Evolution of the avian sex chromosomes and their role in sex determination

Is it the female-specific W chromosome of birds that causes the avian embry o to develop a female phenotype, analogous to the dominance mode of genic s ex differentiation seen in mammals? Or is it the number of Z chromosomes th at triggers male development, similar to the balance mode of differentiatio n seen in Drosophila and Caenorhabditis elegans? Although definite answers to these questions cannot be given yet, some recent data have provided supp ort for the latter hypothesis. Moreover, despite the potentially common fea tures of sex determination in mammals and birds, comparative mapping shows that the avian sex chromosomes have a different autosomal origin than the m ammalian X and Y chromosomes.