Intragenomic Parental Conflict and the Evolution of Genomic Imprinting

Haig and Westoby (1989, 1991) proposed a theory suggesting that genomic imprinting evolved as a consequence of an intragenomic conflict over the allocation of nutrients from mother to offspring. Most of the seed's nutrients are directly transferred from the mother plant, and larger seeds with extra food reserves produce more vigorous seedlings. However, the production of larger food reserves causes greater metabolic costs for the mother, reducing the resources available for future offspring. The pollen parent directly benefits from an increase in seed size (with more vigorous offspring) without experiencing any direct cost. Thus, the intragenomic conflict theory proposes that genes controlling growth and vigor of the seed will be subject to selective forces that bring genes promoting growth under paternal control, whereas genes that restrict growth will come under maternal control. This theory is consistent with many phenotypes that result from the disruption of imprinted genes in animals, where they are required for fetal growth and placental development (Moore and Haig 1991; Tilghman 1999; Arney et al. 2001), but none of the imprinted maize genes described above shows any effect on seed development (Messing and Grossniklaus 1999). Nevertheless, some parent-of-origin effects on the endosperm, which can be considered the functional equivalent of the mammalian placenta because it mediates the transfer of nutrients to the developing embryo, may be related to genomic imprinting (Haig and Westoby 1991; Messing and Grossniklaus 1999; Alle-man and Doctor 2000).

Increasing the dosage of paternally derived genes promotes the growth of both placenta and endosperm, whereas increasing the dosage of maternally derived genes has the opposite effect (Haig and Westoby 1991; Moore and Haig 1991; Surani 1998; Arney et al.2001).Normal endosperm development in several species depends on a 2 m:1p genome ratio (Lin 1984; Haig and Westoby 1991). In some Arabidopsis accessions, interploidy crosses between diploids and tetraploids result in viable triploid embryos. However, a cross of a diploid mother with tetraploid pollen (2n x 4n) results in enlarged seeds with accelerated mitosis and delayed cellularization of the 2 m:2p endosperm. In contrast, a 4n x 2n cross results in small seeds, with reduced endosperm mitosis and precocious cellularization of the 4m:1p endosperm (Scott et al. 1998). These results are consistent with the predictions made from the parental conflict theory. However, there are many other explanations, such as dosage effects or interactions between the cytoplasm of the gametes with the nuclear genome of the resulting zygote, that are also consistent with these observations (Birchler 1993; von Wangenheim and Peterson 2004).

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