Apomixis is a naturally occurring mode of asexual reproduction in angiosperms that leads to embryo and seed formation without a requirement for meiosis or fertilization of the egg. The ability to harness this potential in food crops could significantly alter agricultural practices. Apomixis can be used to effectively harness hybrid vigor because any obligate apomictic plant, even when highly heterozygous, will breed true for its genotype. We are studying apomixis in Pennisetum squamulatum and Cenchrus ciliaris, plants which are members of the grass family and closely related to sexual P. glaucum (pearl millet).
Apomixis is controlled by a single dominant “locus” in both Pennisetum and Cenchrus. The term ASGR (for apospory-specific genomic region) was coined to describe the locus in Pennisetum/Cenchrus. Ninety-nine bacterial artificial chromosome (BAC) clones containing molecular markers (SCARs, RFLPs, and AFLPs) showing total genetic linkage to the aposporous phenotype have been identified from BAC libraries. FISH analysis has confirmed that the ASGR is physically large (approximately 50 Mb in P. squamulatum; Akiyama et al. 2004).
We have isolated and sample sequenced 32 Bacterial Artificial Chromosomes (BACs) that are totally linked with the apomictic trait. The ASGR sequences reveal that the ASGR (1) contains both gene-rich and gene-poor segments, (2) contains several genes that may play a role in apomictic development, (3) has many classes of transposable elements, and (4) does not exhibit large scale synteny with either rice or sorghum genomes but does contain multiple regions of microsynteny with these species (Conner et al., 2008).
Of the potential genes identified in the sample sequencing analysis, we are pursuing the ASGR-BABY BOOM (BBM)-like genes and the ASGR-ResponseRegulator genes as the best potential candidate genes for a role in apomictic development. The ASGR-BBM-like genes’ similarity to other BBM genes suggests a possible role in the induction and/or the maintenance of the unreduced embryo in the aposporous embryo sac. Further study of gene expression and functional knock-down studies of the ASGR-BBM-like genes are in progress to confirm a possible role in the apomictic developmental pathway.
This work is funded by the National Science Foundation (Award 0115911).