Figure 4.

Stages in the evolution of the PAR region of the S. latifolia sex chromosomes. Horizontal lines symbolize X chromosomes at several intermediate stages during its evolution, from the initial state, before additions and rearrangements, to the current state (indicated by the text in boxes), and the additions and rearrangements needed to account for the conclusions described in the text. The ancestral sex chromosome is solid, and the two added regions and genes are shaded. The regions and genes that have been added to the X must also have been added to the Y (see the text), but the Y is not shown in the figure.