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Taxonomy






The majority of taxonomists consider that citrus species belong to the Geraniales order, the Rutaceae family and the subfam- ily Aurantioideae. Rutaceae is one of the 12 families in the Geraniineae suborder, and the Aurantioideae subfamily – one of the seven belonging to the Rutaceae (Engler, 1931) – is rather numerous and comprises the commercial citrus species and also sev- eral important related genera.

Aurantioideae, the ‘Orange’ subfamily, has been subdivided by Swingle into two tribes: Clauseneae with fi ve genera and Citreae with 28 genera including Citrus and related genera, i.e. Fortunella, Poncirus, Eremocitrus, Microcitrus and Clymenia. The taxonomic situation of tribes, sub- tribes, genera and species within the Aurantioideae is controversial, complex and sometimes confusing. Citrus and many


 


 

related genera hybridize readily and have done so in the wild for centuries. In Fig. 3.6, the tribes, subtribes, genera and species are listed after Swingle (1967).

Clauseneae comprise the more primi- tive genera: the fruit usually consists of small, semi-dry or juicy berries, except in Merrillia in which the fruit is of ovoid shape with a thick, leathery exocarp.

The tribe Citreae comprises three sub- tribes: Triphasiinae, Balsamocitrinae and Citrinae; the latter, with 13 genera, has been classifi ed into three groups (Swingle, 1967). Group A ‘the primitive citrus fruit trees’ with fi ve genera, Severinia, Pleiospermium, Burkillanthus, Limnocitrus and Hesper-


 

ethusa, group B ‘near citrus fruit trees’ with only two genera, Citropsis and Atalantia, and group C ‘true citrus fruit trees’ in- cludes six genera, Fortunella, Eremo- citrus, Poncirus, Clymenia, Microcitrus and Citrus.

The fi rst descriptions and classifi ca- tions of citrus species and varieties date back to the 17th century, while the mor- phological description of the trees, the fl oral morphology and biology or the differ- ent uses of the fruit (i.e. citron fruit) date back to ancient times (Teophrastus 310 BC, Virgil 70–19 BC, Pliny 27–79 AD, etc.).

John Baptista Ferrarius, a Jesuit priest of Siena, made the fi rst organic contribution


 

 

 

Fig. 3.6. Aurantioideae subfamily (after Swingle, 1967).


 

 

 

Fig. 3.7. (a) Limon citratus primae notae laevior, Ferrarius (1646) p. 265. (b) Malum citreum vulgare, Ferrarius (1646) p. 61.

 


 

to citrus studies with his book Hesperides, sive de malorum aureorum cultura et usu (1646). The assistance of Cassiano dal Pozzo (1588–1657), who provided the orig- inal drawings and all the information to complete the book, was invaluable. The drawings, done in tempera, form part of Cassiano dal Pozzo’s so-called ‘paper museum’, which is preserved today in the Royal Library of Windsor and in some pri- vate collections.

Ferrarius’s work is divided into four books: in the fi rst one, a table is inserted with the reproduction of various Greek and Roman coins, while the other three books are dedicated to the Hesperides: Aegle, Aretusa and Hesperetusa (the nymphs who guarded the garden of golden apples at the western extremity of the earth). The citron, the lemon and the orange are well described in them.


 

In an attempt to contain all the citrus in only three taxonomic groups, Ferrarius also places the chinotto orange (Aurantium sinense) and the pummelo (Aurantium maximum) with oranges; he puts with oranges and lemons some phenotypes (e.g. Limon citratus and Aurantium citratum) whose names and fruit characteristics indi- cate that they could be interspecific hybrids.

The Ferrarius iconography was cer- tainly created for a taxonomic purpose, but it represents a happy meeting point between fi gurative culture and scientifi c lit- erature. The fruits are drawn to their real size and are often cut into two (Fig. 3.7a and b). Even though the drawings that illus- trate the work are not coloured, the fi neness of the features is such that they succeed in rendering the three-dimensionality of the fruits and their morphological characteris-


 


 

tics with great effectiveness and provide excellent visual documentation (Baldini, 1997).

A few years later, other authors described citrus fruits, although in less detail (Steerbeck, 1682; Hermann, 1687;

Tournefort, 1700).

The fi rst volume of the Nü rnbergische Hesperides by Johann Christoph Volkamer was published in 1708, followed 6 years later by another volume complementary to the fi rst one.

