Mapping Genes to the ASGR-Carrier Chromosome from Apomictic Pennisetum squamulatum Using Heterologous Comparative Genomic Hybridization

Aposporous apomixis has been transferred from Pennisetum squamulatum to its cultivated relative, P. glaucum (pearl millet) through introgressive hybridization.  Single-chromosome addition/substitution lines that contain the Apospory-specific Genomic Region (ASGR) are apomictic (BC8-Apo) while a fraction of their progeny derived through facultative apomixis and lacking the ASGR are sexual (BC8-Sex).  We previously identified transcripts from ovules of BC8-Apo at anthesis that could be mapped to the ASGR-carrier chromosome in the pearl millet background because of sequence divergence between the two species.  Although the ASGR, which is necessary for apomixis, does not show extended macrosynteny to sorghum and rice chromosomes, transcripts from anthesis-stage ovules that were mapped to the ASGR-carrier chromosome, but not necessarily to the ASGR, often showed synteny to sorghum chromosome 2.  Comparative genomic hybridization (CGH) of introgression line DNA to sorghum and maize CGH arrays led to the identification of genes showing a higher copy number in BC8-Apo than BC8-Sex, and a significant number of these also mapped to sorghum chromosome 2, colinear maize chromosome 7, and its duplicate region on maize 2.  Furthermore, CGH of an irradiation mutant lacking the ASGR but retaining a portion of the ASGR-carrier chromosome confirmed synteny of non-ASGR portions of the ASGR-carrier chromosome to sorghum 2 and maize 7 and 2.