Schematic view of the scattered distribution of the poplar MAPKK genes (PtMKKs) over the Populus trichocarpa genome. Six of the 19 poplar chromosomes are presented as vertical bars. In addition, three scaffolds containing a PtMKK gene are shown. PtMKK genes are represented by colored boxes and the color code presented here is also used in the other figures. Recent duplication events between paralogous genes are indicated using dotted lines.