Citrus Germplasm Research Articles

Assessment of Citrus Susceptibility towards Candidatus Liberibacter Asiaticus-Terengganu Isolate Based on Vector and Graft Transmission Tests

Five citrus species namely Citrus reticulata, C. aurantium, C. grandis, C. madurensis and Murraya paniculata were assessed for tolerance by grafting and vector transmission tests toward Ca. L. asiaticus-Terengganu isolate. This study was conducted in an insect-proof glasshouse for a period of six months. Results of this study showed that seedlings of M. paniculata, and C. grandis showed no HLB symptoms six months after inoculation period. They were also showed negative results by polymerase chain reaction (PCR) test and recorded the lowest percentage of reduction rate of plant height and plant biomass. Based on our findings, proved that these two citrus species were resistance to huanglongbing (HLB) or citrus greening infection. However, C. aurantium was showed high tolerance to HLB infection with the trees exhibited mild leaf mottling and green vein and the infected seedlings recovered after 3-4 months after treated with fertilizer. However the pathogen was detected by PCR test in the recovered seedlings. C. reticulata and C. madurensis were very susceptible to HLB infection. Inoculated seedlings showed severe HLB symptoms such as leaf mottling and yellowing, leaf green vein, leaf defoliation, twigs dieback within six months after inoculation. The inoculated seedlings started showing the first symptoms within three months after inoculation. Infected seedlings were also showed positive results with PCR test and recorded the highest percentage of reduction rate of plant height and plant biomass. Hence, this data provide a basis for developing citrus germplasm program for tolerance to HLB disease in Malaysia.

Abundance of Citrus Leafminer Larvae on Citrus and Citrus-related Germplasm

The citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), is a key pest in most citrus-growing regions worldwide. Adult citrus leafminers oviposit primarily on young elongating flush of Citrus as well as other Rutaceae and some ornamental plants. Larvae feed on the epidermal cell layer of developing leaves and injury to leaves provides a pathway for infection by the bacterium Xanthomonas citri subsp. citri (Hasse), the causal agent of Asiatic citrus canker. In this study, we quantified abundance of citrus leafminer larvae on progeny of 87 seed parent genotypes of Citrus and Citrus relatives (family Rutaceae) in the field in East–central Florida to identify those that have low abundance of leafminers. Progeny from the 87 parent genotypes varied in abundance of the leafminer. Progeny of 15 parent genotypes had a high mean abundance of more than six leafminers per flush shoot. All but one of these genotypes were in the Citrus genus. Progeny of 16 parent genotypes had zero, or nearly zero, leafminers, but none were from the Citrus genus. However, many of these 16 genotypes were from genera closely related to true citrus (subtribe Citrinae) and are sexually compatible with Citrus. Progeny of two parent genotypes in the subfamily Toddalioideae and Glycosmis pentaphylla (Retz.) Corr. also had a low abundance of leafminer. Glycosmis pentaphylla also is a poor host for the Asian citrus psyllid, Diaphorina citri Kuwayama, and has biochemical resistance to the citrus weevil, Diaprepes abbreviatus (L.), so this genotype as well as others identified as poor hosts for the leafminer may prove useful in breeding programs aimed at reducing the abundance of multiple insect pests on citrus.

Colonization of Citrus and Citrus-related Germplasm by Diaphorina citri (Hemiptera: Psyllidae)

