Morphological, physiological and molecular characteristics of the seedless 'Hongjiangcheng' sweet orange

Seedless citrus has become one of the breeding goals due to its high edible ratio and convenience in fresh consumption and processing.In this study, the ‘Hongjiangcheng' sweet orange(WT) and its seedless mutant(MT) after cobalt-60 radiation were selected to study the formation metabolism of citrus seedless phenotype. Compared with WT, the MT had altered primary metabolite contents, as indicated by GC-MS analysis. The mature pollen of the MT was mostly distorted and shrunken, and the orange mutant exhibited significantly lower fertility than the WT. Through pollination experiments and paraffin sectioning of the MT, we observed self-compatibility during pollen tube germination in situ, in combination with the absence of natural parthenocarpy and arrested zygotic embryo development at the fourth week after pollination. From transcriptomic analyses of ovules in the fourth week, 815 differentially expressed genes(DEGs) were identified.Furthermore, according to the annotation of gene function and qRT-PCR analysis, Cs4g10930, Cs5g21900 and orange1.1t02243 were identified as candidate genes that may govern the mechanism of seedlessness. Finally, Agrobacterium-mediated transformation verified that the overexpression of Cs4g10930 and Cs5g21900 in Newhall navel orange calli inhibited embryoid production. This study provides a better understanding of seedless formation in citrus and two key genes that may play an important role in the early selection of seedless lines in citrus breeding programs.

Genome of Wild Mandarin and Domestication History of Mandarin

Mandarin （Citrus reticulata） is one of the most important citrus crops worldwide. Its domestication is believed to have occurred in South China, which has been one of the centers of mandarin cultivation for four millennia. We collected natural wild populations of mandarin around the Nanling region and cultivated landraces in the vicinity. We found that the citric acid level was dramatically reduced in cultivated mandarins. To understand genetic basis of mandarin domestication, we de novo assembled a draft genome of wild mandarin and analyzed a set of 104 citrus genomes. We found that the Mangshan mandarin is a primitive type and that two independent domestication events have occurred, resulting in two groups of cultivated mandarins （MD1 and MD2） in the North and South Nanling Mountains, respectively. Two bottlenecks and two expansions of effective population size were identified for the MD1 group of cultivated mandarins. However, in the MD2 group there was a long and continuous decrease in the population size. MD1 and MD2 mandarins showed different patterns of interspecific introgression from cultivated pummelo species. We identified a region of high divergence in an aconitate hydratase （ACO） gene involved in the regulation of citrate content, which was possibly under selection during the domestication of mandarin. This study provides concrete genetic evidence for the geographical origin of extant wild mandarin populations and sheds light on the domestication and evolutionary history of mandarin.