Abstract
The genus Eucalyptus encloses several species with high ecological and economic value, being the subgenus Symphyomyrtus one of the most important. Species such as E. grandis and E. globulus are well characterized at the molecular level but knowledge regarding genome and chromosome organization is very scarce. Here we characterized and compared the karyotypes of three economically important species, E. grandis, E. globulus, and E. calmadulensis, and three with ecological relevance, E. pulverulenta, E. cornuta, and E. occidentalis, through an integrative approach including genome size estimation, fluorochrome banding, rDNA FISH, and BAC landing comprising genes involved in lignin biosynthesis. All karyotypes show a high degree of conservation with pericentromeric 35S and 5S rDNA loci in the first and third pairs, respectively. GC-rich heterochromatin was restricted to the 35S rDNA locus while the AT-rich heterochromatin pattern was species-specific. The slight differences in karyotype formulas and distribution of AT-rich heterochromatin, along with genome sizes estimations, support the idea of Eucalyptus genome evolution by local expansions of heterochromatin clusters. The unusual co-localization of both rDNA with AT-rich heterochromatin was attributed mainly to the presence of silent transposable elements in those loci. The cinnamoyl CoA reductase gene (CCR1) previously assessed to linkage group 10 (LG10) was clearly localized distally at the long arm of chromosome 9 establishing an unexpected correlation between the cytogenetic chromosome 9 and the LG10. Our work is novel and contributes to the understanding of Eucalyptus genome organization which is essential to develop successful advanced breeding strategies for this genus.
Highlights
Eucalypts (Eucalyptus L’Her) belong to Myrtaceae family and are predominantly native to Australia and to its North islands, where they occur in various habitats
Based on the number of clones of each library (36,864 clones for EGC_Ba and 36,864 clones for EGC_Bb) and the size of the E. globulus genome estimated at 1.09 pg/2C (530 Mb/1C) (Grattapaglia and Bradshaw, 1994), the genome coverage of both libraries is 11.2X, and the probability to find a BAC clone harboring a sequence of interest is 99.9% based on the algorithm of Clarke and Carbon (1976)
Using the primers developed for E. gunnii Cinamoyl-CoA reductase (EguCCR, X79566) (Rengel et al, 2009) and E. gunnii GTPase RAC (EguRAC1, DR410036) (Rasmussen-Poblete et al, 2008) a total of eight and seven BAC clones were identified, respectively
Summary
Eucalypts (Eucalyptus L’Her) belong to Myrtaceae family and are predominantly native to Australia and to its North islands, where they occur in various habitats. Plants from this genus are highly diverse and display significant adaptability and phenotypic plasticity. Most species belong to the subgenus Symphyomyrtus Schauer, which is divided in 14 sections (Brooker, 2000) From all of these sections only three are used in plantation forestry for industrial purposes: sect. Maidenaria (E. globulus) which grows in temperate regions (Grattapaglia et al, 2012) These species and their hybrids are among the world’s leading sources of woody biomass and are the main hardwoods used for pulpwood and timber
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