Developmental And Genetic Characters Of Apomixis In Tea Crabapple As Well As Functional Characterization Of Apomixis-related Genes

Apomixis is an attractive trait in developmental biology. Compared with the sexualoffspring, progeny of apomixis are large genetically uniform and without character separation.In the rice and corn breeding, apomixis can fix heterosis. In addition, apple, orange, potatoesand sweet potatoes are typically propagated vegetatively. if apomixis is introduced to them,they would produced asexual seeds which can avoid inconvenience in propagation anddegradation in long-term vegetative propagation. Therefore, apomixis has great potentialapplication in agricultural production. Because of its huge potential utilization inhybridization breeding, both botanists and breeders are interested in elucidating the molecularmechanism underlying apomixis.In this study, developmental and genetic characters were observed and investigated inapomictic tea crabapple. Apomixis-related genes were molecular cloned and functionallycharacterized. The main results are shown as follows:1. Tea crabapple shows high capacity of parthenogenesis and apomeiosisStomatic cells developed into one or more embryo sacs in the ovules of tea crabapple.Aposporous embryo sac initiated development at tetrad stage. Investigations throughcontinuous three years showed that the percentages of parthenogenesis and2n gamete were94%and95%, respectively, in tea crabapple, indicating that it exhibits high capacity toperform parthenogenesis and apomeiosis.2.2n embryo sacs escape fertilization due to developmental delay in apomictic apple speciesEmbryo sacs of tea crabapple showed developmental delay,which was also found both inother two apomictic wild apple species M. Sargentii and M. Xiaojinensis. When embryo sacbecame mature, the stigma turned too senescent to accept pollens. Finally, asexual seedsformed directly from2n embryo sacs without fertilization. This phenomenon was called FLD,which is an early developmental event crucial for apomixis.3. Apomictic reproduction is enhanced by shortening pistil longevityMost of apomictic apple species perform facultative apomixes, i.e. they produce sexualseeds to certain great in addition to apomictic ones. Apomictic seed formation was enhancedby spraying lime sulphur in order to shorten pistil longevity and avoid pollination.4. Apomeiosis and parthenogenesis are genetically independent but collaborate to determineapomixisGenetical analysis showed that apomeiosis and parthenogenesis are geneticallyindependent to each other. They were uncoupled in the hybrid population obtained from a cross between tea crabapple and Maypole. But their multiplication was positively related tothe apomictic capacity and FLD phenomenon. Therefore, apomeiosis and parthenogenesisdetermine apomictic capacity, and FLD reflect the ability of apomixis in apomictic applespecies.5. Screening and cloning of apomixis-related genesAccording to the different ability of apomeiosis and parthenogenesis, hybrid offspringpopulation was divided into four subpopulations. Apomixis genes were screened through anexpression analysis using cDNA libraries of4subpopulations. Depending on the screening,two genes MhFIE and MhMSI1were cloned. Both predicted protein contained WD domainswhich are highly conserved in various plant species. Furthermore, GFP fusion analysisdemonstrated that both MhFIE and MhMSI1proteins were subcellurly localized in nucleus.6. Functional characterization of MhFIE and MhMSI1genesTo further characterize the functions of MhFIE and MhMSI1in planta, the constructscontaining MhFIE and MhMSI1driven by35S promoter were genetically transformed intoarabidopsis, tomato and apple, respectively. In transgenic Arabidopsis and tomato lines,ectopic expression of MhFIE and MhMSI1brought about developmental alterations both invegetative and reproductive growth, indicating that the functions of both genes are importantfor normal growth in higher plants.