Is supernumerary chromatin involved in gametophytic apomixis of polyploid plants?

Gametophytic apomixis, or unreduced embryo sac development that results in asexual reproduction through seeds, occurs in several families of angiosper ms and must be polyphyletic in origin. The molecular mechanisms underlying gametophytic apomixis have not been discovered and are the subject of inten se investigation. A common feature of almost all apomicts is their polyploi d nature. From genetic mapping studies in both monocots and dicots, there i s low genetic recombination associated with a single (rarely two), dominant locus for either aposporous or diplosporous embryo sac formation. In Penni setum squamulatum and Cenchrus ciliaris, some DNA sequences mapping to the apospory locus are unique to apomictic genotypes and apparently hemizygous. This sequence divergence at the apomixis locus could be a consequence of g enome rearrangements and isolation from genetic recombination, both of whic h may have contributed to the definition of a chromosomal region as supernu merary. The possible involvement of supernumerary chromatin, formed as a re sult of interspecific hybridization, in the origin of apomixis, is explored here.