Phylogenetic origin of limes and lemons revealed by cytoplasmic and nuclear markers.

Lemons, limes and citron constitute a group of closely related Citrus species, whose species delimitations and taxonomic relationships are unclear. In order to identify karyotypic s

The relationships among lemons, limes and citron: a chromosomal comparison.

Grapefruit is a group of citrus of recent origin, probably resulting from a cross between pummelo and sweet-orange. Aiming to investigate this putative origin and the genetic variability among grapefruit cultivars, the karyotype of six grapefruits, two pummelos, and one tangelo cultivar (grapefruit × tangerine) were analyzed using sequential CMA/DAPI double staining and FISH with rDNA probes. The karyotypes of grapefruit ‘Duncan’ and ‘Foster’ differ from those of ‘Flame’, ‘Henderson’, ‘Marsh’ and ‘Rio Red’. The former have two chromosomes with a single CMA+ band in both terminal regions (C type chromosome) and six chromosomes with only one CMA+ terminal band (D type), whereas the latter have three C and five D type chromosomes. All accessions investigated exhibited two chromosomes with 5S rDNA but a variable number of 45S rDNA. The two former grapefruits displayed four 45S rDNA sites, whereas the remaining grapefruit cultivars had five. The two pummelos showed identical karyotypes, homozygous for CMA+ bands and their four rDNA sites. From each pummelo chromosome pair one chromosome seems to be present in grapefruit karyotypes. The different grapefruit karyotypes might result from independent crosses between pummelos of different karyotypic constitution and sweet-oranges. The chromosome markers found in the tangelo ‘Orlando’ and the position of their two 45S rDNA confirm the grapefruit ‘Duncan’ and the tangerine ‘Dancy’ as their parents.

Karyotype diversity and the origin of grapefruit

Localization of the 5S and 45S rDNA sites and cpDNA sequence analysis in species of the Quadrifaria group of Paspalum (Poaceae, Paniceae).

5S and 45S rDNA sites are the best mapped chromosome regions in eukaryotic chromosomes. In this work, a database was built gathering information about the position and number of 5S rDNA sites in 784 plant species, aiming to identify patterns of distribution along the chromosomes and its correlation with the position of 45S rDNA sites. Data revealed that in most karyotypes (54.5%, including polyploids) two 5S rDNA sites (a single pair) are present, with 58.7% of all sites occurring in the short arm, mainly in the proximal region. In karyotypes of angiosperms with only 1 pair of sites (single sites) they are mostly found in the proximal region (52.0%), whereas in karyotypes with multiple sites the location varies according to the average chromosome size. Karyotypes with multiple sites and small chromosomes ( 6 µm) more commonly show terminal or interstitial sites. In species with holokinetic chromosomes, the modal value of sites per karyotype was also 2, but they were found mainly in a terminal position. Adjacent 5S and 45S rDNA sites were often found in the short arm, reflecting the preferential distribution of both sites in this arm. The high frequency of genera with at least 1 species with adjacent 5S and 45S sites reveals that this association appeared several times during angiosperm evolution, but it has been maintained only rarely as the dominant array in plant genera.

Non-Random Distribution of 5S rDNA Sites and Its Association with 45S rDNA in Plant Chromosomes.

Karyotypes of 93 individuals belonging to 18 accessions of mandarins, mandarin hybrids and two related species were analysed with the fluorochromes CMA and DAPI, to identify marker chromosomes. The karyotypes revealed highly differentiated banding patterns and could be classified in four groups (I–IV) according to the presence/absence of chromosomes with three bands (type A) or with two bands (one proximal and one terminal, type B, or both terminal, type C). The accessions of group I exhibited the simplest and homozygous karyotypes (lacking chromosome types A, B and C), represented by `Sunki' and `Cleopatra'. Group II (lacking chromosome types A and B) included three accessions of Mediterranean mandarins and `Cravo' mandarin, all of them with very similar and almost completely homozygous karyotypes. All other karyotypes of groups II and III (lacking chromosome type A) were heterozygous for one or more chromosome pairs and most of them seemed to be hybrid derivatives from non-mandarin accessions. Group IV (with chromosome types A and B) was represented only by two heterozygous hybrids (`Murcott' and `King'). The karyotype of most hybrids agrees with one of the possible combinations resulting from chromosome types segregation from their putative ancestor karyotypes, but at least `Orlando' seemed to be a more complex hybrid. Comparing with banding patterns of other Citrus species, those of group I and the Mediterranean mandarins (group II) are the best candidates to represent C. reticulata (sensu Swingle) as a true species.

Chromosomal relationships among cultivars of Citrus reticulata Blanco, its hybrids and related species

Brazil is the world’s largest sweet orange producer, but production in the state of Acre is small and does not meet the local demand, despite having climatic conditions that are adequate for its cultivation. The objective of this study was to evaluate the behavior of the ‘Pera’ orange using different rootstocks under the edaphoclimatic conditions of Rio Branco, Acre, Brazil. We grafted the scion of the ‘Pera’ orange onto nine citrus rootstocks: ‘Cleopatra’ mandarin, TSKFL x CTTR – 013, LVK x LCR – 038, TSKC x CTQT 1439 – 004, LVK x LVA – 009, ‘Indio’citrandarin, ‘Santa Cruz Rangpur’ lime, TSKFL x CTC - 25 – 002, and TSKC x CTSW – 038. The experiment was organized in a random block design with three replicates and two plants per parcel. The following characteristics were evaluated: compatibility between scion and rootstocks, tolerance to drought, production and fruit quality. There was an effect of the rootstocks on the behavior of the ‘Pera’ variety. The ‘Santa Cruz Rangpur’ lime, ‘Indio’ citrandarin, and the hybrid LVK x LCR - 038 rootstocks presented the best performance of the ‘Pera’ orange. Therefore, they constitute an alternative to the ‘Rangpur’ lime and may be incorporated into the current ‘Pera’ orange production system of the region.

W.S. Soares Filho

Papers

The relationships among lemons, limes and citron: a chromosomal comparison.

Chromosomal relationships among cultivars of Citrus reticulata Blanco, its hybrids and related species

Agronomic performance of the ‘Pera’ orange grafted onto nine rootstocks under the conditions of Rio Branco, Acre, Brazil