Enlarging Cells Initiating Apomixis inHieracium praealtumTransition to an Embryo Sac Program prior to Entering Mitosis    

Abstract

Hieracium praealtum forms seeds asexually by apomixis. During ovule development, sexual reproduction initiates with megaspore mother cell entry into meiosis and formation of a tetrad of haploid megaspores. The sexual pathway ceases when a diploid aposporous initial (AI) cell differentiates, enlarges, and undergoes mitosis, forming an aposporous embryo sac that displaces sexual structures. Embryo and endosperm development in aposporous embryo sacs is fertilization independent. Transcriptional data relating to apomixis initiation in Hieracium spp. ovules is scarce and the functional identity of the AI cell relative to other ovule cell types is unclear. Enlarging AI cells with undivided nuclei, early aposporous embryo sacs containing two to four nuclei, and random groups of sporophytic ovule cells not undergoing these events were collected by laser capture microdissection. Isolated amplified messenger RNA samples were sequenced using the 454 pyrosequencing platform and comparatively analyzed to establish indicative roles of the captured cell types. Transcriptome and protein motif analyses showed that approximately one-half of the assembled contigs identified homologous sequences in Arabidopsis (Arabidopsis thaliana), of which the vast majority were expressed during early Arabidopsis ovule development. The sporophytic ovule cells were enriched in signaling functions. Gene expression indicative of meiosis was notably absent in enlarging AI cells, consistent with subsequent aposporous embryo sac formation without meiosis. The AI cell transcriptome was most similar to the early aposporous embryo sac transcriptome when comparing known functional annotations and both shared expressed genes involved in gametophyte development, suggesting that the enlarging AI cell is already transitioning to an embryo sac program prior to mitotic division.

Genomic analyses of primitive, wild and cultivated citrus provide insights into asexual reproduction

“…Previous studies reported that extreme stress induces the initiation of autonomous embryo development in somatic cells in response to exogenous or endogenous signals 39 . Here we identified six nucellar embryo-associated genes that are regulated by stress, which is in agreement with a previous report on apomixis in Hieracium praealtum 40 .…”

Apomictic and Sexual Germline Development Differ with Respect to Cell Cycle, Transcriptional, Hormonal and Epigenetic Regulation

“…Interestingly, for 14 selected meiotic genes and genes expressed in the sexual MMC no evidence for expression was found in the H. praealtum aposporous initial cell [31]. However, although the aposporous and the diplosporous initial cell both give rise to unreduced embryo sacs, cell lineage and developmental fate are markedly different.…”

The Genetic Control of Apomixis: Asexual Seed Formation

“…This approach is complicated by the nature of asexual reproduction that prevents the generation of isogenic lines that differ solely with regard to reproductive mode. Nevertheless, transcriptome comparisons between apomictic and closely related sexual individuals from reproductive tissues at different developmental stages have uncovered pathways potentially central to apomixis including protein degradation, transcription, stress response, and cell-to-cell signaling (Albertini et al 2005;Laspina et al 2008;Sharbel et al 2010;Silveira et al 2012;Okada et al 2013).…”