Even though this monumental work was written in German and printed in Nuremberg, it describes the citrus fruit of the north of Italy and particularly those cul- tivated on the shores of Lake Garda, on the coast of the Brenta and on the Ligurian coast, and can be therefore considered an Italian citrology. The scientifi c approach traces that of Ferrarius: the fi rst parts of both volumes, dedicated to the nymph Aegle, deal with citrons; the second one, dedicated to Aretusa, concerns lemons (Fig. 3.8), and the third one, dedicated to Hesperetusa, explains the orange and pummelo that are distinguished from one another and marked with a specifi c terminology. Even though the number of genotypes represented in Volkamer’s book is greater than those described by Ferrarius, it remains uncertain if each one of them corresponds to a different geno- type.

A complete change of the classifi cation system of citrus is given by Linneus (1737) who, in his work Genera plantarum, cre- ated the genus Citrus, attributing three main species to it: Citrus medica (citrons and lemons), Citrus aurantium (sweet and sour oranges and the pummelo) and Citrus trifoliata.

In 1767, Linneus added in Systema naturae the species Citrus decumana, thus separating the pummelo from the complex species Citrus aurantium.

Linneus collaborated with Osbeck, and together they formulated the binomial names of three species: Citrus grandis, Citrus limonia and Citrus sinensis.

Proceeding with this short review on


 

Fig. 3.8. A lemon, Limon ponzino Regino, as represented by Volkamer (1708) in Nü rnbergische Hesperides.

the classifi cation of the species, we get to 1768, when Burmann raised the lemon to the rank of the species, giving it the name of Citrus limon.

In 1790, while he was studying the citrus of Indochina and Canton, De Loureiro discovered some species such as Citrus nobilis (King mandarin), Citrus madurensis (calamondin) and Citrus mar- garita (later Fortunella margarita Lour. (Swing.)).

In 1813, De Candolle studied and clas- sified Citrus hystrix that was the first species of the Papeda subgenus according to the classifi cation later done by Swingle.

In 1837, Blanco named a new species, Citrus reticulata, a type of mandarin of the Philippines.

In the fi rst years of the 19th century, a


 


lot of authors published papers on the citrus. The fi rst one must have been Giorgio Gallesio who, in 1811, published the Traité du Citrus in Paris. It is a modest work from a publishing aspect, but it stands out for its innovative contribution to citrus taxonomy. The work constitutes the first organic attempt at the systematic botany of the Citrus genus, of its species and of its culti- vated varieties. Citrons, lemons, sour oranges, sweet oranges and their hybrids are well described in the volume.

In the sole illustration of the work, a synoptic table of Citrus, citrus are distrib- uted on the ramifi cations of four leading branches of a symbolic genealogical tree, distributed in four species: Citrus medica cedra (citron), Citrus medica limon (lemon), Citrus aurantium indicum (sour orange) and Citrus aurantium sinense (sweet orange) (Fig. 3.9).

Gallesio’s work should have been accompanied by a citrografi c atlas contain- ing colour tables of the main varieties of citrus. However, the atlas was not actually published and the drawings mentioned in the preface were unknown for almost two centuries; they were found only recently among the papers and the books that Gallesio had left to his Genoan heirs (Baldini, 1996, 1997).

Risso’s work followed in 1813. New classifi cations were elaborated on the dia- gram proposed by Linneus. In particular, Risso distinguished sour oranges with the binomial terminology of Citrus vulgaris and later of Citrus bigaradia (this synonym was used in Mediterranean countries for a long time); Risso changed the Citrus limon of Burmann to Citrus limonium, and included all the sweet orange varieties under the name Citrus aurantium. Risso also created a new species, Citrus limetta, distinguishing it from citrons and lemons. Subsequently he added some new species: Citrus aurata, Citrus peretta, Citrus mellarosa and Citrus rissoi; all these species refer to intermediate types of citrus between limes and pummelo. Finally, the work Risso published in collaboration with Poiteau in 1818 is bound to be of considerable importance: Histoire


naturelle des orangers in which the berg- amot and the ‘lumia’ that were called Citrus bergamia and Citrus lumia were separated, as different species.

The following taxonomists were inter- ested in the systematics of the Aurantioideae at the same time as the scholars already cited and afterwards: Blume (1823), Blanco (1837), Macfadyen

(1837), Tenore (1840), Fortune (1848),

Oliver (1861) and Pasquale (1867).

