Genetic analyses of the inheritance and expressivity of autonomous endosperm formation in
            <i>Hieracium</i>
            with different modes of embryo sac and seed formation

Apomixis in the common dandelion (Taraxacum officinale) consists of three developmental components: diplospory (apomeiosis), parthenogenesis, and autonomous endosperm development. The genetic basis of diplospory, which is inherited as a single dominant factor, has been previously elucidated. To uncover the genetic basis of the remaining components, a cross between a diploid sexual seed parent and a triploid apomictic pollen donor was made. The resulting 95 triploid progeny plants were genotyped with co-dominant simple-sequence repeat (SSR) markers and phenotyped for apomixis as a whole and for the individual apomixis components using Nomarski Differential Interference Contrast (DIC) microscopy of cleared ovules and seed flow cytometry. From this, a new SSR marker allele was discovered that was closely linked to parthenogenesis and unlinked to diplospory. The segregation of apomixis as a whole does not differ significantly from a three-locus model, with diplospory and parthenogenesis segregating as unlinked dominant loci. Autonomous endosperm is regularly present without parthenogenesis, suggesting that the parthenogenesis locus does not also control endosperm formation. However, the high recovery of autonomous endosperm is inconsistent with this phenotype segregating as the third dominant locus. These results highlight the genetic complexity underlying apomixis in the dandelion and underline the challenge of introducing autonomous apomixis into sexual crops.

Section: Introductionmentioning

confidence: 99%

Genetic Dissection of Apomixis in Dandelions Identifies a Dominant Parthenogenesis Locus and Highlights the Complexity of Autonomous Endosperm Formation

Dijk¹,

Camp²,

Schauer³

2020

“…In R35 the dominant LOSS OF PARTHENOGENESIS ( LOP ; Figure 1 B) locus controls fertilization-independent embryogenesis and endosperm formation [ 11 , 12 ]. An additional locus has been identified in D36 termed AutE which enables fertilization-independent endosperm formation [ 13 , 14 ].…”

Section: Introductionmentioning

confidence: 99%

Efficient CRISPR/Cas9-Mediated Knockout of an Endogenous PHYTOENE DESATURASE Gene in T1 Progeny of Apomictic Hieracium Enables New Strategies for Apomixis Gene Identification

Henderson

Goetz

et al. 2020

Self Cite

Most Hieracium subgenus Pilosella species are self-incompatible. Some undergo facultative apomixis where most seeds form asexually with a maternal genotype. Most embryo sacs develop by mitosis, without meiosis and seeds form without fertilization. Apomixis is controlled by dominant loci where recombination is suppressed. Loci deletion by γ-irradiation results in reversion to sexual reproduction. Targeted mutagenesis of genes at identified loci would facilitate causal gene identification. In this study, the efficacy of CRISPR/Cas9 editing was examined in apomictic Hieracium by targeting mutations in the endogenous PHYTOENE DESATURASE (PDS) gene using Agrobacterium-mediated leaf disk transformation. In three experiments, the expected albino dwarf-lethal phenotype, characteristic of PDS knockout, was evident in 11% of T0 plants, 31.4% were sectorial albino chimeras, and the remainder were green. The chimeric plants flowered. Germinated T1 seeds derived from apomictic reproduction in two chimeric plants were phenotyped and sequenced to identify PDS gene edits. Up to 86% of seeds produced albino seedlings with complete PDS knockout. This was attributed to continuing Cas9-mediated editing in chimeric plants during apomictic seed formation preventing Cas9 segregation from the PDS target. This successful demonstration of efficient CRISPR/Cas9 gene editing in apomictic Hieracium, enabled development of the discussed strategies for future identification of causal apomixis genes.

“…Engineering the autonomous formation of the endosperm without fertilization to obtain a complete asexual system may be ideal from a biotechnological perspective. From a biological viewpoint, despite studies showing that a single dominant locus is able to induce the autonomous endosperm phenotype in apomictic Hieracium [144], inducing autonomous endosperm in sexuals might be hard due to the molecular complexity of its development. Autonomous endosperm development differs between dicots and monocots, and the dynamics underlying this complex process rely on genome balance, epigenetic gene regulation, and parent-of-origin effects founded upon the contribution of the male gamete [145].…”

Section: Mimicking Gametophytic Apomixismentioning

confidence: 99%

“…Autonomous endosperm development differs between dicots and monocots, and the dynamics underlying this complex process rely on genome balance, epigenetic gene regulation, and parent-of-origin effects founded upon the contribution of the male gamete [145]. In addition, still unknown genetic modifier elements, protein interactions, and regulatory pathways underlying embryo-endosperm developments add to the complexity [101,146,147] and restrain autonomous endosperm expressivity [144]. Regardless of mutations in fertilization independent endosperm (FIE) and other Polycomb group genes leading to autonomous endosperm development in Arabidopsis [148], orthologs reported in both rice (OsFIE1 and OsFIE2) and maize (ZmFIE1 and ZmFIE2; [149]) produce distinct phenotypes.…”

Section: Mimicking Gametophytic Apomixismentioning

confidence: 99%

Can We Use Gene-Editing to Induce Apomixis in Sexual Plants?

Scheben

Hojsgaard²

2020

Apomixis, the asexual formation of seeds, is a potentially valuable agricultural trait. Inducing apomixis in sexual crop plants would, for example, allow breeders to fix heterosis in hybrid seeds and rapidly generate doubled haploid crop lines. Molecular models explain the emergence of functional apomixis, i.e., apomeiosis + parthenogenesis + endosperm development, as resulting from a combination of genetic or epigenetic changes that coordinate altered molecular and developmental steps to form clonal seeds. Apomixis-like features and synthetic clonal seeds have been induced with limited success in the sexual plants rice and maize by using gene editing to mutate genes related to meiosis and fertility or via egg-cell specific expression of embryogenesis genes. Inducing functional apomixis and increasing the penetrance of apomictic seed production will be important for commercial deployment of the trait. Optimizing the induction of apomixis with gene editing strategies that use known targets as well as identifying alternative targets will be possible by better understanding natural genetic variation in apomictic species. With the growing availability of genomic data and precise gene editing tools, we are making substantial progress towards engineering apomictic crops.