Pseudogamous Apomixis in Maize and Sorghum in Diploid-Tetraploid Crosses

Apomictic seed development is a complex process including formation of unreduced embryo sac,parthenogenetic embryo development from the egg cell,and endosperm formation either autonomously,or due to fertilization of polar nuclei by the sperm(under pseudogamous form of apomixis).In the latter case,an obstacle to the normal endosperm development is disturbance of maternal(m)-to-paternal(p)genomic ratio 2m:1p that occurs in the cases of pollination of unreduced embryo sac with haploid sperms.Usage of tetraploid pollinators can overcome this problem because in such crosses maternal-to-paternal genomic ratio is 4m:2p that provides formation of kernels with plump endosperm.Using tetraploid lines as pollen parents we observed formation of plump kernels on the ears and panicles of diploid maize and sorghum accessions.These kernels had hybrid endosperm and diploid maternal-type embryo or hybrid embryo with different ploidy level(2n,3n,4n).The frequencies of plump kernels on the ear ranged from 0.2-0.3%to 5.7-6.2%counting from the number of ovaries.Maternal-type plants were found in two maize lines,their frequency varying from 10.7 to 37.5%of the progeny plants.In CMS-lines of sorghum pollinated with tetraploid sorghum accessions,the frequency of plump kernels ranged from 0.6 to 14.0%counting from the number of ovaries;the frequency of maternal-type plants varied from 33.0 up to 96.1%.The hybrid nature of endosperm of the kernels that gave rise to maternal-type plants has been proved by marker gene expression and by SDS-electrophoresis of endosperm proteins.These data testify to variable modes of seed formation under diploid×tetraploid crosses in maize and sorghum both by amphi-and by apomixis.Therefore,usage of tetraploid pollinators might be a promising approach for isolation of apomixis in maize and sorghum accessions.

Paternally imprinted LATE-FLOWERING2 transcription factor contributes to paternal-excess interploidy hybridization barriers in wheat∞

Interploidy hybridization between hexaploid and tetraploid genotypes occurred repeatedly during genomic introgression events throughout wheat evolution,and is commonly employed in wheat breeding programs.Hexaploid wheat usually serves as maternal parent because the reciprocal cross generates progeny with severe defects and poor seed germination,but the underlying mechanism is poorly understood.Here,we performed detailed analysis of phenotypic variation in endosperm between two interploidy reciprocal crosses arising from tetraploid(Triticum durum,AABB)and hexaploid wheat(Triticum aestivum,AABBDD).In the paternal‐versus the maternal‐excess cross,the timing of endosperm cellularization was delayed and starch granule accumulation in the endosperm was repressed,causing reduced germination percentage.The expression profiles of genes involved in nutrient metabolism differed strongly between these endosperm types.Furthermore,expression patterns of parental alleles were dramatically disturbed in interploidy versus intraploidy crosses,leading to increased number of imprinted genes.The endosperm‐specific TaLFL2 showed a paternally imprinted expression pattern in interploidy crosses partially due to allele‐specific DNA methylation.Paternal TaLFL2 binds to and represses a nutrient accumulation regulator TaNAC019,leading to reduced storage protein and starch accumulation during endosperm development in paternal‐excess cross,as confirmed by interploidy crosses between tetraploid wild‐type and clustered regularly interspaced palindromic repeats(CRISPR)–CRISPR‐associated protein 9 generated hexaploid mutants.These findings reveal a contribution of genomic imprinting to paternal‐excess interploidy hybridization barriers during wheat evolution history and explains why experienced breeders preferentially exploit maternal‐excess interploidy crosses in wheat breeding programs.