Most fruit tree species are characterized by a high degree of heterozygosity, long juvenility, large size, and sometimes self-incompatibility. For these reasons, progress in breeding programs through conventional methods (i.e., generations of crossing and selection) is time-consuming and limited by available space for field experiments. Biotechnological techniques such as production of haploids offer new opportunities for genetic research in breeding programs (Höfer and Grafe, 2003). Haploids and di-haploids (DHs) are important for genetic and developmental studies as well as for plant breeding. They have potential use in mutation research, genetic analysis, transformation, and in the production of homozygous cultivar(s) that can be used to produce F1 hybrids for exploitation of heterosis (Germana, 2009). DHs are also very useful for genome mapping, providing reliable information about the location of major genes and quantitative trait loci for economically important traits (Khush and Virmani, 1996). Several methods have been tested for induction of haploids, including anther and microspore culture and in situ parthenogenesis by irradiated pollen followed by in vitro culture of immature embryos in fruit trees (Germana, 2006).
Haploid production through in situ parthenogenesis has been reported in fruit trees species such as apple [Malus ×domestica (Höfer and Lespinasse, 1996; Zhang and Lespinasse, 1991)], pear [Pyrus communis (Bouvier et al., 1993)], sweet cherry [Prunus avium (Höfer and Grafe, 2003)], kiwifruit [Actidinia deliciosa (Chalak and Legave, 1997, Pandey et al., 1990)], and clementine [Citrus reticulata (Ollitrault et al., 1996)]. In mandarin ‘Fortune’ (Citrus clementina × C. tangerina) and ‘Ellendale’ (C. reticulata × C. sinensis), haploid induction was successfully performed through in situ parthenogenesis followed by immature embryo culture (Froelicher et al., 2007). The selection of an efficient radiation dose, the optimization of the pollination method, the seed collection time, the developmental stage, the culture media, and culture conditions are all important factors affecting the success of this technique and the number of haploid embryos rescued (Germana, 2009).
The purpose of this research was to evaluate the use of gamma-irradiated pollen for production of parthenogenetic haploid plants in persian walnut. Viability of irradiated pollen, percent fruit set, parthenogenetic embryos formation, and the first production of haploid plants in walnut are reported.
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