Huanglongbing (HLB) is a serious and devastating disease of citrus caused by Candidatus Liberibacter spp. and vectored by the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae). The disease has the potential to greatly limit the production of citrus in Florida and other citrus-growing regions worldwide. Current control of D. citri and HLB is inadequate, but the identification and incorporation of D. citri resistance traits from uncultivated Citrus spp. and Citrus relatives is seen as a potential disease management strategy. In this study, seedlings of 87 Rutaceae seed-source genotypes, primarily in the orange subfamily Aurantioideae, were assessed in the field for their propensity in a free-choice situation for infestations of natural south Florida populations of D. citri. The majority of test populations surveyed hosted all three life stages of D. citri; however, there were significant differences among the test populations in the mean ranks for D. citri eggs (F = 3.13, df = 86, P < 0.0001), nymphs (F = 9.01, df = 86, P < 0.0001), and adults (F = 4.21, df = 86, P < 0.0001). The only sampled test population that was completely avoided by all life stages of D. citri was seedlings of Casimiroa edulis Llave et Lex, commonly known as white sapote, which was one of the few plants included in the study belonging to the Rutaceae subfamily Toddalioideae. Although not completely avoided, very low levels of D. citri were found on two surveyed test populations of Poncirus trifoliata (L.) Raf, seedlings of ‘Simmon's trifoliate’ and ‘Little-Leaf’. Poncirus trifoliata, the trifoliate orange, readily forms hybrids with Citrus spp., is commonly incorporated into rootstock varieties, and has been used in breeding-advanced scion material. The identification of partial resistance in P. trifoliata to D. citri could prove useful in future citrus breeding efforts aimed at reducing the incidence and spread of HLB. Determining if there is true resistance to D. citri in this and other germplasm sources with a low propensity for infestations in free-choice situations will require no-choice experiments.

DNA content, morphometric and molecular marker analyses of Citrus limonimedica, C. limon and C. medica for the determination of their variability and genetic relationships within the genus Citrus

This work investigated the fingerprinting and phenotyping of Citrus germplasm; species selected were of historical importance belonging to Citrus limonimedica Lush. and its supposed ancestors, along with some other species of the Citrus genus. An integrated approach based on the exploitation of nuclear DNA content, morphological traits and molecular markers, such as RAPD fingerprints and ITS-based SNPs, was employed. We studied a core collection of 54 distinct accessions, including 43 genotypes of the Citrus species (18 species or supposed species) and 11 genotypes of the Poncirus genus, which was used as the reference outgroup. Morphological trait analysis and statistical analysis of DNA content and markers were useful for reconstructing a Citrus phylogeny. In particular, our experiments aimed at estimating the genetic variation within and the genetic relatedness among C limon (L.) Burm., C. limonimedica and C. medica L. to shed light on the hybrid origin hypothesis of C. limonimedica. The results of the multidisciplinary analyses allowed us to confirm a remarkable differentiation between Poncirus and Citrus genera and to highlight a close relationship among the three investigated Citrus species but a distinct difference between these three species and other species in the Citrus genus. RAPD fingerprints and ITS polymorphisms enabled us to point out a variation gradient between lemon and citron, with C. limonimedica as a possible intermediate species. Some accessions of C. medica and C. limonimedica that deviate from such a trend suggest recurrent introgression and/or hybridisation with other species of Citrus.

Somatic hybridization for citrus rootstock breeding: an effective tool to solve some important issues of the Mediterranean citrus industry

The prevalence of sour orange rootstock in the southern and eastern part of the Mediterranean Basin is presently threatened by the spread of Citrus Tristeza Virus (CTV) and its main vector Toxoptera citricida, combined with abiotic constraints such as drought, salinity and alkalinity. The search for alternative CTV-resistant rootstocks that also withstand the other constraints is now considered an urgent priority for a sustainable citrus industry in the area. Complementary progenitors can be found in citrus germplasm to combine the desired traits, particularly between Poncirus and Citrus genera. The production of somatic hybrids allows cumulating all dominant traits irrespective of their heterozygosity level, and would appear to be an effective way to solve the rootstock challenge facing the Mediterranean citrus industry. This paper presents the results obtained during a regional collaborative effort between five countries, to develop new rootstocks by somatic hybridization. New embryogenic callus lines to be used for somatic hybridization have been created. Protoplast fusions have been performed at CIRAD and IVIA laboratories, focusing on intergeneric combinations. Analysis of ploidy level by flow cytometry and molecular markers confirmed the acquisition of new interesting tetraploid somatic hybrids for six combinations. Diploid cybrids with intergeneric (Citrus×Poncirus) nucleus and C. reticulata or C. aurantifolia mitochondria were also identified for four combinations. The agronomical performance of a pre-existing somatic hybrid between Poncirus trifoliata and Citrus reticulata was validated in calcareous soils in Morocco. Somatic hybridization is now integrated into the breeding programs of the five Mediterranean countries.

