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Citrus Tristeza Virus Resistance Gene (Ctv) – A Single Dominant Allele






CTV is the most signifi cant viral pathogen of citrus worldwide, inducing two major serious disease syndromes: ‘quick decline’ and ‘stem-pitting’ (Garnsey and Lee 1988). The former may cause tree death resulting from phloem necrosis below the graft union of the infected scions on sour orange (C. aurantium L.) rootstock; the latter will decrease tree vigour, fruit size and quality, and productivity regardless of rootstock (Bar-Joseph et al., 1989). There is no known source of broad resistance to CTV among Citrus species. The CTV resistance gene was characterized as a single dominant gene (Yoshida 1985, 1993) and designated Ctv (Gmitter et al., 1996); it is found only in Poncirus trifoliata (the trifoliate orange), a sexually compatible species related to Citrus.

Several populations were used initially to evaluate the inheritance of CTV resist- ance and susceptibility by enzyme-linked immunosorbent assay (ELISA) to detect the virus in rootstocks and in the grafted hybrids, using either a monoclonal anti- body or polyclonal antibodies (Garnsey et al., 1981, 1987; Gmitter et al., 1996). A min- imum of four repeated ELISAs over time were run independently at two laboratories to ensure identical results for individuals from a larger population, consisting of 65


 

 

T08 B11

O07 X18 W18 H01 O16 F06

A

3.1 Ctv 1.6 1.6 1.6 1.5 4.7

 

 

T08 O07 Ctv B11 X18 W18 H01 O16 F06

B

1.7 1.8 0.4 0.6 0.6 0.7 1.4 1.3

 

Fig. 13.2. The fi rst genetic linkage map of the Ctv region, based on the R family (C. grandis and P. trifoliata backcross to C. grandis), containing eight linked RAPD markers (a). The consensus map of the Ctv region based on the R family and the other four families (b). MAPMAKER 3.0 and JOINMAP 1.3 were used for mapping, respectively. All RAPD markers were identifi ed initially using bulked segregant analysis (BSA; Michelmore et al., 1991).

 

 


BC1 hybrids of ‘Thong Dee’ pummelo (C. grandis L.) ´ USDA 17-40 (‘Thong Dee’ pummelo ´ Pomeroy trifoliate orange (P. tri- foliata)). It was designated the R family and used as the primary marker screening and genetic mapping population. The fi rst local- ized map derived from the R family, as well as the consensus map based on all popula- tions used, is presented in Fig. 13.2 (Gmitter et al., 1996). This map started the powerful engine to move the exploration of the gene forward.

Because of limitations in using quickly identifi ed random amplifi ed polymorphic DNA (RAPD) markers in genomic library screening and other applications, the amplified fragments were cloned, sequenced, and used to develop the specifi c sequence characterized amplified region (SCAR) markers linking the Ctv gene (Deng et al., 1997). In this stage, a preliminary screening of a BAC library using those spe- cifi c markers resulted in several individual BAC clones in this region (Gmitter et al., 1997; Chen et al., 1999). Continued marker development and mapping in a larger segre- gating population led to an increase of marker density on the genetic map, and development of additional BAC libraries harbouring larger inserts were screened so that both the genetic and physical maps in this region were integrated (Deng et al., 2001a, b; Fig. 13.3). Finally, BAC contigs


that spanned both the resistance and sus- ceptibility allelic regions were constructed, additional fi ne mapping was conducted to delimit the gene region, and full sequences of both allelic chromosome regions were determined.

Similar work has been conducted at the University of California at Riverside, and Texas A & M University (Fang et al., 1998), and a full-length sequence of the resistant chromosome region has been published by Yang et al. (2003). This sequence reveals numerous RGCs, as well as other gene sequences and mobile genetic elements, based on gene prediction software. Many of these candidate genes have been cloned into transformation vectors, and several are already confi rmed to have been integrated into the genomes of CTV-susceptible citrus lines for genetic complementation assays (F. G. Gmitter, T. E. Mirkov and M. L. Roose, unpublished data). To date, how- ever, the gene or genes ultimately responsi- ble for the broad-spectrum resistance of the Mendelian gene, Ctv, have not yet been con- fi rmed.

 

 

Citrus Nematode Resistance Gene (Tyr1)






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