Distribution of G/C tracts along each chromosome of C. elegans and C. briggsae*. X axis in each graph represents one chromosome whose length was normalized to 1 and Y axis is the number of G/C tracts. Each G/C tract from the left end to the right end of one chromosome was numbered sequentially. The diamond spots on the X axis marked the edge of the genetically defined central gene cluster . * Positions of G/C tracts in C. briggsae were predicted by the method described in text.