Knots in the family tree: evolutionary relationships and functions of knoxhomeobox genes

Knotted-like homeobox (knox) genes constitute a gene family in plants. Clas s I knox genes are expressed in shoot apical meristems, and (with notable e xceptions) not in lateral organ primordia. Class II genes have more diverse expression patterns. Loss and gain of function mutations indicate that kno x genes are important regulators of meristem function. Gene duplication has contributed to the evolution of families of homeodomain proteins in metazo ans. We believe that similar mechanisms have contributed to the diversity o f knox gene function in plants. Knox genes may have contributed to the evol ution of compound leaves in tomato and could be involved in the evolution o f morphological traits in other species. Alterations in cis-regulatory regi ons in some knox genes correlate with novel patterns of gene expression and distinctive morphologies. Preliminary data from the analysis of class I kn ox gene expression illustrates the evolution of complex patterns of knox ex pression is likely to have occurred through loss and gain of domains of gen e expression.