Single nucleus sequencing reveals spermatid chromosome fragmentation as a possible cause of maize haploid induction

Abstract

Production of maternal haploids using a conspecific haploid inducer is routine and highly efficient in maize. However, the underlying mechanism of haploid induction (HI) is unclear. We develop a method to isolate three nuclei from a pollen grain and four microspores from a tetrad for whole-genome sequencing. A high rate of aneuploidy is observed at the three-nucleus stage (6/22 pollens) rather than at the tetrad stage (1/72 microspores) in one HI line CAU5. Frequent aneuploidy is also observed in another two inducer lines, but not in two regular lines, which implies that HI may be associated with pollen aneuploidy. We further sequence the individual embryos and endosperms of 88 maize kernels crossing between regular and inducer lines. Genome-wide elimination of the CAU5-derived chromosome is identified in eight of 81 embryos. Together, these results suggest that continuous chromosome fragmentation occurring post meiosis in the gametophyte may cause haploidy of the embryo.

Synthetic Apomixis an Old Enigma to Preserve Hybrid Vigor

“…Moreover, [14] multiplex editing of three (REC8, PAIR1 and OSD1) key meiotic genes in hybrid rice leads to the production of clonal diploid gametes and tetraploid seeds. Next, editing of the MTL [4,59] gene involved in fertilization results in the induction of haploid seeds in hybrid rice. By simultaneous editing of these four endogenous genes in hybrid rice using the CRISPR/Cas9 system, plants able to propagate clonally through seeds were obtained.…”

“…Wang et al, [14], reported the MiMe does not significantly reduced the seed production whereas, the mutation in MTL leads toward haploid induction at expense of seed production in rice. Similarly, in Maize the large scale chromosome fragmentation caused reduction in fertility owing to post-meiosis in the mtl gametophyte [4]. The fragmentation leads toward the imbalance between genome of both maternal and paternal side.…”

“…Furthermore, the knockout of ZmTMS5 and OsTMS5 in maize and rice, respectively resulted in thermosensitive lines. These lines contain fertile female part and are successfully utilized for hybrid seed production [4]. Moreover, a transgene free maize male sterile line was developed through the targeted mutagenesis of MS8 gene and proved as a valuable source for hybrid development [5] (Figure 1).…”

Clonal seeds in hybrid rice using CRISPR/Cas9

“…First, we showed that multiplex editing of three key meiotic genes 5,6 in hybrid rice leads to the production of clonal diploid gametes and tetraploid seeds. Next, editing of the MATRILINEAL ( MTL ) gene that involved in fertilization 7,8 results in the induction of haploid seeds in hybrid rice. By simultaneous editing of these four endogenous genes in hybrid rice using the CRISPR/Cas9 system, we obtained in one generation plants able to propagate clonally through seeds.…”

“…This ploidy doubling must be prevented to achieve apoximis. Recently, it was reported that the MATRILINEAL ( MTL ) gene, a sperm-specific phospholipase, triggers haploid induction in maize 7,8 . To test whether the homologous gene could be manipulated to induce haploid in self-fertilized hybrid rice, we edited the MTL gene in CY84 (Fig.…”