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Nucellar Embryony






Nucellar embryony is a heritable trait found in some citrus varieties. Nucellar embryony is an adventitious form of apomictic repro- duction, wherein the somatic cells of the nucellus tissue are initialized to enter into an embryonic pathway of development. Development of nucellar embryos was stud- ied in detail by Koltunow et al. (1995), Kobayashi et al. (1979) and Wakana and Uemoto (1988). The nucellar embryos develop from nucellar initial cells that orig- inate in the nucellus that surrounds the embryo sac (Fig. 5.2). These cells occur in all portions of the nucellus, but many migrate to the micropylar end so in many varieties nearly all nucellar embyos are found at the micropylar end (Wakana and Uemoto, 1988). Nucellar embryos generally initiate development before fertilization, about ten days before anthesis, but this may vary with the genotype. Many embryo sacs contain several to many nucellar initial cells. As these begin to divide and grow, the number of growing embryos diminishes. At later stages, the larger embryos tend to be located at the micropylar end of the embryo sac (Fig. 5.2).

In citrus, nucellar embryony does not prevent normal sexual reproduction (Esan and Soost, 1977; Wilms et al., 1983). This results in trees bearing fruit with seeds con- taining two classes of embryos: (i) zygotic embryos from a fertilization event; and (ii) nucellar embryos that are genetically iden- tical to the maternal parent. Many geno- types with nucellar embryony also have a high frequency of multiple embryos, since many cells in the nucellus initiate embryo- genesis and several embryos may mature. A seed may contain two or more seedlings that germinate, but not every seed produced


 

 

 

Fig. 5.2. Citrus ovules, about 60 days after pollination, of (a) a variety with a high level of nucellar embryony and (b) a monoembryonic (sexual) variety.

 

 


by a tree with nucellar embryony has mul- tiple mature embryos.

In a genotype that produces seeds by nucellar embryony, normal sexual repro- duction can also occur. Such a genotype can produce several different types of seeds (Wakana and Uemoto, 1988): (i) seeds with one mature, sexual embryo; (ii) seeds with one mature nucellar embryo; (iii) seeds with multiple mature nucellar embryos; and (iv) seeds with one mature sexual and one or more mature nucellar embryos. Seeds may also contain immature embryos that cannot germinate normally. This is because not only does adventitious apomixis occur without preventing normal sexual reproduction, but also its success depends on fertilization for endosperm for- mation (Esan and Soost, 1977).

Nucellar embryo growth is arrested at the late globular stage without endosperm development (Koltunow et al., 1995). Most fertilization events involve a double fertilization: (i) of the polar nuclei to form the endosperm; and (ii) of the egg to form a zygote. Developmental competition between the zygotic and nucellar embryos, as well as the genotype of the zygote, will affect maturation and, in seeds with mature nucellar embryos, immature zygotic


and nucellar embryos are often present but do not germinate (Ueno et al., 1967; Esan and Soost, 1977; Koltunow et al., 1995).

The number and type of embryos pro- duced may vary from tree to tree and also at different positions on a single tree (Nasharty, 1945; Parlevliet and Cameron, 1959). This variation has been suggested to be controlled by minor genes, pollen source and environmental conditions (Frost, 1926; Parlevliet and Cameron; 1959, Khan and Roose, 1988). A study specifi cally designed to determine the origin of seedlings pro- duced by open-pollinated Poncirus trifoli- ata cultivars with the genotype of nucellar embryony found that the majority of the mature embryos that germinate are geneti- cally identical to the maternal parent (Khan and Roose, 1988). The proportion of nucel- lar seedlings has been estimated in many citrus rootstock cultivars using isozymes and other genetic markers, with most culti- vars having more than 90% nucellar seedlings (Table 5.1; Roose and Kupper, 1992), but this sample is biased toward genotypes producing a high proportion of nucellar embryos because citrus rootstocks are selected to have a high percentage of nucellar seedlings.


 

 

Table 5.1. Percentage of nucellar seedlings in citrus rootstocks as detected by isozyme analysis.

 

Rootstock Percentage zygotic References
Pomeroy trifoliate 14.1 Khan and Roose, 1988
Rubidoux trifoliate 4.6 Khan and Roose, 1988
Trifoliate orange 2.6, 21.6 Anderson et al., 1991; Moore and Castle, 1988
Flying Dragon trifoliate orange 18.0, 29.7 Moore and Castle, 1988; Khan and Roose,
     
Swingle citrumelo 9.3, 9.6, 15.2, 17.7 Ashari et al., 1988; Moore and Castle, 1988;
    Xiang and Roose, 1988
Sacaton citrumelo 38.7, 39.7 Xiang and Roose, 1988; Xiang and Roose,
     
C-32 citrange 5.7, 19.7 Xiang and Roose, 1988; M. L. Roose and R.
    S. Kupper, personal communication
Carrizo citrange 0.0 Anderson et al., 1991;
Troyer citrange 0.0, 0.8 Moore and Castle, 1988; Anderson et al., 1991
Uvale citrange 1.3 Moore and Castle, 1988
Yuma citrange 36.1, 50.6 Moore and Castle, 1988, Xiang and Roose,
     
Amblycarpa 4.9 Moore and Castle, 1988
Cleopatra mandarin 0.8 Anderson et al., 1991
Cuban shaddock 12.0 Moore and Castle, 1988;
Rough lemon 2.1, 5.0, 5.5 Anderson et al., 1991; Moore and Castle,
    1988; Xiang and Roose, 1988
Milam 0.0 Moore and Castle, 1988
Sour orange 0.0 Moore and Castle, 1988
Sweet orange 0.0, 0.8 Moore and Castle, 1988; Anderson et al., 1991
Taiwanica 34.8 Xiang and Roose, 1988
Volkamer lemon 16, 23.7, 27.3 Moore and Castle, 1988; Xiang and Roose,
     
Yuma Ponderosa 20.0 Xiang and Roose, 1988

At least 94% of zygotic seedlings from self-pollination should have been detected based on the number of enzyme loci studied, except in sweet orange (87%) and Cleo (50%).

 



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