| The female gametophyte in Arabidopsis thaliana, and other Angiosperms, develops from a single haploid spore within the sporophytic ovule into a four-celled structure at maturity. Double fertilization of the central cell and egg cell of the female gametophyte by the two sperm cells of the male gametophyte gives rise to the endosperm and embryo, respectively. Despite the central role of the female gametophyte in sexual plant reproduction, little is known about the molecules involved in the development and reproductive functions of the female gametophyte.; To begin to understand the genetic requirements for a functional female gametophyte, I performed a genetic screen for gametophyte mutants that have an effect on the female gametophyte. Based on the fact that female gametophyte-lethal mutations will fail to transmit to the next generation through the female, two criteria were used: 50% reduced seed set and segregation distortion. I isolated 27 mutants in A. thaliana that affected the female gametophyte. Analysis of these mutants demonstrated that all stages of the haploid phase could be disrupted by T-DNA mutagenesis including late in megasporogenesis. I characterized the phenotypes of the terminal stage of female gametophyte development in these mutant lines using confocal laser scanning microscopy. Next, I was able to isolate the flanking plant DNA in 11 of these lines.; Detailed analysis of two mutants isolated (fem19 and fem40) revealed a complex relationship between the female gametophyte and surrounding sporophyte. Two fem19 lines (Df(4R) In(4R) fem19-1 and Df(4R) fem19-2) contained major chromosomal rearrangements that deleted many genes. Surprisingly Df(4R) fem19-2 had modest effects on the female gametophyte demonstrating that the haploid phase could partially tolerate a 440kb deletion. The surrounding diploid tissue may supply the gene functions that are absent in the mutant haploid gametophytes. The fem40 line appears to have a disruption of a single gene that encodes a protein component of the U3 small nucleolar ribonucleoprotein required for rRNA processing. Approximately half of the female gametophytes develop at a slower pace and never develop to maturity. The mutant female gametophyte may receive some maternal stores to compensate for loss of FEM40 activity, but these stores are insufficient to support normal development. |
