Female gametophytic mutants of Arabidopsis thaliana identified in a gene trap insertional mutagenesis screen

In plants, the male and female gametophytes represent the haploid generation that alternates with the diploid sporophytic generation. Male and female gametophytes develop from haploid micro- and megaspores, respectively. In flowering plants (angiosperms), the spores themselves arise from the sporophyte through meiotic divisions of sporogenous cells in the reproductive organs of the flower. Male and female gametophytes contain two pairs of gametes that participate in double fertilization, a distinctive feature of angiosperms. In this paper, we describe the employment of a transposon-based gene trap system to identify mutations affecting the gametophytic phase of the plant life cycle. Mutants affecting female gametogenesis were identified in a two-step screen for (i) reduced fertility (seed abortion or undeveloped ovules) and (ii) segregation ratio distortion. Non-functional female gametophytes do not initiate seed development, leading to semi-sterility such that causal or linked alleles are transmitted at reduced frequency to the progeny (non-Mendelian segregation). From a population of 2,511 transposants, we identified 54 lines with reduced seed set (2%). Examination of their distorted segregation ratios and seed phenotypes led to the isolation of 12 gametophytic mutants, six of which are described herein. Chromosomal sequences flanking the transposon insertions were identified and physically mapped onto the genome sequence of Arabidopsis thaliana. Surprisingly, the insertion sites were often associated with chromosomal rearrangements, making it difficult to assign the mutant phenotypes to a specific gene. The mutants were classified according to the process affected at the time of arrest, i.e. showing mitotic, karyogamic, maternal or degenerative phenotypes.