Abstract
Apomixis is able to fix traits through the formation of maternal clonal seeds. Apomixis has been observed in diverse plant families, however rarely in crop plants. Transferring apomixis into crops to retain heterosis is desirable. In order to develop a successful apomixis-transfer technology, the molecular mechanisms of apomixis need to be understood. Many model systems for studying apomixis exist for example; Hieracium spp., Poa pratensis, Boechera holboellii, Hypericum perforatum, Paspalum spp., Ranunculus spp., and Taraxacum officinale are utilized to study various aspects of apomixis. Hieracium subgenus Pilosella is a model for studying the apospory type of apomixis.
In this study two species of Hieracium subgenus Pilosella and a mutant accession that lost the ability for parthenogenesis and autonomous endosperm development were used to generate a reference transcriptome from reproductive tissue. The transcriptome is composed of 147,632 contigs, 50% of which were annotated with orthologous genes and their probable function. This transcriptome provides a solid basis for further studying apomixis.
The transcriptome was used to identify transcripts differentially expressed during apomictic and sexual seed development. Gene Ontology enrichment analysis of differentially expressed transcripts showed that epigenetic regulation of gene expression was the main difference between apomictic seed development and sexual seed development. Epigenetic regulation of gene expression was enriched in transcripts differentially expressed during sexual seed development, whereas it was not enriched in transcripts differentially expressed during apomictic seed development.
Genes that mark key developmental stages, i.e. aposporous embryo sac development and seed development, were also identified through their enhanced expression at those stages. These genes can be used to investigate the relevant developmental stages. In addition, genes active in the apomictic plant and inactive in the mutant pre-pollination stage, as the locus responsible for autonomous seed development (LOP) is deleted in the mutant, were identified. These can serve as candidates for characterizing the LOP locus.
In summary, a reference transcriptome was produced from apomictic Hieracium undergoing the key apomictic events of apomeiosis, parthenogenesis and autonomous endosperm development; as well as from a mutant accession that is not capable of parthenogenesis and autonomous endosperm development, at pre-pollination and post-pollination stages. The diverse origin of the transcriptome makes it a valuable novel resource for apomixis research.