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Sperm Morphology of The Australasian Hydromyine Rodents and the interactions between the Spermatozoon and Oocyte at the time of Fertilisation

William Breed , Chris Leigh, Emily Roycroft, Ingrid Ahmer

Abstract

This paper reviews the morphology of the spermatozoon of Australasian hydromyine rodents together with the interactions that take place between the sperm and egg at the time of fertilisation in the Plains mouse, Pseudomys australis. Unlike most murid rodents, most hydromyine rodents in the 6 divisions including the Plains mouse have a highly complex sperm head that contains two prominent ventral processes in addition to the apical hook. These processes are largely composed of filamentous actin albeit that in a few species of the New Guinea Pogonomys Division the nucleus extends for most of the length of the lower process. Since this complex sperm form is present in most species of hydromyine rodents in all 6 divisions it suggests an evolutionary origin of several million years ago. There are, nevertheless, a few species in Pogonomys Division that have a sperm head that lacks these ventral processes, whereas in the Pseudomys Division highly divergent sperm heads have evolved in a few species that lack both the ventral processes. In the Plains mouse these ventral processes bind to the outer surface of the zona pellucida that surrounds the recently ovulated oocyte at the time of fertilisation. This binding may enhance stabilisation of the sperm head at this time thus enhancing sperm head attachment to the zona pellucida after which time the acrosome reaction may occur and passage through the zona pellucida take place and fertilisation result.

RD25012  Accepted 18 March 2025

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