Gametophytic Abortion in Heterozygotes but Not in Homozygotes: Implied Chromosome Rearrangement during T-DNA Insertion at the ASF1 Locus in Arabidopsis

T-DNA insertional mutations in genes have conferred huge benefits to the research community, greatly facilitating gene function analyses. However, the insertion process can cause chromosomal rearrangements. Here, we show an example of a likely rearrangement following T-DNA insertion in the ( ) gene locus on chromosome 5, so that the phenotype was not relevant to the gene of interest, . ASF1 is a histone H3/H4 chaperone involved in chromatin remodeling in the sporophyte and during reproduction. Plants that were homozygous for mutant alleles or were developmentally normal. However, following self-fertilization of double heterozygotes ( hereafter ), defects were visible in both male and female gametes. Half of the and ovules displayed arrested embryo sacs with functional megaspore identity. Similarly, half of the and pollen grains showed centromere defects, resulting in pollen abortion at the bi-cellular stage of the male gametophyte. However, inheritance of the mutant allele in a given gamete did not solely determine the abortion phenotype. Introducing functional ASF1B failed to rescue the and mediated abortion, suggesting that heterozygosity in the gene causes gametophytic defects, rather than the loss of . The presence of reproductive defects in heterozygous mutants but not in homozygotes, and the characteristic all-or-nothing pollen viability within tetrads, were both indicative of commonly-observed T-DNA-mediated translocation activity for this allele. Our observations reinforce the importance of complementation tests in assigning gene function using reverse genetics.