MAPPING DIPLOSPOROUS APOMIXIS IN TETRAPLOID TRIPSACUM - ONE GENE OR SEVERAL GENES

Polyploids in Tripacum, a wild relative of maize, reproduce through th e diplosporous type of apomixis, an asexual mode of reproduction throu gh seeds. Diplosporous apomixis involves both the failure of meiosis a nd the parthenogenetic development of the unreduced gametes, resulting in progenies that are exact genetic copies of the mother plant. Apomi xis is believed to be controlled by one single dominant allele, respon sible for the whole developmental process. Construction of a linkage m ap for the chromosome controlling diplosporous apomixis in Tripsacum w as carried out in both tetraploid-apomictic and diploid-sexual Tripsac um species using maize restriction fragment length polymorphism (RFLP) probes. A high level of collinearity was observed between the Tripsac um chromosome carrying the control of apomixis and a duplicated segmen t in the maize genome. In the apomictic tetraploid, there was a strong restriction to recombination, as compared to the corresponding genomi c segment in sexual plants and maize. This suggests that apomixis, alt hough inherited as a single Mendelian allele, might really be controll ed by a cluster of linked loci. The analysis also revealed the tetraso mic nature of the inheritance of the chromosomal segment controlling a pomixis, which contradicts the usually accepted hypothesis of an allop olyploid origin of apomictic species. The implications of these data f or the transfer of apomixis into cultivated crops are discussed, and a new approach to studying the genetics of apomixis, based on comparati ve mapping, is proposed.