Isozyme and DNA Analyses of Local Citrus Germplasm on Amami Islands, Japan

Genetic relationships among local Citrus accessions on the Amami islands, Japan, were evaluated using isozyme, random amplified polymorphic DNA (RAPD) and cleaved amplified polymorphic sequence (CAPS) of chloroplast DNA (cpDNA) analyses. Four loci were detected for three enzymes examined in isozyme analysis. Four and three kinds of genotypes were detected in glutamate oxaloacetate transminase (GOT)-1 and GOT-2, respectively. At least six genotypes were observed in peroxidase (PX). All accessions showed the same genotype except for one accession of superoxide dismutase (SOD)-3. In general, accessions that belong to the same species or type showed identical genotypes; however, some diversity of genotypes was observed within the same species and type. On the basis of the RAPD data, genetic relationships were assessed using NJ cluster analysis. From this result, accessions could be classified into three major clusters, A, B, and C. Cluster A included C. nobilis (Kunenbo), C. keraji (‘Keraji-Kikaijima’ and Kabuchii), C. oto (Oto), ‘Keraji-Kakeromajima’, and ‘Oto-Okinoerabujima’. Cluster B included only control accessions. Cluster C could be divided into four subclusters as follows: subcluster C. depressa (Shiikuwasha), some Sour orange relatives such as ‘Fusuu’ and ‘Kusa’, ‘Kikaimikan-Okinoerabujima’, and ‘Shimamikan’. Accessions were classified into three types in cpDNA analysis. Type I included C. nobilis (Kunenbo), C. keraji (‘Keraji-Kikaijima’ and Kabuchii), C. oto (Oto), ‘Keraji-Kakeromajima’, ‘Oto-Okinoerabujima’, and C. rokugatsu. Type II was composed of C. depressa (Shiikuwasha), ‘Kikaimikan-Okinoerabujima’, ‘Kusa’, and ‘Shiikuu’. Type III consisted of only ‘Shimamikan’. All mandarin accessions that were determined to belong to cluster A in RAPD analysis were included in type I of cpDNA analysis. Meanwhile, mandarins in types II and III in cpDNA analysis consisted of accessions in cluster C of RAPD analysis.

Characterisation of citrus rootstock germplasm introduced as seeds to Australia from the People's Republic of China

Seeds were collected from a range of citrus rootstock types in China and sent to Australia for evaluation. Before evaluation proceeded, however, the seeds and their seedlings were characterised for uniformity and trueness-to-type. The seeds were cultured in vitro and/or germinated in sand beds to assess whether or not they were mono- or polyembryonic. Seedlings were then rogued on the basis of vigour and morphology before isozyme profiles were developed from leaf extracts to assess seedling uniformity within introductions. Isozyme profiles were also compared to those of the source trees, as well as hybrids generated between some of them in China, to assess and verify trueness-to-type. The degree of variability between the seedlings propagated from the imported seeds and their source trees in China differed between accessions, which highlighted some of the issues that need to be considered when importing citrus germplasm as seeds. Regardless of whether the seedlings could be shown to be identical to their source tree in China, three uniform seedlings of polyembryonic types were established in arboreta as Australian source trees to provide propagules for evaluation and possibly commercial use.

Utility of RAPD, ISSR, IRAP and REMAP markers for the genetic analysis of Citrus spp.

The application and informativeness of RAPD, ISSR, IRAP and REMAP markers to study the genetic diversity and relationship among the Citrus and its relative genotypes were investigated. High levels of polymorphism were observed for all four marker systems. The RAPD technique generated the highest number of polymorphic bands and average number of polymorphic band per assay unit. Average limit of the discriminating power was very close to its actual discriminating power of each marker. The highest and the lowest values of effective number of patterns were obtained from the marker REMAP (5.94) and RAPD (4.48), respectively. Correlation between the genetic similarities matrices were estimated from all four markers using the Mantel matrix correspondence test, and results showed significant correlations among the RAPD, ISSR, IRAP and REMAP similarity matrices. The highest correlations were found comparing RAPD and ISSR markers, whereas RAPD and REMAP ( r = 0.143) markers were poorly correlated. To assess the usefulness of the overall information provided by these marker data for establishing phylogenetic relationships and Citrus germplasm classification, cluster analysis was performed. All four techniques, solely or in combination, discriminated the genotypes very efficiently and generated a high similarity in dendrogram topologies, although some differences were observed. The linkage analysis was conducted based on the segregation of 38 RAPD, 13 ISSR, 19 IRAP and 9 REMAP loci in 96 progeny of an intergeneric cross Citrus sinensis and Poncirus trifoliata. Among the 81 studied loci 65 loci distributed on five linkage groups. Comparing the result obtained with RAPD, ISSR, IRAP and REMAP markers in this study, IRAP and REMAP proved to be as a reliable molecular marker for fingerprinting, mapping and diversity study of Citrus and its relatives.

