Genetic diversity in the orange subfamily Aurantioideae. I. Intraspecies and intragenus genetic variability

Abstract

Despite the great economic importance of citrus, its phylogeny and taxonomy remain a matter of controversy. Moreover pathogens of increased virulence and dramatic environmental changes are currently spreading or emerging. The objectives of the present paper, measuring genetic variability and studying its pattern of distribution, are crucial steps to optimize sampling strategies in the search of genotypes that tolerate or resist these threatening factors within the huge array of Citrus and Citrus related species. Their intraspecific and intrageneric variability was studied comparatively by means of ten enzymatic systems using eight different measures. The analysis of ten enzymatic systems allowed us to distinguish all the species and all but one artificial hybrid. The species with the lowest genotypic variability are C. myrtifolia, C. deliciosa (willow leaf mandarin), C. paradisi (grapefruit), C. limon (lemon) and C. sinensis (sweet orange), while Severinia buxifolia shows the highest value. A broad spectrum of heterozygosity values was found in the collection. Lemons (C. limon, C. meyeri, C. karna, C. volkameriana), limes (C. aurantifolia, C. limettioides, C. lattifolia) and C. bergamia show a very high percentage of heterozygosity which indicates an origin through interspecific hybridization. Two main factors limit genetic intraspecific variability: apomictic reproduction, where nucellar embryos are much more vigorous than the zygotic ones, and nurserymen selecting against variability in the seedling stage of the rootstocks or in propagating the scion cultivars vegetatively. Additionally, self-pollination appears in some species mainly used as rootstocks which would explain their low heterozygosity values. Genetic differences between species and genera are in general high, which suggests that adaptation might have played an important role during the evolution of the orange subfamily.

Development and characterization of polymorphic microsatellite markers for the sweet orange (Citrus sinensis L. Osbeck)

“…It recognized that sweet oranges have a narrow genetic basis and that most morphological characters origi- 94 Polymorphic microsatellite markers for the sweet orange nated through mutations, propagation of sweet oranges being by vegetative propagation as is the case for the majority of citrus species (Herrero et al, 1996;Bretó et al, 2001). The similarity between the sweet orange cultivars investigated by us can be seen in the dendrogram shown in Figure 2, this close relationship supports the view that most sweet orange cultivars arose through mutation.…”

“…The low intraspecific diversity found in cultivated species such as sweet orange contrasts with the high variability of agriculturally important traits such as ripening period and color and size of fruits (Herrero et al, 1996). Because of these factors and the general lack of citrus molecular markers the distinction between cultivars is still based mainly on morphological traits, especially fruit traits (Fang and Roose, 1997).…”

Molecular characterization of sour orange (Citrus aurantium) accessions and their relatives using SSR and SRAP markers

“…Sour orange produces seeds containing both nucellar and zygotic embryos, and therefore, uniformity is almost complete (Siragusa et al, 2006). Understanding the taxonomy, phylogenetic relationships, and genetic variability in Citrus is critical for the determination of genetic relationships, characterization of germplasm, control of genetic erosion, design of sampling strategies for core collections, establishment of breeding programs, and registration of new cultivars (Herrero et al, 1996).…”

Genetic Diversity in Citrus

“…Elucidating relationships, taxonomy, and diversity is important for developing breeding strategies, conserving biodiversity, and improving breeding efficiency. Also understanding genetic variability in citrus is critical for characterizing germplasm, controlling genetic erosion and the registration of new cultivars (Herrero et al, 1996;Barkley et al, 2006).…”