Are There No Mammal Species with Xo Males-And If Not, Why Not?

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Abstract
There are 5 species of mammals in which an XO, or apparently XO condition has been reported, although not all claims are verified. In the case of the microtine rodents Ellobius lutescens and Microtus (Chilotus) oregoni, the female also possesses a single sex chromosome and the same chromosome number as the male. This may involve an attached XX in the female and an attached XY in the male (Ellobius) operating as a balanced lethal mechanism. If the male Microtus has a similar attached XY chromosome, we conclude that no mammal truly lacks a Y in the male sex. Loss of the Y in mammals is presumably excluded by its powerful male-determining role, whereas in certain other groups of animals (Coleoptera) repeated evolutionary losses of the Y have occurred without masculinizing individuals that were otherwise female. "Loss" probably involves translocation to the X or to an autosome. The existence of three or four instances of XY1Y2 species in the mammalia (whereas no X1X2Y species is known) may likewise indicate that small portions of the X are more apt to be dispensable in this group than similar segments of the Y (the origin of an XY1Y2 mechanism by centric fusion will involve deletion of a segment of the X, while a centric fusion which leads to an X1X2Y system involves deletion of a piece of the Y).