indeterminate gametophyte1 (ig1) mutants have an indeterminate number of cells in the mature embryo sac. ig1 embryo sacs undergo extra rounds of free nuclear divisions resulting in extra eggs, extra central cells, and extra polar nuclei within central cells (Figure 5). Thus, wild-type ig1 function is most likely involved in limiting the number of free nuclear nuclear divisions. In the absence of ig1 function syncytial development is indeterminate.

The exact number and placement of nuclei at cellularization is variable in ig1 mutants. The phenotypes of ig1 embryo sacs suggest a position-based determination of cellular identity. The ability of the extra cells and nuclei to function as egg cells or polar nuclei, for example, may depend on their position in the embryo sac. Many of these defective embryo sacs give rise to abnormal seeds (Figure 6). Abnormalities include polyembryony, heterofertilization, miniature endosperms, and early abortion of seeds. Additionally ig1 restricts the embryogenic potential of cells that lack one of the two parental genomes. ig1 mutant embryo sacs produce haploid progeny, of both maternal and paternal origin, at a higher rate than wild type. The same seed phenotypes as in ig1 have been used to identify two more mutants with extra polar nuclei.

I have recently cloned the ig1 gene using a combination of directed tagging and comparative mapping approaches. The ig1 gene encodes a member of the LATERAL ORGANS BOUNDARY (LOB) protein family. Projects to identify targets of the ig1 gene and proteins that interact with IG1 are in progress.

Figure 6. Ear of an ig1-O/ig1-O homozygous female pollinated by a homozygous wild type male. Many (but fewer than half) of the seeds are abnormal, including miniature and aborted kernels, which are easily recognized. The endosperm phenotype reflects the number of polar nuclei in the central cell before fertilization.