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Eliezer E. Goldschmidt

The citron ( Citrus medica L.) is believed to be one of the ancestors of the citrus group ( Barrett and Rhoads, 1976 ), but its study was somewhat neglected during the 20th century, presumably because of its limited commercial use. However, renewed

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Elisabetta Nicolosi, Stefano La Malfa, Mohamed El-Otmani, Moshe Negbi and Eliezer E. Goldschmidt

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Mikeal L. Roose, Claire T. Federici and Gregory P. Copenhaver

To assess genetic diversity in the UC Riverside Citrus Variety Collection, all accessions of pummelo (59), citron (24), and trifoliate orange (48) were studied for RFLP variation using 11-18 cDNA probes that had previously been shown to reveal polymorphism in a broad range of citrus germplasm. Inheritance studies have shown that these probes hybridize to at least 20 loci. The taxa studied are believed to represent biological species rather than hybrids. Citrons were nearly monomorphic and most appeared homozygous at all of the loci studied. Pummelos were very polymorphic and highly heterozygous. Trifoliate orange, an important source of disease resistance in rootstock breeding, was nearly monomorphic but moderately heterozygous (17% of loci). Most accessions of trifoliate orange have evidently differentiated only by mutation. One multilocus probe separated trifoliate orange accessions into 3 groups. Two new trifoliate orange accessions had novel alleles at some loci. The use of genetic markers to recognize hybrid accessions classified as members of species will be discussed.

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Godfrey P. Miles, Ed Stover, Chandrika Ramadugu, Manjunath L. Keremane and Richard F. Lee

percentages for each seed parent (pedigrees were not determined for individual seedlings) from each of the progenitor Citrus species that gave rise to all cultivated citrus: citron ( C . medica ), pummelo ( C . maxima ), mandarin ( C. reticulata ), and

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Xiaomeng Li, Rangjin Xie, Zhenhua Lu and Zhiqin Zhou

derived from AFLP data are presented in Table 3 . When interspecific mean distances within genus Citrus were compared, C. medica and C. paradisi appeared to be the most distant pair (0.0592, Table 3 ), while the least interspecific distance was

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Yuan Yu, Chunxian Chen, Ming Huang, Qibin Yu, Dongliang Du, Matthew R. Mattia and Frederick G. Gmitter Jr.

in five population groups. The five groups are depicted using the following color codes: red, Citrus maxima ; green, Citrus reticulata ; blue, Citrus medica ; yellow, Poncirus trifoliata ; and purple, Fortunella . Interspecific introgressions

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Dafna Langgut

citrus types are citron ( C. medica ), pummelo ( C. maxima ), and mandarin ( C. reticulata ). This argument was made four decades ago by several scholars ( Barrett and Rhodes, 1976 ; Scora, 1975 ) based on morphological and biochemical characteristics

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O. Gulsen and M.L. Roose

Chloroplast DNA (cpDNA) restriction fragment length polymorphisms were used to study the parentage of lemons [Citrus limon (L.) Burm. f.] and several other putative hybrids. The 30 citrus accessions studied included nine lemons, three pummelos [C. maxima (Burm.) Merrill], three citrons (C. medica L.), three mandarins (C. reticulata Blanco, C. clementina Hort. Ex. Y. Tan., and C. sunki Hort. ex. Tan.), and single representatives of 13 other taxa. Four different fragments of citrus cpDNA were amplified by polymerase chain reaction using four universal chloroplast primers and amplification products were digested with four endonucleases, Rsa I, Dra I, Hae III, and Mbo I. A total of 104 different restriction fragments were scored and used for phylogenetic analysis by parsimony. The three taxa which have been proposed as ancestral Citrus L. sp., C. medica, C. maxima, and C. reticulata, had distinct cpDNA patterns. Pummelo contributed the chloroplast genome to lemons, sweet orange [C. sinensis (L.) Osbeck], and Bergamot orange (C. bergamia Risso and Poit.), and mandarin contributed the chloroplast genome to rough lemons (C. jambhiri Lush.), Rangpur lime (C. limonia Osbeck), and `Mexican' lime [C. aurantifolia (Christm.) Swing.]. Data suggest that the particular accessions of these ancestral species that were studied were not directly involved in the hybridization events that created these hybrids, since they have similar but not identical cpDNA restriction fragments. Using inter-simple sequence repeat markers that amplified from nuclear genomic DNA, a set of samples including pummelos, citrons, mandarins, sour oranges (C. aurantium L.), and `Samuyao' papeda (C. micrantha Wester) were tested as possible parents of lemons. Sour orange and citron together had all nuclear and chloroplast fragments found in lemon and are therefore proposed to be the maternal and paternal parents, respectively, of many commercial lemon cultivars, including `Lisbon', `Eureka', `Villafranca', and `Monachello'.

