Parental contribution to plant embryos.

Unequal contribution of the parental genomes to the developing embryo can occur by a variety of mechanisms. In Dropsophila, for example, maternally produced transcripts determine the embryonic formation of body axes (St. Johnston and Nüsslein-Volhardt, 1992). Inequality in parental contribution of a different nature is achieved by genetic imprinting, in which either the maternal or paternal allele of a given gene pair is silenced. In Drosophila, studies with compound chromosomes consisting of either two maternally inherited or two paternally inherited arms established that no imprinting occurs (Strommen, 1982), even among early patterning genes. In mammals, imprinting can involve either the paternal or maternal copy of a gene and is thought to reflect an additional level of control over fetal growth (Tilghman, 1999). Indeed, many of the genes found to be imprinted appear to affect the growth and development of the fetus. Several recent papers have addressed the importance of maternal contributions to embryo development in plants and have also provided evidence for the existence in plants of paternal silencing during embryogenesis. In plants, the gametophytic (haploid) phase begins with the formation of the megaspore mother cell embedded deep in the ovule primordium and with formation of microspores in the developing anther. The sporophytic phase Parental Contribution to Plant Embryos