‘Garbí’ Mandarin: A New Late-maturing Triploid Hybrid

Citrus is the most extensively produced fruit tree crop in the world (FAO, Food and Agriculture Organization, 2009). There are two clearly differentiated markets: fresh fruit and processed juice. In 2007, the main citrus fruit-producing countries were China (17.9%), Brazil (17.8%), the Mediterranean countries (17.1%), and the United States (8.5%) (FAO, Food and Agriculture Organization, 2009). These areas account for more of two-thirds of the total production of citrus fruits. In the Mediterranean area, citrus fruits are primarily produced for the fresh fruit market. Spain is the principal producer in the area with a total planted area of 330,000 ha and a production 6.3 million tons. Seedlessness is one of the most important characteristics for mandarin on the fresh fruit market, because consumers do not accept seeded fruits. Parthenocarpy is an essential trait for seedless fruit production, and this characteristic is present in citrus germplasm. Triploidy gives rise to seedless commercial cultivars. However, triploid plants have very low fertility, are generally sterile, and do not induce seeds in other cultivars by crosspollination (Frost and Soost, 1968). Several methods have been developed to obtain triploid citrus (Navarro et al., 2002; Ollitrault et al., 2008). One exploits natural events of polyploidization such as 2n gametes using embryo rescue and flow cytometry to select triploids in 2x · 2x crosses. Second meiotic division restitution has been proposed for diploid megagametophyte development in clementine (Luro et al., 2004), whereas Chen et al. (2008) proposed first meiotic division restitution in sweet oranges. In Spain, the structure of mandarin cultivars poses several problems. Mandarins include satsumas, clementines, and mandarin hybrids. Satsumas are harvested from the beginning of September until the beginning of November. They are cultivars with a high degree of parthenocarpy and have sterile pollen and ovules. Clementines are the most representative cultivars of mandarin in Spain. They are picked from mid-September until mid-February. They are self-incompatible, but pollen and ovules are viable, being able to pollinate and to be pollinated by other cultivars. Mandarin hybrids, like ‘Nova’ [C. clementina · (C. paradisi · C. tangerina)], ‘Fortune’ (C. clementina · C. tangerina), and ‘Ortanique’ tangor (natural hybrid between mandarin and C. sinensis), were introduced in the Spanish citriculture to cover the demand of late-season mandarins by international markets. They enable the harvesting period to be extended until May. These hybrids are also self-incompatible, but pollen and ovules are viable and cross-pollinate with clementines, producing fruits with seeds in both groups of cultivars. A triploid breeding program has been carried out in Spain since 1996 based on sexual hybridization, embryo rescue, and ploidy analysis by flow cytometry (Navarro et al., 2002). The objective is to produce new high–quality, late-season and seedless triploid mandarin hybrids. In this article, we describe ‘Garbi’ mandarin, a new triploid hybrid developed within this program characterized by its high-quality, late-season ripening and seedless fruits.

Comparative genome-wide segregation analysis and map construction using a reciprocal cross design to facilitate citrus germplasm utilization

