A more complex situation exists for group B MPKs, where all group B PtMPKs contain six exons, while only two out of five group B AtMPKs (AtMPK4 and AtMPK12) display this organization. Nevertheless, except in PtMPK5-1, the phases of the various introns found in these gene models are also perfectly conserved. These gene models might thus share a common evolutionary history. On the other hand, AtMPK5, AtMPK11 and AtMPK13 all possess four exons with variable intron phase combinations. These combinations are not observed in any poplar MPKs. This suggests that the Arabidopsis group B MPK gene family has evolved differently than the corresponding poplar family. It is possible that one of the ancestral genes that gave rise to the present group B AtMPK family was not transmitted to poplar when these species diverged or that the precursor gene was lost during poplar evolution. Alternatively, the generation of the four exon configurations observed in Arabidopsis was the result of a duplication event that followed species divergence.