The evolution of genomic imprinting.

We explore three possible pathways for the evolution of genomic imprinting. (1) Imprinting may be advantageous in itself when imprinted and unimprinted alleles of a locus confer different phenotypes. If a segment of DNA is imprinted in the gametes of one sex but not in those of the other, it might lead to effects correlated with sexual dimorphism. More fundamentally, in certain organisms, sex determination might have evolved because of imprinting. When imprinting leads to chromosome elimination or inactivation and occurs in some embryos but not in others, two classes of embryos, differing in the number of functional gene copies, would result. A model for sex determination based on inequality in the actual or effective copy-number of particular noncoding, regulatory sequences of DNA has been proposed (Chandra, Proc. natn. Acad. Sci. U.S.A. 82. 1165-1169 and 6947-6949, 1985). Maternal control of offspring sex is another possible consequence of imprinting; this would indicate a potential role for imprinting in sex ratio evolution. (2) Genes responsible for imprinting may have pleiotropic effects and they may have been selected for reasons other than their imprinting ability. Lack of evidence precludes further consideration of this possibility. (3) Imprinting could have co-evolved with other traits. For instance, gamete-specific imprinting could lead to a lowered fitness of androgenetic or gynogenetic diploids relative to the fitness of 'normal' diploids. This in turn would reinforce the evolution of anisogamy. The reversibility of imprinting raises the possibility of occasional incomplete or improper erasure.(ABSTRACT TRUNCATED AT 250 WORDS)