Embryonic growth and the evolution of the mammalian Y chromosome. I. The Y as an attractor for selfish growth factors.

The fitness of a mammalian zygote is affected by its probability of implantation and of postimplantation maintenance as well as the level of transplacental and transmammary uptake of resources. As with paternally expressed imprinted genes, in a species in which females are not obligately monogamous, a Y-linked sequence that can positively alter any of the above parameters could spread in a population even if it harms the prospects of other embryos. Such a selfish Y-linked gene could act as a sex ratio distorter. In contrast to autosomal imprinted loci, the patrilineal inheritance of the Y ensures that selfish Y-linked growth-promoting genes need not evolve a means to ensure correct parent-dependent expression rules. Thus, as the conditions for both their initial evolution and spread are relatively relaxed, the mammalian Y chromosome is expected to be an attractor for growth-promoting genes. Data from mice and humans indicate that, as expected and in contrast to the Y of flies, the mammalian Y harbours growth factors, sex ratio factors and multiple foetally expressed genes. The accumulation of Y-linked genes may also be explained in terms of sexual antagonism. Sexual antagonism and the model presented here are not mutually exclusive.