In 1875, the work of Hooker, Flora of British India, was published in which the citrus and related genera found a botanical place in 13 genera and 43 species (Hooker attributed only four species to the Citrus genus).

In 1888, Bonavia published a volumi- nous treaty on the oranges, lemons and other citrus of India and Ceylon and, subse- quently, in 1890, he added the publication of an atlas. Bonavia’s theories on the mor- phology and evolution of citrus were extremely clever and original, but they diverged widely from the standard of other taxonomists (Swingle, 1967).

In 1896, Adolph Engler published a classifi cation of the Aurantioideae that was subsequently examined and fi xed in 1931. In this second edition of the Die natü r- lichen Pfl anzenfamilien, Engler, in an effort to describe the Rutaceae family, divided the subfamily of the Aurantioideae into 29 genera including 180 species, 11 of which belong to Citrus.

Around the same time as Engler’s work, other studies were carried out on the Aurantioideae by several taxonomists, such as Bailey (1895, 1903) or Guillaumin (1911). The latter, for example, worked on the citrus of Indochina and he re-examined the previous citrus taxonomic classifi cation and divided the Aurantioideae into 11 genera and 44 species, six of which belong to Citrus.

The Swingle and Tanaka systems

Two different classification systems are commonly accepted for the citrus taxon-


 

Fig. 3.9. The synoptic table of Citrus of Gallesio.

 


omy: the system of Swingle (1943, 1967) and that of Tanaka (1954, 1961).

Despite their collaboration, between 1915 and 1930, the differences in their ideas are obvious; their classifi cations rep-


resent two different taxonomic conceptions and their contrast is clearly emphasized just by observing the subdivision within the genus Citrus.

Between 1912 and 1926, W. T. Swingle,


 


of the United States Department of Agriculture, proposed a new system for the classifi cation of the Aurantioideae, with a series of publications which were defi ni- tively coordinated in ‘The botany of Citrus and its wild relatives’, which appeared in the fi rst volume of the The Citrus Industry edited by Reuther et al. (1967). Swingle accepted the Engler classification and divided the genus Citrus into two subgenera: Citrus (in 1943, 1st edition of the The Citrus Industry, Eucitrus) and Papeda, which included, respectively, ten and six species.

The two subgenera were separated according to their morphological character- istics and to the chemical components of fl owers, leaves and fruits. To be more pre- cise, the species of the fi rst subgenus have fl owers with grouped and perfumed sta- mens, and a small petioles no larger than three-quarters of the whole leaf; the fruit has edible fl esh with little or no bitter oil. On the contrary, the Papeda have small fl owers with free separate stamens, large winged petioles, that can be very big, reach- ing even larger dimensions than those of the same lamina; the fruit is inedible due to the high content of bitter oil in its fl esh.

In 1926, almost at the same time as Swingle’s studies, Marcovitch published his classifi cation which divided the Citrus into three groups, Aurantium, Intermedium and Medica, and 20 species.

From 1915 to 1961, Tyozaburo Tanaka supplied a considerable contribution to the classifi cation of the Aurantioideae and pub- lished a series of information-packed papers on the taxonomy entitled ‘ Revisio aurantiacearum ’.

In 1954, in Species problems in Citrus, he published a classifi cation of the genus Citrus in which two subgenera, Archicitrus and Metacitrus, eight sections, 13 subsec- tions, eight groups, two subgroups, two microgroups and 145 species were distin- guished. Seven years later, in 1961, he added two new subsections, another group and 12 new species to his system, taking the total to 157 species.

He hypothesized that the citrus origi- nated in Asia about 30 million years ago


from C. hystrix, C. latipes, C. macroptera

and C. combara.

Tanaka’s classifi cation is more com- plex compared with that of Swingle because of the far greater number of species included in each subgenus; on the contrary, Swingle’s classifi cation appears easier to understand although it does not provide an exhaustive description of citrus systemat- ics. Even so, the Swingle system is the most used, but there are some cases in which it is amplified with the inclusion of some species belonging to Tanaka’s system.

There is a big difference between the two systems regarding the taxonomic clas- sifi cation of mandarins. This is particulary relevant because it concerns a group that includes many genotypes that are widely cultivated and of great economic impor- tance. So, Swingle includes in the species Citrus reticulata all the mandarins except

C. tachibana, a wild species from Japan, and C. indica, a wild species from India, whereas Tanaka separates mandarins into

36 species. Swingle affi rms that in some cases the sole distinguishing characteristic is the size of the leaves or of the fruits, and affirms that such differences are easily infl uenced by different factors: the root- stock, the humidity and the soil fertility.

