in this group. Understanding these processes could give insight into broader studies on reproductive strategies and evolutionary biology in rodents.