Citrus genetic resources are rich but underutilized in breeding because their complex reproductive biology and the scarceness of inheritance studies on agronomic traits. Here, we investigated the genomic distribution of segregation distortion regions, the inheritance of organelle DNA and colinearity between scion citrus linkage maps by using a reciprocal cross design. The parents were Fortune, a hybrid mandarin from C. clementina, and Chandler, a hybrid pummelo from C. grandis that largely differ in fruit size, taste and colour. The inheritance of organelle DNA was studied in 201 hybrids by using four organelle DNA markers, and the linkage maps were based on 174 of those hybrids. Around ten percent of the seedlings derived from the pummelo as female parent showed the same organelle markers as those of the mandarin, indicating a possible exception to their expected maternal inheritance in citrus. Most segregation distortion affects just the allele frequencies, generally representing differences in pollen fertilization success, as a likely consequence of the presence of gametal factors affecting the functionality of gametes and pollen-pistil interactions. The large extension of colinearity found when comparing the C. grandis and C. clementina linkage maps to those previously reported for rootstock species (C. aurantium and P. trifoliata), will be helpful to infer the position of orthologous genes and QTLs in citrus species and for a more useful genetic characterization of citrus germplasm collections.

Production of tetraploid plants of non apomictic citrus genotypes

Ploidy manipulation in Citrus is a major issue of current breeding programs aiming to develop triploid seedless mandarins to address consumer demands for seedless fruits. The most effective method to obtain triploid hybrids is to pollinate tetraploid non apomictic cultivars with pollen of diploid varieties. Such non apomictic tetraploid lines are not found in the citrus germplasm and need to be created. In this work we describe a new methodology based on in vitro shoot-tip grafting combined with treatment of the micro-grafted shoot-tip with colchicine and oryzalin to achieve chromosome doubling and a dechimerization procedure assisted by flow cytometry. Stable tetraploid plants of Clemenules, Fina and Marisol clementines and Moncada mandarin have been obtained directly from shoot tip grafting combined with colchicine and oryzalin treatments or after dechimerization of mixoploids plants (2x-4x). These stable tetraploid plants have been used in 4x x 2x hybridizations, to recover over 3,250 triploid hybrids in 3 years.

ASSESSMENT OF GENETIC VARIATION IN CITRUS GERMPLASM OF SOUTH OF IRAN USING SSR MARKERS

In this study, eight SSR markers were used to assess the genetic variation among 48 citrus genotypes (12 orange, 6 grapefruit, 20 mandarin and 10 lemon) from the Citrus repository of the Agricultural Research Institute of South of Iran. From each accession, a full-grown tree was selected and genomic DNA was extracted from young leaves using a CTAB procedure. DNA amplification was performed and SSR banding patterns were evaluated in silver-stained polyacrylamide gels. The allele bands were transformed into binary data of presence-absence and resulting matrices were processed by NTSYS software. In total, 32 alleles were detected using the eight SSR primer pairs. The number of putative alleles per primer pair ranged from three to seven with an average of four. Polymorphic Information Content (PIC) values varied from 0.31 to 0.82. Based on the marker data, cluster analysis separated the Citrus collections into two groups. This study detected genetic similarities and differences between citrus germplasm of south of Iran.

Produção e qualidade de frutos de cultivares de laranja-doce no norte do Paraná

O objetivo do trabalho foi a caracterização de acessos de laranja-doce [Citrus sinensis (L.) Osbeck] do Banco Ativo de Germoplasma de Citros (BAG Citros) do IAPAR, em Londrina-PR. Foram estudados os acessos I-02 'Piralima'; I-03 'Barão'; I-11 'Natal', I-16 'Hamlin', I-17 'Seleta-Vermelha', I-60 'Natal', enxertados sobre limão- 'Cravo' (Citrus limonia Osbeck). As plantas foram conduzidas em espaçamento de 7,0 m x 7,0 m e sem irrigação. Os dados de produção (de 1983 a 1997) e as características físico-químicas dos frutos (de 1984 a 2000), como massa (MF), sólidos solúveis totais (SST), acidez titulável total (ATT), ratio (SST/ATT), rendimento em suco (Suco) e índice tecnológico (IT) foram submetidos à análise de variância, complementada pelo teste de Tukey, a 5% de probabilidade. O acesso I-16 'Hamlin' foi o mais produtivo (218,1 kg por planta) e diferiu-se dos demais. Para a massa do fruto, destacaram-se os acessos I-17 'Seleta-Vermelha' (208,8 g) e I-11 'Natal' (142,0 g), sem diferença entre si. Com relação às características químicas dos frutos, os acessos apresentaram resultados semelhantes, dentro dos padrões considerados adequados para as cultivares, exceto o acesso I-17, 'Seleta- Vermelha', que teve índice tecnológico menor que 2,0 kg.caixa-1 de SST.

