Identification of different cDNA clones of the Apomixis-specific gene-1 homolog involved in aposporous embryo sac formation in guineagrass (<i>Panicum maximum</i> Jacq.)

Apomixis is the reproduction mode in which only the mother's genes are transmitted from generation to generation. If this trait can be put to practical use, it is expected to be a truly epoch-making breeding method, as seed production costs can be greatly reduced by fixing F1 hybrids. This study aimed to isolate the aposporous apomixis gene and analyze how the gene (s) will be expressed from/in the aposporous guineagrass (Panicum maximum Jacq.). A new classification method using the ovary length as an index was developed to sample different developmental stages of ovaries and buds in obligate sexual plants and apomicts. A cDNA library was derived from the ovaries of aposporous accession N68/96-8-o-11 staged at the appearance of aposporous initial cells (AICs) to isolate AIC stage-specific genes. Using differential screening, four AIC stage-specific cDNA clones obtained from ten thousand of plaques by Northern blot hybridization showed the same start codon and sequences, ranging in lengths from 577 to 1182 bp. The characteristics and their homologies of the four cDNA clones are similar to Apomixis-specific gene-1 (ASG-1), indicating that they are the different cDNA clones of Apomixis-specific gene-1 homolog (ASG-1H). In situ expression analysis detected signals without distinguishing between ASG-1 mRNA and the ASG-1H on gene expression specifically in AIC, AIC-derived embryo sacs, and root tips and shoot apical meristems of aposporous accession. The finding and identification of ASG-1H expressed at the times of AIC appearance and AIC-derived embryo sac formation, may represent an initial step towards isolating an apospory gene.