Jing Zhou and John Larkin, Biological Sciences
Genetic Mapping of Siamese Phenotypic Modifiers

Arabidopsis trichomes (leaf hairs) are cells that extend from the epidermis of leaves and sepals. They are large and easily visible, making it easy to identify mutant phenotypes. During development, wild type (WT) trichomes exit the mitotic cycle and enter an endoreplication cycle. After such a switch, DNA replication occurs without nuclear and cellular division, resulting in cells with more than 2C DNA content. This characteristic makes trichomes well-studied models for cell differentiation.

The siamese (sim) mutation is recessive and produces multicelluar trichomes. It seems to trigger extra rounds of cell division that are not observed in WT trichomes, suggesting that the gene encodes a cell cycle regulator. It is likely that the SIAMESE (SIM) gene product interacts with other cell cycle components.

Homozygous sim seeds have been mutagenized with ethyl methane sulfonate to identify genes functioning together in the same pathway. In particular, several mutations that result in strongly enhanced multicellularity have been recovered, and they are thus called enhancer of sim (ens). We present here phenotypic characterization of the mutations. We also determined their chromosomal location using bulk segregant analysis. Because the Arabidopsis genome is sequenced, we can isolate these sim phenotypic modifiers based on their chromosomal location. This may allow us to identify other cell cycle components that interact with the SIM gene product and help to explain the complexity of plant cell cycle regulation.