Inferences on the genome structure of progenitor maize through comparativeanalysis of rice, maize and the domesticated panicoids

Corn and rice genetic linkage map alignments were extended and refined by t he addition of 262 new, reciprocally mapped maize cDNA loci. Twenty chromos omal rearrangements were identified in maize relative to rice and these inc luded telomeric fusions between rice linkage groups, nested insertion of ri ce linkage groups, intrachromosomal inversions, and a nonreciprocal translo cation. Maize genome evolution was inferred relative to other species withi n the Panicoideae and a progenitor maize genome with eight linkage groups w as proposed. Conservation of composite linkage groups indicates that the te trasomic state arose during maize evolution either from duplication of one progenitor corn genome (autoploidy) or from a cross between species that sh ared the composite linkages observed in modern maize (alloploidy). New evid ence of a quadruplicated homeologous segment on maize chromosomes 2 and 10, and 3 and 4, corresponded to the internally duplicated region on rice chro mosomes 11 and 12 and suggested that this duplication in the rice genome pr edated the divergence of the Panicoideae and Oryzoideae subfamilies. Charti ng of the macroevolutionary steps leading to the modern maize genome clarif ies the interpretation of intercladal comparative maps and facilitates alig nments and genomic cross-referencing of genes and phenotypes among grass fa mily members.