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Zenaida Viloria and Jude W. Grosser

Interploid hybridization was conducted using `Key' lime [Citrus aurantifolia (Cristm.) Swing.], `Lakeland' limequat hybrid [C. aurantifolia × Fortunella japonica (Thumb.) Swing.], Palestine sweet lime (C. limettioides Tan.), `Etrog' citron (C. medica L.), and seven lemon [C. limon (L.) Burm. F.] varieties as female progenitors and five allotetraploid somatic hybrids {`Hamlin' sweet orange [C. sinensis (L.) Osbeck] × `Femminello' lemon (C. limon)]; `Key' lime × `Valencia' sweet orange (C. sinensis); `Valencia' sweet orange × rough lemon (C. jambhiri Lush); Milam lemon (purported C. jambhiri hybrid) × `Femminello' lemon (C. limon); and `Valencia' sweet orange × `Femminello' lemon} and two autotetraploids [`Giant Key' lime (C. aurantifolia) and `Femminello' lemon] as pollen progenitors. A few tetraploid × diploid crosses were also performed. Thirty-five parental cross combinations were accomplished in 2000, 2001, and 2002. The breeding targets were seedlessness, cold-tolerance, and disease resistance. Triploid hybrids were recovered through embryo culture. Generation of triploid citrus hybrids was affected by several factors including sexual compatibility, cross direction, embryo developmental stage, pollen viability, as well as horticultural practices and climatic conditions. Efficiency of triploid hybrid production was higher in diploid × tetraploid crosses than the reciprocal. Many more triploid hybrids were generated from lemon seed progenitors compared to the other acid citrus fruit progenitors. `Todo el Año', `Lisbon', and `Limonero Fino 49' showed the highest sexual compatibility. Embryo germination rate and normal plant recovery were also higher in lemons as compared to the other seed progenitors. Low winter temperatures might have affected the hybrid production efficiency from tropical acid fruit progenitors. A total of 650 hybrids (mostly triploid) were transferred to soil. The novel genetic combinations of these progenies should be valuable for the genetic improvement of acid citrus fruit (lemons and limes).

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G. Reforgiato Recupero, G. Russo and S. Recupero

The breeding of citrus triploid hybrids started at Istituto Sperimentale per l'Agrumicoltura spell out in Acireale, Italy, in 1978 (Starrantino and Reforgiato, 1981). The strategy used has been to cross a monoembryonic 2x female parent with a 4x male parent. The imbalance between the embryo and endosperm ploidy (3:4) makes seeds from such crossings incapable of germinating in vivo. However, in vitro cultivation has been used to rescue the embryos. In this paper we report the main characteristics of triploid hybrids from 22 different parents, including sweet orange [Citrus sinensis (L.) Osbeck], mandarin (C. reticulata Blanco), clementine (C. reticulata Blanco), grapefruit (C. paradisi Macf.), pummelo [C.maxima], tangor (C. reticulata × C. sinensis), lemon [C. limon (L.) Burm. f.], citron (C. medica L.), and Fortunella hindsii (Champ.) Swing. Tetraploid parents are duplex because they originate from the doubling of chromosome number of the nucella or of other somatic tissues. The segregation and recombination process results in triploid hybrids with characteristics that are nearer to the 4x parent than the 2x one. This strategy is important in obtaining seedless hybrids similar to a parent after generations of backcrossing.