Gene expression analysis at the onset of aposporous apomixis in Paspalum notatum

Apomixis is a route of asexual reproduction through seeds, that progresses in the absence of meiosis and fertilization to generate maternal clonal progenies. Gametophytic apomicts are usually polyploid and probably arose from sexual ancestors through a limited number of mutations in the female reproductive pathway. A differential display analysis was carried out on immature inflorescences of sexual and apomictic tetraploid genotypes of Paspalum notatum, in order to identify genes associated with the emergence of apospory. Analysis of ∼10,000 transcripts led to the identification of 94 high-quality differentially expressed sequences. Assembling analysis, plus validation, rendered 65 candidate unigenes, organized as 14 contigs and 51 singletons. Thirty-four unigenes were isolated from apomictic plants and 31 from sexual ones. A total of 45 (69.2%) unigenes were functionally categorized. While several of the differentially expressed sequences appeared to be components of an extracellular receptor kinase (ERK) signal transduction cascade, others seemed to participate in a variety of central cellular processes like cell-cycle control, protein turnover, intercellular signalling, transposon activity, transcriptional regulation and endoplasmic reticulum-mediated biosynthesis. In silico mapping revealed that a particular group of five genes silenced in apomictic plants clustered in a rice genomic area syntenic with the region governing apospory in Paspalum notatum and Brachiaria brizantha. Two of these genes mapped within the set of apo-homologues in P. notatum. Four genes previously reported to be controlled by ploidy were identified among those expressed differentially between apomictic and sexual plants. In situ hybridization experiments were performed for selected clones.