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A differential dosage hypothesis for parental effects in seed development

Dilkes BP, Comai L.
Department of Biology, University of Washington, Seattle, WA 98195, USA.

Parent-of-origin effects generate phenotypes that depend on the direction of a cross. This phenomenon occurs frequently during angiosperm seed development, where maternal influence is most common (Alleman and Doctor, 2000). Various genetic mechanisms can contribute to parent-of-origin effects during seed development, including (1) the disproportionate maternal contribution to the endosperm, (2) plastidic and cytoplasmic inheritance, (3) expression of genes in the gametophytes and gametes, and (4) differential expression of parental alleles in the developing seed. In addition, the maternal sporophyte influences seed development by providing nutrients and other resources to the seed (Lynch and Walsh, 1998; Alleman and Doctor, 2000). Thus, parent-of-origin effects can be due to genetic or environmental differences between the parents attributable to processes acting at several distinct stages of development (Figure 1). Many parent-of-origin effects have been proposed to result from transcriptional imprinting, the differential expression of an allele when transmitted through the pollen or egg germline. Indeed, many genes are differentially expressed during seed development (Vielle-Calzada et al., 2000; Baroux et al., 2001; Weijers et al., 2001; Guo et al., 2003, 2004). A minority of imprinted genes are subject to the complete silencing of one parental allele, a condition we refer to as binary imprinting. The apparent contrast between the two categories of imprinted genes, differentially imprinted versus binary, creates difficulties in understanding the evolutionary and mechanistic relationship between the two fates. An example of such difficulties is that binary imprinting of growth-regulating genes is considered the sole stable outcome of conflict of interest between parents (Haig and Westoby, 1991, 1989; Haig, 1997; Wilkins and Haig, 2003), yet conflict of interest could potentially explain cases of differential imprinting. In this essay, we discuss how both types of imprinting are forms of dosage regulation and suggest that parent-of-origin phenomena can most easily be understood when considered in the light of a differential dosage hypothesis. [1]

  1. Dilkes BP & Comai L (2004) A differential dosage hypothesis for parental effects in seed development. Plant Cell 16: 3174-80 PubMed
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