‘Ougan’ mandarin is an ancient and peculiar Chinese native citrus cultivar, grown in the south of Zhejiang. It is a very tasty and easy-to-peel mandarin, cultivated for over 1000 years in China. Additionally, it has good shelf life at room temperature and has been considered to have medicinal application because its fruit are rich in vitamins, flavonoids, monoterpenoids, etc. (Yang, 2002). The wild type is seedy with each fruit containing ≈10 seeds. Seedlessness is an attractive trait for fresh fruit and a convenience for the consumer. In fact, many economically important commercial citrus cultivars worldwide are seedless. They originated from conventional breeding programs, from bud sport selections or chance seedling variation (Deng, 2005). The seedless cultivars from the bud sport mainly belong to three types: Satsuma mandarin (C. unshiu Marc.) (Malidzan et al., 2004), navel orange (C. sinensis Osbeck) (Yi et al., 2003), and Clementine mandarin (C. reticulata Blanco) (Bono et al., 1996; Russo, 2004). Each type includes dozens of cultivars with 4- to 6-month-long harvest periods. Seedless ‘Ougan’ mandarin is desired from a bud sport mutation found in Zhejiang Province in 1996. This mutant maintains all horticultural traits of the wild-type ‘Ougan’ mandarin.
Various researchers have achieved male-sterile materials by disrupting pollen development through induced mutation or genetic transformation (Daniell, 2002; Goetz et al., 2001; Guerineau et al., 2003). In citrus, with the aim of seedlessness, Li et al. (2002, 2003) introduced the chimeric ribonuclease gene into Ponkan mandarin (C. reticulata) and Valencia sweet oranges (C. sinensis) by agrobacterium [Agrobacterium tumefaciens (Smith & Towns.) Conn. EHA105] transformation. Guo et al. (2004) conducted somatic fusion of embryogenic suspension-derived protoplasts of Satsuma mandarin, a male-sterile cultivar, with leaf protoplasts of seedy types of citrus, in an attempt to generate seedless cybrids. Male sterility in citrus generally leads to seedlessness because of the parthenocarpy characteristic of most citrus genotypes.
Knowledge of male sterility in citrus is limited because woody plants have long life cycles, making them a difficult group to study. Yamamoto et al. (1993, 1995) investigated the relationship between male sterility and seedlessness in citrus and found that seedlessness in diploid citrus generally was related to male sterility or to self-incompatibility. They reported that cytoplasmic male sterility in C. unshiu was due to nuclear-cytoplasmic interaction (Yamamoto et al., 1997). Furthermore, Hu et al. (2006) reported that the timing and cell-specificity of mitochondrial gene expression were differentially regulated during male gametophyte development in C. unshiu and suggested that this was probably associated with its pollen abortion. In ‘Ougan’ mandarin, little evolution about male sterility has been conducted. Zeng et al. (2005) reported that rare, naturally released pollens and difficulty in pollen germination were responsible for the seedlessness of the mutant ‘Ougan’ mandarin. However, the development of mature pollen grains from the microspore mother cell (MMC) in the seedless mutant ‘Ougan’ mandarin is unknown.
In the current study, the development of anthers of both the mutant and the wild-type ‘Ougan’ mandarin were studied to determine the onset and structural manifestations of pollen abortion, with particular attention to the distribution of polysaccharides, lipids, and callose during anther development.
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