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

369 The effects of multiple floral promotion pathways on flowering time in winter-annual
Arabidopsis thaliana accessions
Christopher Schwartz, Richard Amasino
UW-Madison
The timing of flowering is key for reproductive success, and plants have elaborate systems to control when a given plant
will flower in a specific environment. To better understand how flowering is induced in the wild, we are utilizing the natural
variation in flowering time of Arabidopsis accessions collected from around the world. We have surveyed over 300 natural
accessions of Arabidopsis for flowering time and determined that approximately 45% of Arabidopsis accessions are late
flowering (winter-annuals). A subset of accessions was chosen based upon phenotype and geographic location to represent
the flowering time variability observed in nature. We tested the effects of three different floral promotion pathways on these
accessions to gain an understanding of the role each pathway plays in promoting flowering in different accessions, which
are presumably adapted to different local environments. To determine the effect of the shade avoidance response, plants
were grown under cool white light supplemented with far-red light. For characterizing the vernalization response, seedlings
were cold treated for variable lengths of time and the effect on subsequent flowering time determined. Finally, the plant
hormone GA was applied to the accessions to assay the responsiveness of these accessions to GA induced flowering. We
found substantial variability in the effects of a given treatment on flowering time; in addition the effects across the multiple
treatments were poorly correlated, supporting the idea that these are largely independent or separate pathways. Based upon this
data, we generated multiple F2 populations from accessions that display contrasting behavior for sensitivity to a given floral
promotion pathway. In some cases, we see Mendellian segregation, implying that a single locus is responsible, while in other
F2 populations, we see a continuous phenotypic distribution suggesting multiple causative loci. In the far-red supplementation
experiments, designed to elicit the shade avoidance response, all the F2 populations segregated in a 3:1 manner, suggesting
that a single large effect locus was responsible for the differences in sensitivity to far-red light. Preliminary mapping using
multiple segregating populations indicates that the same locus is responsible for the far-red sensitivity in many different
accessions. In addition, when the same accessions are grown outside in the summer in Wisconsin, the outdoor flowering
phenotype is highly correlated to the flowering time observed when far-red light supplementation is included, suggesting
that this response is physiologically relevant and important for reproductive success in the wild.
370 High resolution mapping of genome variations between polyploid and diploid Arabidopsis
species
Shin-Han Shiu 1 , Xu Zhang 2 , Justin Borevitz 2
1
Michigan State University, 2 University of Chicago
Polyploidization results in an immediate increase in gene dosage and essentially releases the whole genome
from purifying selection. Based on studies of paleopolyploidy in plants and yeasts, most genes are lost within tens
of millions of years of the polyploidization event but some seem to be preferentially retained. We are interested in
understanding the differential retention of genes and the rate of gene loss in polyploid species. However, without
polyploid genome sequences, the extent and the nature of sequence variation in these species are virtually unknown.
We set out to obtain a high resolution map of sequence variation in a relatively recently formed polyploid, Arabidopsis
suecica (As) by comparing As to its parental species, Arabidopsis thaliana and Arabidopsis arenosa. The genomic
DNA of As and its parents was isolated and hybridized independently to Arabidopsis whole genome tiling arrays, 4
biological replicates each. Among > 3 million features on the tiling array, 112,035 are significantly different between As
and its parental species. Although we predicted that hybridization intensities should be higher for the parental genomes
due to gene losses in the polyploid, nearly half of the variable features have intensities significantly higher in As.
Most (60%) of the sequence variation is present in the intergenic regions. As expected, genic features tend to be less
variable than intergenic ones. However, pseuogenes, retroposons, and transposons have significantly higher proportions
of variable features (after normalizing by number of features) than intergenic sequences, suggesting they may have been
purged or deleted soon after the polyploidization event. Among features located within genic regions, those overlapping
with tandemly duplicated genes also tend to have a higher level of variation. In addition, many genes with significantly
more variable features are related to those preferentially lost since the most recent paleo-polyploidization event in
Arabidopsis (~25-45 Mya). Assuming these highly variable genes have been deleted in As, a polyploid formed ~20,000
years ago, our finding suggests that differential gene loss occurs rather rapidly after the polyploidization event.