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Introduction. The development of new improved culti- vars is the key for the economic success of the agricultural marketing of any crop






The development of new improved culti- vars is the key for the economic success of the agricultural marketing of any crop. Cultivars with unique traits facilitate expansion of crop areas and attract small growers interested in developing niche markets to avoid the competition with large and well established growers. Conventional breeding has been the most used method to create cultivars in most crops; however, for citrus this is not very practicable. Conventional breeding has been irrelevant for the development of improved cultivars for the most economically important citrus species, specifi cally sweet orange, grape- fruit and lemon. Barriers such as sterility (Soost and Cameron, 1975) and self- and cross-incompatibility (Soost, 1969) prevent important crosses from being successful. Additionally, the widespread nucellar embryony (Frost and Soost, 1968; Soost and Cameron, 1975) in conjunction with high heterozygosity makes the production of a large segregating population for the selec- tion of a specifi c trait an almost impossible task.

The development in the last decade of


 

molecular methods for the introduction of genes into several citrus species (Gutierrez et al., 1997; Bond and Rose, 1998; Dominguez et al., 2000; Yang et al., 2000) opened up new opportunities for the cre- ation of improved citrus varieties in the near future. Many horticulturally important traits, however, are polygenically encoded and, therefore, are not amenable to be trans- ferred by the currently available genetic transformation procedures. Additionally, for the gene to be transferred by genetic transformation, it needs to be isolated, and currently the availability of genes of horti- cultural importance for citrus is still very limited. Somatic hybridization by proto- plast fusion is a method that allows combi- nation of the entire genome of two different species. This has been an excellent method in citrus for combining closely (Louzada et al., 1992; Grosser et al., 1998) and distantly related genotypes (Grosser et al., 1988, 1990; Louzada et al., 1993), overcoming incompatibility barriers. Fertility seems to be no problem among several somatic hybrids produced to date (J.W. Grosser, Florida, 2002, personal communication). This method has been very successful in producing tetraploid breeding parents to be


 

© CAB International 2007. Citrus Genetics, Breeding and Biotechnology (ed. I.A. Khan) 261


used in interploid crosses targeting seedless triploid cultivars. In addition, it has been of great importance for the production of new genotypes to be tested directly as root- stocks. For direct use as scion varieties, however, the tetraploid nature of the somatic hybrids seems to be, in most cases, less benefi cial probably due to the overex- pression of some genes. The production of citrus somatic hybrids containing the com- plete genome of a recipient species, and only one or a few chromosomes, or chro- mosome segments from a donor species would be of great interest for the citrus industry; so far there are no reports on the production of such hybrids.

To achieve partial genome transfer, usually asymmetric somatic hybridization is performed by irradiating donor proto- plasts with high doses of UV, X- or [g]-rays, before fusion, to induce donor chromosome elimination or recombination in fusion products. Sometimes, most donor chromo- somes are eliminated in the asymmetric hybrids (Vlahova et al., 1997; Yemets et al., 2000), and at other times only a few are eliminated (McCabe et al., 1993), and, fur- ther, donor chromosomes may undergo fragmentation and recombine with the recipient genome (Lie et al., 1999; Tian and Rose, 1999; Tian et al., 2002). Highly asym- metric hybridization, however, is more common when distantly related species are used as parents, being more diffi cult to accomplish when the parents are from the same genus (Dudits et al., 1987). Asymmetric hybridization involving dis- tantly related species is important to introgress genes from incongruent species into cultivated species. For citrus, it would also be of great interest to have a method that would facilitate the transfer of one or a few chromosomes from one species to a closely related one, for the production of new cultivars in one step without further sexual hybridization. Additionally, it would probably be more profi table if partial genome transfer could be performed with- out the use of irradiation to minimize the damage to the donor genome. Partial nuclear genome transfer in citrus is still to


 

 

be explored since hybrids with very limited chromosome numbers from a donor species have never been produced. In this chapter, we will discuss the methodology of micro- protoplast-mediated chromosome transfer (MMCT) as a new approach for partial genome transfer in citrus.

 

 


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