Resposta diferencial de espécies e de híbridos de citros à leprose

O objetivo deste trabalho foi buscar novas fontes de resistência à leprose-dos-citros, no Banco Ativo de Germoplasma do Centro APTA Citros Sylvio Moreira, Instituto Agronômico, em Cordeirópolis, SP. Foram utilizadas plantas obtidas por sementes de 26 acessos, infectadas com o vírus da leprose-dos-citros (Citrus leprosis virus - CiLV), por meio do seu vetor Brevipalpus phoenicis. O aparecimento de lesões, a partir de 21 dias após a inoculação, foi observado em 11 dos genótipos testados (42,3%). Quinze espécies, entre elas Citrus pennivesiculata e C. celebica, comportaram-se como altamente resistentes, enquanto outras, como C. keraji, foram mais suscetíveis que o padrão C. sinensis. Os dados mostraram grande variação de respostas de Citrus spp. à leprose, com elevado número de espécies resisentes, que podem ser utilizadas como fonte de resistência à doença em programas de melhoramento.

Resistance to Citrus Canker Caused by Xanthomonas axonopodis pv. citri in an Accession of Citrus sp. Similar to Grapefruit.

Citrus canker is caused by Xanthomonas axonopodis pv. citri. All grapefruit (Citrus paradisi Macf.) cultivars are highly susceptible to the A-group strains of Xanthomonas axonopodis pv. citri. A type of grapefruit (Citrus sp.) called Dalan Dan (DD) has shown high resistance in the field in Misiones, Argentina where it is widely planted. The DD is Accession No. NC678 in the Campinas Citrus Germplasm Collection (Centro APTA Citrus Sylvio Moreira, Cordeiropolis, SP, Brazil) and Accession No. CCC-618 Dalan Dan in the Argentina Citrus Germplasm Collection at INTA (Concordia, Entre Ríos). Molecular studies completed in Brazil have shown high similarities among grapefruit varieties and DD (1). We report here the characterization of quantitative resistance to citrus canker in DD. Quantitative resistance in citrus is expressed as lower number of lesions per leaf area (3). Lesions per square centimeter were calculated after 3 to 4 weeks of two sets of inoculation of three inoculum levels (5 × 102, 2.5 × 103, and 5 × 103 cells per ml) in leaves of developing leaf flushes. The period of greatest susceptibility in DD was 15 to 23 days for the three inoculum levels; 1.35 ± 0.34 to 0.01 ± 0.01, 4.98 ± 0.91 to 0.91 ± 0.48, and 9.85 ± 2.17 to 1.91 ± 1.37 lesions per cm2, respectively. By contrast, the period of greatest susceptibility in grapefruit cv. Duncan was 15 to 29 days for the same three inoculum levels; 2.78 ± 0.46 to 1.22 ± 0.39, 17.29 ± 2.79 to 5.55 ± 0.93, and 22.87 ± 3.27 to 11.95 ± 3.55 lesions per cm2, respectively. The results shown here suggest that the DD citrus accession is susceptible to citrus canker for a very short time during the development of leaf flushes, which results in very high resistance in the field. This citrus germplasm material could be valuable as a source of resistance to improve other varieties or as a replacement of grapefruit due to the similar fruit and juice qualities.

Characterization of citrus germplasm including unknown variants by inter-simple sequence repeat (ISSR) markers

Abstract Inter-simple sequence repeat (ISSR) markers were used to study phylogenetic relationships among 33 citrus genotypes including several undefined local or native varieties as well as some known varieties in the Fars Province of Iran. Based on 234 total ISSR fragments and 209 polymorphic fragments, a genotype similarity dendrogram was constructed using UPGMA cluster analysis. The 33 genotypes were classified into six major groups with, within-group similarity values of ≥0.65. Group 1 included Citrus sinensis and Citrus reticulata, group 2 common sour orange and ‘Peach’ sour orange, Group 3 ‘Bakraee’ and Volkameriana and three selections of sweet lime. ‘Bakraee’ is an undefined variety with morphological similarities to both sweet lime and mandarin. It is probably a hybrid between these two varieties. Group 4 included Lisbon lemon (Citrus limon) and two undefined selections ‘Rock lemon’ and ‘Pear-shaped lemon’. They might be mutant or hybrid lemons. Group 5 included a single genotype (D3) with little molecular similarity to other genotypes. Group 6 could be divided into two subgroups. One including citron (Citrus medica) and three local varieties of ‘Otroj’, ‘Bidkhoni’ from Darab and ‘Bidkhoni’ from Fassa. Based on their morphological and molecular similarities with citron, they are regarded as citron variants. The other subgroups of group 6 included Citrus aurantifolia, Citrus latifolia and 11 undefined genotypes with close morphological and molecular similarities to lime. The latter were considered as lime variants.

