Genetic analysis of the apospory-specific genomic region (ASGR) in Pennisetum squamulatum
MetadataShow full item record
Pennisetum squamulatum reproduces by apospory, a type of apomictic reproduction where non-generative nucellar cells develop into an unreduced embryo sac. Apospory behaves as a dominant Mendelian trait in P. squamulatum and a genomic region from a single chromosome, called the apospory-specific genomic region (ASGR), is sufficient for inheritance of the trait. However, the ASGR is physically large (>50Mb), highly heterochromatic, hemizygous, and recombinationally suppressed. These characteristics have hindered high-resolution genetic mapping and map-based cloning of apomixis genes. In order to aid physical map construction, additional molecular markers linked to the ASGR are needed. The first study focused on development of molecular markers based on the Long Terminal Repeats (LTRs) of retrotransposons which are abundant in the ASGR. Sequence-specific amplified polymorphisms (SSAP) were identified using LTR-specific primers paired with restriction site targeted primers. Over 60% of the SSAP markers generated were linked with apomixis and strongly clustered within 9 cM. Low recombination observed among SSAP markers was reinforced after converting five SSAP markers into Sequence Characterized Amplified Regions (SCARs). In a second study, one candidate gene for apomixis identified through sample sequencing of ASGR-linked Bacterial Artificial Chromosome (BAC) clones, ASGR-BABY-BOOM-like, was characterized by gene silencing using RNA interference (RNAi). Fifty-nine transgenic tetraploid pearl millet plants were generated from 16 independent lines and crossed with P. squamulatum (male parent) to combine the silencing transgene with the ASGR-BABY-BOOM-like gene. Expression of ASGR-BABY-BOOM-like was reduced in three out of 14 F1plants with the genotype of ASGR/RNAi, and embryo development in these three reduced plants was significantly delayed. Over-expression of this gene in Arabidopsis also caused ectopic organ development on leaves. These combined phenotypes suggest that ASGR-BABY-BOOM-like plays a role in organ/embryo initiation and may be necessary for parthenogenetic development of the egg.