This genus includes grapefruit, citron, lemon, lime, orange and shaddock, but the precise number of its species is not reliably known, as some claim only four to five species, while others regard up to 145 species as belonging to this genus. Everett (1980–82) includes 16 species with botanical names of some of the common fruits such as C. aurantifolia (swingle, lime), C. aurantium L. (sour or Seville orange), C. limon Burm (Lemon), C. limonia (Mandarin lime), C. medica L. (citron), C. reticulata Blanco (C. nobilis, mandarin, satsuma orange, tangerine) and C. sinensis Osbeck (common sweet orange). Because of much confusion in delimiting and distinguishing citrus species from each other and from their hybrids, many researchers have not given the botanical names but common names. Therefore, presently common names have been retained to avoid the existing confusion and confounding of details. Citrus species hybridise easily with each other and many such hybrids produce apomictic seeds. The apomixis in hybrids has rendered pollen production nonuseful and extravagent. The mst has persisted through apomictic seed formation and extensive vegetative propagation. Therefore mst is most rampant in this genus. However, not much is known about the genetic control and female fertility of such mst’s. Due to apomixis, in some of them seeds are frequently produced. The mst is rampant in C. aurantifolia, C. limon, C. medica, C. sinensis, C. reticulata C. unshiu and C. yatsushiro. In many of these, the microsporogenesis breakdown stage is known (Table 47.1) but not the genetic control of the sterility and status of female fertility. In a few, male meiosis is known, and is given below.
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