Genetic organization of the mating-type loci of smut fungi. Genes are indicated by arrows with the arrow denoting the direction of transcription. Related genes are denoted by the same color and respective gene functions are explained in the lower part of the Figure. *indicates that the relative order and orientation of these genes have not been determined. In the tetrapolar species, U. maydis and S. reilianum, the a and b specific sequences reside on different chromosomes, while they are linked by spacer regions (which are not drawn to scale and whose length is indicated) in the bipolar species U. hordei and S. scitamineum. The black bars on top of the figure indicate the regions of the b locus, which covers the two homeodomain protein genes bE and bW, and the a locus (that expands to different length in the different loci, indicated by a broken line) from the lba gene to the rba gene. Sequence information was obtained from the following Accession Numbers: AF043940, AM118080, AF184070, AF184069, Z18531, AJ884588, AJ884583, AJ884590, AJ884585, AJ884589, AJ884584, U37796, M84182, AACP01000083 and AACP01000013.