75 Integrating Membrane Transport with Male Gametophyte ... - TAIR

307 Genetic Control of the Interploidy Hybridization Barrier
Brian Dilkes, Isabelle Henry, Renee Weizbauer, Luca Comai
Biology Department, University of Washington, Seattle WA 98195
Parental contributions to seed development are dramatically asymmetrical. This asymmetry is manifest in the failure of
some crosses involving gametes of different ploidy. Multiple mechanisms have been proposed to explain this phenomenon
including nuclear to cytoplasm communication, parent-dependent epigenetic imprinting, and gametophytic effects. We
are analyzing the genetic control of the interploidy barrier in Arabidopsis using natural variation and gene knockouts.
We identified variability for survival of seeds following paternal excess (diploid x tetraploid) crosses in Arabidopsis
thaliana. Variation in maternal tolerance was mapped in two RIL populations (Ler x Col and Ler x Cvi). In the Ler x Col
RILs, a main effect QTL, Dr. Strangelove, controlled 15% of the phenotypic variation. Genetic analyses of this QTL in
segregating populations has confirmed this large effect and fine mapping refined our estimate of QTL position. Mutants
of candidate genes have been tested. Diploid loss of function mutations in the WRKY transcription factor ttg2, increased
seed viability in paternal excess crosses by three-fold. Variability in the paternally-encoded determinant of interploidy
seed failure has also been tested in both RIL and backcross populations of 4x Col x Wa-1. A single locus of major effect
was identified on chromosome 1. We are currently testing mutations in candidate genes within the QTL interval for their
effect on the pollen determinant of this ploidy barrier.
B.P.D. and L.C. alternate address: UC Davis Genome Center, University of California at Davis, Davis CA 95616
308 Analysis of the functional interaction between CO and TFL2 at the FT locus
Sara Farrona, George Coupland, Franziska Turck
Max Planck Institute for Plant Breeding Research
FT plays a main role in the regulation of the transition to reproductive development in Arabidopsis thaliana. The gene
is able to integrate the inputs from different flowering pathways, such as the photoperiod, vernalization and autonomous
pathway. One component of the photoperiod pathway is the CO gene, which encodes a protein containing zinc-finger and
CCT domains. The accumulation of the CO protein in long day conditions promotes the up-regulation of FT expression.
It has been proposed that CO acts like a transcription factor with FT as direct target. Since attempts to demonstrate a
physical interaction between CO and the FT locus have so far been unsuccessful, the exact mechanism of FT regulation
by CO remains unknown.
Another regulator of FT expression is TFL2/LHP1/TU8. This gene encodes a protein with two characteristic domains:
a cromo- and a chromoshadow domain. It has been characterized as the only Arabidopsis homologue of mammalian and
Drosophila HP1. HP1 protein has been described as component of the heterochromatin although recently a wider role in
chromatin structure has been proposed. In Arabidopsis, TFL2 is expressed in the SAM and RAM, in the hypocotyl, and,
as FT and CO, in the vascular tissue of young leaves and cotyledons. In a wild type plant CO is required for transcriptional
activation of FT, whereas FT is constitutively expressed in the vasculature of tfl2/lhp1 mutant plants. The data indicate
that CO counteracts the function of TFL2 at the FT locus.
In order to study the mechanistic interplay between TFL2 and CO, we obtained different transgenic lines: epitope
tagged TFL2 (35S::TFL2-HA), CO overexpressor line (35S::CO) and a double transgenic that express both constructs.
We carried out chromatin immunoprecipitacion experiments in each of these lines with antibodies against the HA epitope
to analyse the binding of TFL2-HA protein in a long region that contains the FT locus and the neighbouring genes. These
and further results will be presented at the meeting.