Tanaka gave special consideration to the cultivated species (not wild) and, in 1928 at the International Congress of Horticulture of Vienna, he proposed the adoption of the term Hortulanorum next to the name of the taxonomist (e.g. Citrus clementina Hort., ex Tanaka).

In order to heal the rift between the American school of Swingle and the Japanese school of Tanaka, Hodgson pro- posed a new classifi cation in 1961. He increased the number of the species from 16 to 36, dividing them into four groups: ‘acid fruits’, ‘orange group’, ‘mandarins group’ and ‘other’.

 

 

Modern taxonomic systems

All the citrus taxonomic classifications elaborated in the past were based only on


 


 

the morphological and anatomical differ- ences and on the geographical area of origin.

Subsequently, from the beginning of the last century, chemical characters have also been used for this purpose. Swingle (1943) fi rst mentioned the possibility of using glycosides as a taxonomic marker, in addition to classical morphological charac- ters. After Swingle, several researchers employed various techniques in order to analyse the biochemical composition of dif- ferent parts of the citrus plant i.e. leaves, flowers and fruits; in particular, these include long chain hydrocarbon profi les (Nagy and Nordby, 1972), flavonoids (Tatum et al., 1974), leaf and rind oils (Malik et al., 1974), polyphenol oxidase- catalysed browning of young shoots (Esen and Soost, 1978) and seed teguments (Gorgocena and Ortiz, 1988), root peroxi- dase isoenzymes (Button et al., 1976), leaf isozymes (Soost and Torres, 1982; Hirai et al., 1986), and fraction I protein in leaves (Handa et al., 1986).


 

Taxonomic studies received a boost from the work of Barrett and Rhodes in 1976: ‘A numerical taxonomic study of affi nity relationships in cultivated Citrus and its close relatives’. Barrett and Rhodes performed a comprehensive phylogenetic study that evaluated 146 morphological and biochemical tree, leaf, fl ower and fruit characteristics. This study suggested that only three citrus types, namely the citron, Citrus medica, the mandarin, Citrus reticu- lata, and the pummelo, Citrus grandis (now called C. maxima Burm. Merrill), consti- tuted true or valid species. Similar results have been communicated by Scora in 1975 and, in 1988, he added another true species, the Citrus halimii, a species previously described by Stone et al. in 1973 as a new species from Malaya and peninsular Thailand.

Barrett and Rhodes postulated relation- ships among citrus, indicating a probable origin of cultivated citrus and some Citrus and related genera. In Fig. 3.10, some of the affi nity relationships according to Swingle


 

Fig. 3.10. Affi nity relationships between some Citrus species and relatives after Swingle (1943) and Barrett

and Rhodes (1976).


 


(1943) and Barrett and Rhodes (1976) are listed.

Moreover, modern techniques have been instrumental in deciphering the taxo- nomic situation in Citrus; the development of various biochemical and molecular markers has provided some answers regard- ing the relationships of various citrus types. In particular, DNA markers, with their phe- notypic neutrality, abundance and impervi- ousness to environmental conditions, have been most useful.

The concept of the true valid species and the other genotypes derived from hybridization between them has gained fur- ther support from various studies using bio- chemical and molecular markers, including isozymes (Torres et al., 1978; Fang et al., 1993; Herrero et al., 1996), organelle genome analysis (Green et al., 1986; Yamamoto et al., 1993; Luro et al., 1995; Nicolosi et al., 2000), microsatellites (Fang and Roose, 1997; Fang et al., 1998), RFLP (restriction fragment length polymor- phism), RAPD (random amplifi ed polymor- phic DNA) and SCAR (sequence characterized amplified region) analyses (Luro et al., 1992; Federici et al., 1998; Nicolosi et al., 2000).

Even though different authors have studied the phylogenetic relationships in Citrus, the work carried out by the group of

M. L. Roose, University of California appears very interesting because of the large number of genotypes investigated and the results obtained (Federici et al., 1998). They analysed the relationships among 88 accessions of Citrus and related genera using RFLP and RAPD analysis. The main results obtained suggest that C. maxima has some affiliation with some papedas; C. medica clustered with C. indica; Fortunella is not separate from the genus Citrus; and mandarin species do not cluster in the groups Tanaka used. Another important result is that of C. ichangensis (a Papeda species in the Swingle classifi cation, but in the Metacitrus subgenera of Tanaka) that is assumed to be a distinct species, very dif- ferent from most other Citrus species, loosely aligned with C. hystrix and C.


micrantha, but not easily placed in relation to other species. Moreover, the researchers arrived at the conclusion that it is not appropriate to place C. ichangensis into the subgenus Metacitrus of Tanaka with all the mandarins.

