Introduction. Citrus and related genera have 18 chromo- somes in diploid somatic cells (Frost, 1925a; Nakamura, 1929)

Citrus and related genera have 18 chromo- somes in diploid somatic cells (Frost, 1925a; Nakamura, 1929). The basic chromo- some number, x=9, is invariant in Citroideae/Aurantioideae (Stace et al., 1993) and largely conserved in Rutaceae (Smith-White, 1954). Variant poloidies of different types have been reported (Guerra, 1984).

There have been spontaneously occurred tetraploids from nuclellar seedling (Frost, 1925b). Tetraploid clones were generated by artifi cial induction using colchicine (Barrett, 1974; Oiyama and Okudai, 1986). Naturally occurring triploid ‘Tahiti’ lime, triploids between tetraploid and diploid crosses, and spontaneously occurring triploids from diploid crosses have been reported (Krug and Bacchi, 1943; Oiyama et al., 1991; 1980). The percentage of triploid obtained was higher in the smaller seed with less than 0.1 g weight. The triploid seedless grapefruit cultivars, ‘Oroblanco’ and ‘Melogold’ have been released (Soost and Cameron, 1980, 1985).

Pentaploid, hexaploid as well as

tetraploids were obtained from crosses between triploid and diploid (Esen and Soost, 1972a). These unexpected ploidy levels may have arisen from the functioning of doubly unreduced female gametes (Esen and Soost, 1973). Oiyama and Kobayashi (1993) obtained haploids from diploid ‘Clementine’ and ‘Lee’ pollinated with pollen from a triploid plant.

Haploid plantlet regeneration through gynogenesis in Citrus clementina Hort. ex Tan., cv. Nules, has been induced by in vitro pollination with pollen from ‘Oroblanco’, a triploid grapefruit hybrid (Gemanà and Chiancone, 2001).

Interspecific hybridization, ploidy level and the mono/polyembryonic nature of the variety in question may also con- tribute to the frequency of polyploid proge- nies (Cameron and Soost, 1969; Wakana et al., 1981).

Aneuploids such as trisomics and monosomics are usually convenient tools for the genome mapping of the specifi c chromosomes. From the crossing between diploid and tetraploid, Esen and Soost (1972b) detected various aneuploids including 2n=22, 24, 25, 28, 29, 30, 31, 33,

© CAB International 2007. Citrus Genetics, Breeding and Biotechnology (ed. I.A. Khan) 151

37, 38, 39, and 41. In the crosses between diploid with triploid pollen, a number of trisomics (2n=19) were obtained (Sharma and Bal, 1957; Oiyama and Kobayashi, 1993). Several types of meiotic irregulari- ties capable of producing aneuploid gametes have been reported by researchers (Raghuvanshi, 1962a, b; Naithani and Raghuvanshi, 1963).

Meiotic behavior of somatic hybrids provide valuable information for their prac- tical utilization in citrus breeding pro- grams. Generally abnormal tetrad formation and irregular chromosome behavior with univalent or multivalent pairing occur in somatic hybrid plants. Meiotic abnormali- ties such as chromosome bridges and chro- mosomes orientated away from the equatorial plate are frequently observed in somatic hybrids resulting in different sizes of pollen grain (Chen et al., 2004).

Karyotype analysis in most plants has been identifi ed by the length, and the posi- tions of centromere and secondary constric- tion of each chromosome. Kandaelaki (1938) attempted to classify the metaphase chromosomes into 3 groups i.e. distinc- tively unequal-armed chromosomes, chro- mosomes with only slightly unequal-armed chromosomes, and with satellites. However, the staining methods used tradi- tionally with aceto-carmine, aceto-orcein or Feulgen’s solution were less informative to reveal detailed structure under the usual optical microscope because the mitotic

chromosomes are very small (1.0-4.0 µm) and most of them are similar in morphology (Krug, 1943).

Recent developments in genomics require high resolution karyotype analysis so that chromosome map could provide the integrated information of physical mapping in Citrus. The techniques of karyotype analysis have been highly improved by the development of enzymatic maceration of specimens, fl uorochrome staining for the structure analysis of chromosomes, and FISH on the mapping of the specifi c DNA sequences on chromosomes.

Citrus breeding programs need improved chromosomal analysis. Cultivated Citrus species have been hybridized with some wild relatives such as Murraya, Severinia, Atalantia, and Swinglea, in order to introduce desirable traits, mainly resistance to pests and pathogens (Barrett, 1977; Motomura et al., 1995). The identifi cation of chromosomes of different genomes could be a simple method of identifying citrus hybrids and is thus important for future work (Cameron and Frost, 1968).