Developmental expression of ASG-1 during gametogenesis in apomictic guinea grass (Panicum maximum)

We have used Western blue-visualized in situ-hybridization (ISH) to monitor the expression of apomixis-specific gene-1 (ASG-1, GenBank accession number AB000809) during gametogenesis in obligate-sexual and facultative-apomictic (aposporic) genotypes of guinea grass (Panicum maximum). The in situ-analysis revealed that ASG-1 is not expressed in the ovule during early floral development in both, the facultative apomicts (A1 stage) and the obligate sexuals (S1 stage). With the appearance of the aposporous initial cell(s) in the ovule of the apomictic type (A2-1 stage), ASG-1 expression is strong and specific to this apomixis-specific cell. ASG-1 expression continued through different stages of aposporous embryo sac development (A2-2 stage), indicating that the gene may play a role in this developmental process. Regular embryo sacs in sexual types did not show hybridization signals (S2 stage). However, strong ASG-1 expression was detected in immature pollen grains and young embryos in both reproductive types, suggesting that ASG-1 may be an allele derived from the obligate-sexual wild type. Expression in pollen grains faded with maturation. In a heterologous system, using Paspalum notatum, a facultative-aposporic tropical grass (bahia grass), identical results were obtained. The results are discussed in view of the fact, that ASG-1 shows some homology to genes known to be seed- or embryo-specific or involved in processes related to cell growth.