Comparative Physical Mapping of the Apospory-Specific Genomic Region in Two Apomictic Grasses: Pennisetum squamulatum and Cenchrus ciliaris

Shailendra Goel^*, Zhenbang Chen^*^,1^,2, Yukio Akiyama^*^,1^,3, Joann A. Conner^*, Manojit Basu^*^,4, Gustavo Gualtieri^*^,5, Wayne W. Hanna and Peggy Ozias-Akins^*^,6

^* Department of Horticulture, University of Georgia Tifton Campus, Tifton, Georgia 31793-0748 and Department of Crop and Soil Sciences, University of Georgia Tifton Campus, Tifton, Georgia 31793-0748

^⁶ Corresponding author: Department of Horticulture, University of Georgia Tifton Campus, 2356 Rainwater Rd., Tifton, GA 31794.
E-mail: pozias{at}uga.edu

In gametophytic apomicts of the aposporous type, each cell ofthe embryo sac is genetically identical to somatic cells ofthe ovule because they are products of mitosis, not of meiosis.The egg of the aposporous embryo sac follows parthenogeneticdevelopment into an embryo; therefore, uniform progeny resulteven from heterozygous plants, a trait that would be valuablefor many crop species. Attempts to introgress apomixis fromwild relatives into major crops through traditional breedinghave been hindered by low or no recombination within the chromosomalregion governing this trait (the apospory-specific genomic regionor ASGR). The lack of recombination also has been a major obstacleto positional cloning of key genes. To further delineate andcharacterize the nonrecombinant ASGR, we have identified eightnew ASGR-linked, AFLP-based molecular markers, only one of whichshowed recombination with the trait for aposporous embryo sacdevelopment. Bacterial artificial chromosome (BAC) clones identifiedwith the ASGR-linked AFLPs or previously mapped markers, whenmapped by fluorescence in situ hybridization in Pennisetum squamulatumand Cenchrus ciliaris, showed almost complete macrosynteny betweenthe two apomictic grasses throughout the ASGR, although withan inverted order. A BAC identified with the recombinant AFLPmarker mapped most proximal to the centromere of the ASGR-carrierchromosome in P. squamulatum but was not located on the ASGR-carrierchromosome in C. ciliaris. Exceptional regions where syntenywas disrupted probably are nonessential for expression of theaposporous trait. The ASGR appears to be maintained as a haplotypeeven though its position in the genome can be variable.