Many authors have investigated the relationships in the mandarin group; among these, the studies carried out by Li et al. (1987, 1988, 1992) on the basis of isozyme data are particularly interesting. The authors suggest that it would be better to divide man- darins into ‘three primary species’, the wild species C. mangshanensis, C. daoxianensis and C. chuana. The mangshan wild man- darin, C. mangshanensis, should be a transi- tional wild species of mandarin from one type of ichang papeda, C. ichangensis.

As also appears from the results of the literature cited above, the taxonomy of mandarins is very complex and is in con- tinual development, so that new species are still emerging among the wild species placed in the primary centre of origin.

To clarify phylogenetic relationships among Citrus and its relatives, Nicolosi et al. (2000) employed different molecular markers such as RAPDs, SCARs and chloro- plast DNA analysis. The analysis based on the RAPD and SCAR data of the 40 citrus genotypes, including four related genera, allowed the elaboration of different dendro- grams which all indicated that the genus Citrus is quite distant from the related genera Poncirus, Microcitrus and Eremocitrus, but not from Fortunella (Figs

3.11 and 3.12 were obtained, respectively, by PAUP and polymorphism parsimony). Moreover, a clear separation between the two subgenera indicated by Swingle, Citrus and Papeda, was observed, with the excep- tion of C. indica and C. celebica.

The three valid species belonging to Citrus, citron, mandarin and pummelo were separated into three distinct clusters, and each one also comprises some other hybrid genotypes.

In Fig. 3.11, the fi rst cluster, the Citron cluster, comprises, of course, C. medica and also C. aurantifolia, C. macrophylla, C. limon, C. bergamia, C. limettioides, C.


Fig. 3.11. A 50% majority rule consensus tree (PAUP tree) for 40 genotypes of Citrus and related genera derived from bootstrap analysis (500 replications) of RAPD and SCAR data with confi dence levels for arms (Nicolosi et al., 2000).

 


jambhiri, C. limonia and C. volkameriana. The second, called the Mandarin cluster, consists of all the mandarin and mandarin- like accessions as well as C. tachibana and

C. paradisi, C. aurantium, C. sinensis and

C. junos. The third is the Pummelo cluster with C. grandis and C. celebica, a papeda species. Moreover, other clusters were sep- arated as reported in Fig. 3.11. Analysing the second dendrogram (Fig. 3.12), that was elaborated using another analysis system


(polymorphism parsimony), the position of both C. aurantifolia and C. macrophylla is different: they moved from the Citron clus- ter to the Micrantha cluster; C. micrantha is considered as another species which con- tributed to the origin of the cultivated citrus. The clustering differences between the two phylogenetic trees compared with several genotypes may be explained by their hybrid origin. Besides the position of

C. aurantifolia and C. macrophylla, C.


Fig. 3.12. Dendrogram (PP tree) for 40 genotypes of Citrus and related genera obtained by Polymorphism Parsimony analysis of RAPD and SCAR data (Nicolosi et al., 2000).

 


sinensis, C. aurantium and C. paradisi also stay in different clusters and form a sub- cluster of the Mandarin cluster in the fi rst dendrogram while they stay with Pummelo in the second one.

The chloroplast DNA (cpDNA) data


resulted in a different phylogenetic tree which can be explained by the different nature of the two genomes: nuclear (total) DNA and cpDNA. The latter is conservative and maternally inherited and its analysis permits us to trace backwards to the original


 


 

type. Each cluster located in Fig. 3.13, a phylogenetic tree elaborated by PAUP, includes accessions ranging from cultivated types to ancient ones. By analysing the den- drogram, it is possible to observe that the four related genera form a single cluster each and are distant from each other and from Citrus, as observed in 1986 by Green et al.

In contrast to total DNA data, in the analysis of the dendrogram obtained after


 

the cpDNA data, the Citrus are not sepa- rated in the two subgenera suggested by Swingle but most of the Citrus genotypes analysed fall into two subgenera described by Tanaka, Archicitrus and Metacitrus, except for the Citron cluster, including C. medica and C. indica, which belong to two separate subgenera, i.e. C. medica to Archicitrus and C. indica to Metacitrus in the section Acrumen.

