Equivalent parental contribution to early plant zygotic development.

Hybrid vigor or heterosis results from the combination of genetically distant genomes at fertilization, and as well as being of major commercial importance, it is held to contribute significantly to fitness [1]. Activation of the paternal genome marks the transition from maternal to zygotic control of development, but a reported delay of paternal-genome activation in flowering plants [2-4] and animals [5, 6] excludes heterosis from impacting on very early development. We have analyzed the allele-specific expression of 25 genes after fertilization of the egg in maize and show immediate equivalent parental genomic contribution to the zygote. Every gene expressed before the first cell division of the zygotes showed paternal transcripts. Sequence comparisons indicate that these genes are involved in a range of processes and are distributed throughout the genome. Our findings confirm that some plant species have evolved a strategy to activate the paternal genome immediately after fertilization, in contrast to the situation in other plants and in animals. Such an extensive activation of the paternal genome very early in development is consonant with observations of high levels of heterosis in early hybrid maize embryos [7, 8], indicating a significant impact of this sexual strategy on fitness.