Morphological Variation in the Sperm Head of Ryukyu Long-Furred Rat,Diplothrix legata

Abstract

Ryukyu long-furred rats (Diplothrix legata) are large rodents distributed only on three small islands in Japan: Amami-Oshima Island, Tokuno-shima Island, and Okinawa-jima Island (Iwasa 2009). These rats are an endangered species (Ministry of Environment, Japan 2012; IUCN 2014), and little is known about their ecological and physiological characteristics (Iwasa 2009). For the management or conservation of the Ryukyu longfurred rats, it is important to know such information. Reproductive traits are important features to consider the maintenance of populations of an endangered species from standpoint of conservation biology. The relationships between sperm morphology and inbreeding have been reported for endangered mammals (O’Brien et al. 1987; Gomendio et al. 2000; Gage et al. 2006). The ­individual­coefficient­of­inbreeding­is­inversely­related­ to morphologically normal spermatozoa frequency in the endangered gazelles (Gazella cuvieri); this species has a­ high­ inbreeding­ level,­with­ inbreeding­ coefficients­ of­ approximately 0.15 (Gomendio et al. 2000). Males with few normal form spermatozoa are known to have low fertility (Kruger et al. 1988; Guzick et al. 2001). Mammalian­ spermatozoa­ have­ species-specific­ morphology.­ The sperm head of domestic mammals, including bull, stallion,­ pig,­ and­ dog,­ is­ generally­ flattened­ and­ ovalshaped (Masuda 1995). In contrast, most rodents have falciform sperm head with an apical hook, and many morphological sperm variations among species have been reported (Breed and Yong 1986; Breed 2004). For example, the laboratory mouse (Mus musculus) and the striped field­mouse­(Apodemus agrarius) have two ventral processes in the sperm head, which are absent in the laboratory rat (Rattus norvegicus; Lee and Mori 2006). Furthermore, the desert mouse (Pseudomys desertor) has three apical hooks in the sperm head, whereas only one hook is present in most other species (Breed 1983). However, the sperm morphology of Ryukyu long-furred rats has not yet been reported. Therefore, this study aimed to investigate the morphology of spermatozoa in Ryukyu long-furred rats by using phase-contrast and scanning electron microscopy.