In mammals, gender follows from the development of the embryonic gonads as testis or ovaries and is under genetic control. In males, testis development is dependent on the inheritance of the Y Chromosome-encoded Sry gene. It is believed, as a result of transgenic experiments, that Sry is the only gene encoded on the Y Chr that is necessary to switch the genital ridges towards the testicular developmental pathway (Koopman et al. 1990). An exception to this rule is described in a recent report describing the absence of Sry in two species of an arvicoline rodent, Ellobius, which have an unconventional gonosome complement (Just et al. 1995).

In this context, Tokudaia osimensis osimensis, a small murine rodent from the Amami Island (part of the Ryukyu Archipelago in the south of Japan), is an intriguing mammal. Indeed, in this particular species, the Ryukyu Spiny Rat, males and females have an identical karyotype (2n 25, XO; Honda et al. 1977). This small (head and body length from 12 to 18 cm) spiny rat is a rare mammal (Nowak 1991), believed by IUCN (1978) to be seriously endangered by the general destruction of natural arcas on the Ryukyu Islands. Tokudaia osimensis is fully protected in Japan. The presence or the absence of Sry on either the X Chr or on one of the autosomes of this particular mammal was, therefore, investigated.