Chapter 19, Problem 37P

In the plant Arabidopsis thaliana, every flower is constructed of four concentric whorls of modified leaves. The first whorl (whorl 1) consists of four green leaf-like sepals, whorl 2 is composed of four white petals, whorl 3 is made of six stamens bearing the pollen that houses the male gametes (sperm), and whorl 4 contains the two carpels, within which lie the ovules that hold the female gametes (eggs). As shown in the diagram that follows, a shorthand description of the wild-type flower pattern is: sepal, petal, stamen, carpel.

Scientists wanted to understand how this pattern of whorls arises. They generated Arabidopsis strains homozygous for randomly induced mutations and screened them for mutant flowers with an abnormal order or selection of floral organs. The interesting mutants identified fell into three phenotypic classes: (1) carpel, stamen, stamen, carpel; (2) sepal, sepal, carpel, carpel; (3) sepal, petal, petal, sepal.

The investigators found that all of the class 1 mutants were alleles of the same gene which they called APETELA2 (AP2). Class 2 mutants were alleles of either one of two genes, which were named APETELA3 (AP3) and PISTILLATA (PI). Finally, the class 3 mutants represented a single gene, AGAMOUS (AG). Molecular analysis showed that all four genes encode transcription factors.

Based on the phenotypes of AP2, AP3, PI, and AG mutants (all of them are null alleles) and the fact that all four gene products are transcription factors, investigators formulated the following model for differentiation of the four flower whorls from four equivalent whorl precursor cell groups: AP2 protein determines sepals; AP2 + AP3 + PI determines petals; AP3 + PI + AG determines stamens; AG determines carpels. In addition, AP2 and AG proteins repress each other’s transcription.

a.	Complete the chart that follows to show how the model predicts the phenotype of flowers homozygous for mutations in each of the four genes. In the chart, the colored boxes represent genes that are expressed; the white boxes are genes that are not expressed.
b.	Scientists tested the flower patterning model with RNA in situ hybridization experiments using cDNA probes for each of the four genes. The goal was to see whether each gene’s mRNA was expressed in the precursor cells for each of the four whorls. What results would you predict with each probe on wild-type flowers? on AP2 mutants? on AG mutants?
c.	Another way the researchers tested their model for flower patterning was by making double mutants. What phenotypes does the model predict for each of the six double mutant combinations? [Hint: It will be helpful to expand the chart shown in part (a) for each possible double mutant.]
d.	Are the roles of the four genes described in this problem more similar to those of the segmentation genes or to those of the homeotic genes from Drosophila described in the text?