As reported previously, Tanaka sug-


 

 

 

Fig. 3.13. A 50% majority rule consensus tree for 27 genotypes of Citrus and related genera derived from bootstrap analysis (500 replications) of cpDNA data with confi dence levels for arms (Nicolosi et al., 2000).


 


gested that citrus originated from C. hystrix,

C. latipes, C. macroptera and C. combara. The presence in the dendrogram (Fig. 3.13) of C. latipes, C. hystrix and C. macroptera, respectively the first species in the Pummelo cluster and the other two in the Micrantha cluster, might indicate that the ancient maternal relationship is in the clus- ter. Unfortunately, C. combara was not included in this analysis. Citrus ichangen- sis, that is considered by Li et al. (1992) to be an ancestor of the mandarin through an intermediate genotype, in the result with cpDNA analysis, is clustered with all the mandarin accessions and C. tachibana that is considered to be a kind of wild mandarin species of Japan.

 

 

Origin

Which genotypes are the parents of the main citrus species? This question has interested many researchers, and a lot of hypotheses have been formulated.

It is bound to be a diffi cult task to go back to the relationships of different citrus species, considering the large number of genotypes that can be found in different citrus world areas.

A lot of research has already been car- ried out using both classical and innovative methods. The citrus germplasm is immense, and it is important to study the main centre of origin and spread. Naturally, in order to get a better understanding of the genetic origin, it is necessary to know about those ancestral species.

At this point some results obtained by different methods with regard to the origin of some citrus species will be reported. The species analysed are those considered to be of primary importance due to their econom- ical and territorial spread.

The hypotheses of Barrett and Rhodes, and Scora regarding the three valid species found support in several other pieces of research; the pummelo, the citron and the mandarin are considered to be the ancestors of most of all the other citrus genotypes.

The group of sweet oranges (C. sinensis


Osbeck), also called by different names such as Portugal oranges or Malta oranges, were already known in the past to Ferrarius (1646) as Aurantium vulgare medulla dulci and to Volkamer (1708) as Aurantium fructu dulci (both authors distinguished them from sour oranges). Linneus (1753) placed the sweet orange as a variety of sour orange, naming it Citrus aurantium var. sinensis. Today, the separate origin of sweet and sour oranges is well accepted. As regards the sweet orange, the most widely distributed citrus species, full agreement on its hybrid origin has been reached among different researchers. Although the pres- ence of a lot of varieties originated by muta- tions, sweet oranges are thought to be hybrids between a mandarin genotype with pummelo (Barrett and Rhodes, 1976; Torres et al., 1978; Scora, 1988; Fang and Roose, 1997; Nicolosi et al., 2000).

The sour orange (C. aurantium L.) has a separate, parallel origin; as is well known, sour oranges are easily distinguished from sweet varieties by many morphological and taste characteristics, such as their broadly winged petioles, the distinctly different odour of the essential oil, the taste of the fruit, and so on. Also, different names have been used for sour oranges: Citrus vulgaris Risso (1813), Citrus bigaradia Risso and Poiteau (1818) and others; Citrus aurantium

L. is the scientifi c binomial adopted after the description provided by Linnaeus in 1753. A hybrid origin can certainly be attributed to this species: it is believed by Barrett and Rhodes to be of predomin- antly C. reticulata genotype introgres- sed with genes from C. grandis, and this idea is further supported by different molecular analyses. Moreover, cpDNA analyses have revealed that, as expected, the pummelo is the maternal parent due to its monoembryonic nature (Nicolosi et al., 2000).

In spite of its uncertain geographical origins, it seems certain that the grapefruit (C. paradisi Macf.) is very closely related to the pummelo; in fact, the grapefruit derives from a backcross between it, or an ancestral type, and a sweet orange (Barrett and


 


 

Rhodes, 1976; Torres et al., 1978; Scora, 1988; Nicolosi et al., 2000).