Distribution, biology, ecology and control of the psyllid Diaphorina citri Kuwayama, a major pest of citrus: A status report for China

The Asiatic citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Psyllidae) is a major pest of citrus in China. Its status derives, not from the damage it causes, but from its role as the only known vector in China of huanglongbing, a phloem-limited bacterial disease of international importance. The disease can devastate orchards within a few years of planting. It also poses a major threat to endangered indigenous citrus germplasm in Asia and Australasia. The distribution, biology, ecology and control of the psyllid in China are reviewed in these contexts. Constraints and challenges related to control of the vector in China are discussed.

Assessing genetic diversity and population structure in a citrus germplasm collection utilizing simple sequence repeat markers (SSRs)

Twenty-four simple sequence repeat (SSR) markers were used to detect molecular polymorphisms among 370 mostly sexually derived Citrus accessions from the collection of citrus germplasm maintained at the University of California, Riverside. A total of 275 alleles were detected with an average of 11.5 alleles per locus and an average polymorphism information content of 0.625. Genetic diversity statistics were calculated for each individual SSR marker, the entire population, and for specified Citrus groups. Phylogenetic relationships among all citrus accessions and putative non-hybrid Citrus accessions were determined by constructing neighbor-joining trees. There was strong support for monophyly at the species level when hybrid taxa were removed from the data set. Both of these trees indicate that Fortunella clusters within the genus Citrus but Poncirus is a sister genus to Citrus. Additionally, Citrus accessions were probabilistically assigned to populations or multiple populations if their genotype indicated an admixture by a model-based clustering approach. This approach identified five populations in this data set. These separate analyses (distance and model based) both support the hypothesis that there are only a few naturally occurring species of Citrus and most other types of Citrus arose through various hybridization events between these naturally occurring forms.

Genetic resources of Citrus of north-eastern India and their potential use

The north-eastern India is highly diverse in Citrus germplasm (23 taxa including their 68 varieties). The climate and soil factors are highly favourable for plant growth and fruit quality. Inspite of the high potentialities, the Citrus had remained more or less a family need based cultivated plant. The large variation in traditional agro-ecosystems of the region has evolved and preserved highly valuable genetic resources under different species. The value of certain taxa as rootstocks has been outlined. The conservation of germplasm and research gaps has been emphasised.

Regeneration and characterization of somatic hybrid plants of Citrus sinensis (sweet orange) and Citrus micrantha, a progenitor species of lime

Most commercial citrus fruit species and cultivars are inter- and intra-specific hybrids. Conventional hybridization in citrus is largely handicapped by apomixis and long juvenility. As an alternative, somatic hybridization via protoplast fusion has been employed to create novel citrus germplasm. Witches' broom disease of lime (WBDL) emerged in Oman during the 1970s, which has been spreading to the neighboring countries. The disease is extremely destructive. A possible solution to the WBDL problem is to develop resistant hybrids. Resistance is available among the citrus relatives of lime i.e., sweet orange. Somatic hybrids combining sweet orange with lime have been produced but the fruit characteristics are different from lime. Herein, we report the development of somatic hybrids between a progenitor of lime (Citrus micrantha) and sweet orange (Citrus sinensis) in efforts to recreate a lime-like fruit using sweet orange as a donor of disease resistance gene(s). Successful somatic hybridization was verified by ploidy analyses using flow cytometry and RAPD analyses. This is the first report of using a progenitor species in somatic hybridization experiments in efforts to resynthesize an improved lime.