The evolution of X-linked genomic imprinting

We develop a quantitative genetic model to investigate the evolution of X-i mprinting. The model compares two forces that select for X-imprinting: geno mic conflict caused by polygamy and sex-specific selection. Genomic conflic t can only explain small reductions in maternal X gene expression and canno t explain silencing of the maternal X. In contrast, sex-specific selection can cause extreme differences in gene expression, in either direction (lowe red maternal or paternal gene expression), even to the point of gene silenc ing of either the maternal or paternal copy. These conclusions assume that the Y-chromosome lacks genetic activity. The presence of an active Y homolo gue makes imprinting resemble the autosomal Pattern, with active paternal a lleles (X- and Y-linked) and silenced maternal alleles. This outcome is lik ely to be restricted as Winked alleles are subject to the accumulation of d eleterious mutations. Experimental evidence concerning X-imprinting in mous e and human is interpreted in the light of these predictions and is shown t o be far more easily explained by sex-specific selection.