It is more diffi cult to go back to the exact genetic origin of the lemon. There are divergent hypotheses regarding it: both Swingle (1943) and Malik et al. (1974) con- sidered the lemon to be a hybrid of citron and lime. Later, Barrett and Rhodes specu- lated that the lemon is a complex trihybrid of citron, pummelo and a species of Microcitrus, but one carrying a greater pro- portion of citron genes acquired by further introgression from the citron. Torres et al. (1978) excluded the hypotheses of Swingle and Malik et al. because, using isozyme analyses, they found that the W allele in the isozyme phosphoglucose isomerase was absent both in citron and lime, so they indi- cated the origin of lemon to be sour orange

´ lime. The sour orange was also suggested

as a candidate parent by Hirai and Kozaki (1981), and also molecular marker data indicate that the lemon originated from citron and sour orange, the latter being the maternal parent (Nicolosi et al., 2000). Using other molecular markers, inter- sequence simple repeat (ISSR) markers, Fang et al. (1998) suggested a possible poly- phyletic origin for the lemon.

Different considerations were also put forward for the acid Mexican lime (C. aurantifolia Christm.): a trihybrid inter- generic cross involving the citron, the pum- melo and a Microcitrus species was considered by Barrett and Rhodes (1976), while Torres et al. (1978) indicated that this lime may derive from a cross between citron and a papeda species. Using RAPD and SCAR markers, Nicolosi et al. (2000) found that the citron and C. micrantha (a papeda species that produces inedible fruit) should be the parents; also RFLP data sup- port the notion that the citron is a parent (Federici et al., 1998).

Similar results were obtained for C. macrophylla: Nicolosi et al. (2000) assumed that C. micrantha and C. medica were involved in its origin, while Swingle was of a different opinion. He considered C. macrophylla to be a hybrid of C. celebica, or some different species of the subgenus


 

Papeda, with C. maxima. Also, the results obtained by Federici et al. (1998) indicate that C. macrophylla is closely related to C. micrantha.

The sweet lime C. limettioides (Palestine sweet lime) was considered by Webber (1943) to be a hybrid of the Mexican lime with a sweet lemon or a sweet citron. Barrett and Rhodes agreed on the involvement of the Mexican lime as a parent, but they thought that a sweet orange rather than a lemon or citron was involved. A different hypothesis emerges from molec- ular marker analysis (Nicolosi et al., 2000) which indicates that the Palestine sweet lime might be a backcross hybrid between sweet orange and citron.

Moreover, the citron is involved in the origin of other important citrus hybrids such as the bergamot (C. bergamia Risso and Poiteau), the Volkamer lemon (C. volkameriana Ten. and Pasq.), the rough lemon (C. jambhiri Lush.) and the Rangpur lime (C. limonia Osbeck). The bergamot orange is a hybrid of C. aurantium ´ C. medica (Scora, 1988; Deng et al., 1996), and the Volkamer lemon has a similar parallel origin. However, as reported by Nicolosi et al. (2000), in this genotype a greater pro- portion of mandarin genes was observed, probably due to backcrossing with man- darin. The citron and the mandarin are involved in the origin of the rough lemon and the Rangpur lime.

The highly varied group of mandarin includes numerous species, most of which derive from intergeneric and interspecifi c crosses, while others, which are surely commercially important, derive from man- made crosses.

The clementine is of great importance in the Mediterranean citrus industry. Despite fairly divergent theories about its origin, the consensus is that it is a tangor, a hybrid between orange and mandarin. Trabut (1902) referred to the discovery of an interesting new specimen among the seedlings bred by Father Clè ment Rodier at the orphanage of the Father of the Holy Spirit in Misserghin, near Oran, in Algeria. The Algiers Horticultural Society called it


 


clementine, the name suggested by Trabut, who also put forward the mandarin and the ‘Granito’ sour orange as the parents of the clementine. Webber (1943) and Chapot (1963) disagreed with this theory. Webber held the clementine to be a variant within the mandarin-like group or, if it were an interspecifi c cross, a mandarin and sweet orange hybrid. Chapot, on the other hand, found some remarkable similarities to the Canton mandarin. However, it seems strange that the Canton mandarin should have escaped the notice of such an expert on the far-eastern citrus industry as Tanaka,


who claimed he had only seen the clemen- tine under cultivation and coined the term Citrus clementina Hortulanorum for it (1954). Webber’s hypothesis on the inter- specifi c cross between mandarin and sweet orange is supported by the results obtained using molecular markers (Deng et al., 1996; Nicolosi et al., 2000). A similar origin is considered by these authors for other geno- types such as tankan and satsuma, while they have some doubts as to the origin of Murcott and King, as to whether it is the sour or sweet orange that is involved.


 






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