EMBRYONIC GROWTH AND THE EVOLUTION OF THE MAMMALIAN Y-CHROMOSOME .1. THE Y AS AN ATTRACTOR FOR SELFISH GROWTH-FACTORS

The fitness of a mammalian zygote is affected by its Probability of im plantation and of postimplantation maintenance as well as the level of transplacental and transmammary uptake of resources. As with paternal ly expressed imprinted genes, in a species in which females are not ob ligately 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 ac t as a sex ratio distorter. In contrast to autosomal imprinted loci, t he patrilineal inheritance of the Y ensures that selfish Y-linked grow th-promoting genes need not evolve a means to ensure correct parent-de pendent expression rules. Thus, as the conditions for both their initi al evolution and spread are relatively relaxed, the mammalian Y chromo some is expected to be an attractor for growth-promoting genes. Data f rom 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 g enes may also be explained in terms of sexual antagonism. Sexual antag onism and the model presented here are not mutually exclusive