During pollen development each product of meiosis undergoes a stereotypical pattern of cell divisions to give rise to a three-celled gametophyte, the pollen grain. First an asymmetric mitosis generates a larger vegetative cell and a smaller generative cell, then the generative cell undergoes a second mitosis to give rise to two sperm cells. It is unknown how this pattern of cell divisions is controlled. We have identified an Arabidopsis gene, SIDECAR POLLEN, which is required for the normal cell division pattern during pollen development. In the genetic background of the NoO ecotype, sidecar pollen heterozygotes have about 45% wild-type pollen, 48% aborted pollen and 7% pollen with an extra cell. Homozygous sidecar pollen plants have about 20% wild-type pollen, 53% aborted pollen and 27% extra-celled pollen. Similar ratios of sidecar pollen phenotypes are seen in the Columbia ecotype but sidecar pollen is a gametophytic lethal in the Landsberg erecta ecotype. Thus this allele of sidecar pollen shows differential gametophytic penetrance and variable expressivity in different genetic backgrounds. The extra cell has the cell identity of a vegetative cell and is produced prior to any asymmetric microspore mitosis. Pollen tetrad analysis directly demonstrates that SIDECAR POLLEN is indeed expressed in male gametophytes. To our knowledge, scp is the first male gametophytic mutation to be described in Arabidopsis.

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