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.

Floral sex allocation and reproductive success within inflorescences of Hosta ventricosa, a pseudogamous apomict

Aims Within inflorescences of sexually reproducing hermaphrodites,the production of ovules,fruits and seeds commonly declines from basal(early-opening)to distal(late-opening)flowers,while pol-len production remains constant or only changes slightly,with the result that distal(late-opening)flowers become functionally male.However,few empirical studies have specifically examined whether or not changes in allocation to pollen production actually lead to changes in the number of seeds sired,a more direct measure of male fitness.in pseudogamous apomicts,fitness depends on the number of seeds produced;thus,a contrasting pattern of variation in the pollen-to-ovule(P/o)ratio within inflorescences might be expected.Methods We investigated floral sex allocation and reproductive success within racemes of Hosta ventricosa,a pseudogamous apomictic hermaphrodite possessing flowers that open acropetally.We quan-tified variations in pollen number,ovule number,the P/o ratio and fruit and seed production,from 2007 to 2011,among flowers within racemes of four populations of H.ventricosa in southwest China.Ecological causes for fruit and seed production were evaluated by observing patterns of pollen deposition,flower removal and sup-plemental pollination.Important Findings Pollen number,ovule number and the P/o ratio declined from basal-to-distal positions in all sampled populations(years).Fruit and seed production decreased distally in most populations(years).low fruit and seed set of distal flowers was not due to pollen limitation because pollen deposition never declined distally and the low fruit and seed set of distal flowers remained even after supplementary pollination was provided.The flower-removal experiment indicated that inter-fruit competition for resources among flowers was common.The low P/o ratio of distal flowers in H.ventricosa might be favored because they were unable to obtain fitness by donat-ing pollen and siring seeds on other plants.our study may help to understand the adaptive significance of sex allocation among flow-ers within inflorescences of sexually reproducing hermaphrodites.