Plant Genome III Conference

DUPLICATIONS WITHIN THE MAIZE GENOME: EVOLUTIONARY ORIGINS AND IMPLICATIONS FOR GENE EXPRESSION.

Past studies with molecular markers revealed that Z . mays contains large chromosomal segments which are duplicated elsewhere within the genome, despite the general diploid inheritance identified for most maize genes [T. Helentjaris et al., Genetics II 8, 353 (1988)]. 1 have recently compiled the available data for sequence distribution within maize and the evidence confirms the model that most, if not all, of the genome exists as two copies in rearranged forms with respect to each other. The evidence is still not clear whether this duplicated genome arose originally via internal duplication with subsequent rearrangement or by fusion of two distinctive genomes. Comparative map data demonstrate that two sets of maize chromosomes (1+2+3+4+6 and 5+7+8+9+10) each contain one genome equivalent to the rice genome, providing a testable model, whereby closely related species (sorghum, Coix, etc.) can be evaluated as possible progenitor species by comparing their genetic maps with that of the two partial maize genomes. Nevertheless, Z. mays has clearly evolved in a quite different manner from other polyploids which have maintained nearly identical subgenomes in terms of both structure and gene expression. Interestingly, loci detected by expressed genes seem to fit these general duplication patterns more precisely than nongenic DNA and comparison f DNA sequence data for copies of duplicated genes may provide some further insights into this process. Preliminary examination of genomic DNA around duplicated genes suggests that expressed regions themselves are more highly conserved while surrounding DNA and larger introns have significantly diverged to the point that their relationships are no longer recognizable. Further examination of synonymous substitution rates for several duplicate genes reveals that two classes of duplicate genes may exist with significantly different rates of divergence, a fact which may also provide some insights into the duplication process (J. Doebley, unpublished).