Abstract
The objective of this study was to produce citrus somatic asymmetric hybrids by fusing gamma-irradiated protoplasts with iodoacetamide-treated protoplasts. Protoplasts were isolated from embryogenic suspension cells of grapefruit (Citrus paradisi Macfad.) cultivars Ruby Red and Flame, sweet oranges (C. sinensis Osbeck) 'Itaboraí', 'Natal', Valencia', and 'Succari', from 'Satsuma' (C. unshiu Marcow.) and 'Changsha' mandarin (C. reticulata Blanco) and 'Murcott' tangor (C. reticulata x C. sinensis). Donor protoplasts were exposed to gamma rays and receptor protoplasts were treated with 3 mmol L-1 iodoacetamide (IOA), and then they were fused for asymmetric hybridization. Asymmetric embryos were germinated, and the resulting shoots were either grafted onto sour orange, rough lemon or 'Swingle' (C. paradisi x Poncirus trifoliata) x 'Sunki' mandarin rootstock seedlings, or rooted after dipping their bases in indol-butyric acid (IBA) solution. The products were later acclimatized to greenhouse conditions. Ploidy was analyzed by flow cytometry, and hybridity was confirmed by amplified fragment length polymorphism (AFLP) analysis of plantlet DNAsamples. The best treatment was the donor-recipient fusion combination of 80 Gy-irradiated 'Ruby Red' protoplasts with 20 min IOA-treated 'Succari' protoplasts. Tetraploid and aneuploid plants were produced. Rooting recalcitrance was solved by dipping shoots' stems in 3,000 mg L-1 IBA solution for 10 min.
Highlights
Sexual hybridization in most citrus species is very complicated
Asymmetric somatic hybridization using X‐ or γ‐irradiation, on the other hand, has great potential for scion improvement because it allows partial genomic transfer (Rasmussen et al, 2000) as chromosome elimination is induced by high radiation doses
Protoplasts were isolated from habituated embryogenic suspension cells of grapefruit (Citrus paradisi Macfad.) cultivars Ruby Red and Flame, sweet oranges (C. sinensis Osbeck) 'Itaboraí', 'Natal', Valencia' and 'Succari', 'Satsuma' (C. unshiu Marcow.)
Summary
Sexual hybridization in most citrus species is very complicated. Somatic hybridization via protoplast fusion, on the other hand, is a powerful tool in genetic breeding because it circumvents such sexual restraints (Grosser & Gmitter Junior, 2005). Even though symmetric somatic hybrids have great potential for rootstock improvement and as tetraploid breeding parents in interploid crosses, they may not have direct application as scion cultivars as they may present complex genetic constitution. Asymmetric somatic hybridization (donor-recipient fusion) using X‐ or γ‐irradiation, on the other hand, has great potential for scion improvement because it allows partial genomic transfer (Rasmussen et al, 2000) as chromosome elimination is induced by high radiation doses. Colony formation of irradiated cells tends to be avoided (Derks et al, 1992)
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