Abstract
A generalized concept in the Citrus genus is that highly polyembryonic varieties produce only a small number of hybrids. Small zygotic embryos congregate primarily near the micropyle, while nucellar embryos organize near or away from the micropyle. In the present study, the authors determined the number of polyembryonic seeds, embryos per seed, and the largest embryo (LE) position in five citrus cultivars: C35 citrange, Volkamer lemon, Amblycarpa mandarin, Minneola tangelo, and Valencia orange. The percentage of nucellar seedlings obtained exclusively from the LE per seed was then calculated. The polyembryony percentage varied largely between genotypes, from 65 to 98 %, and the mean number of embryos ranged from 2.9 to 4.6. The chalaza contained up to 87 % of the LE. Out of 30 primers, 17 Simple sequence repeats [SSRs] (AG14, ATC09, CAT01, CCSM147, CCSM18, CCSM13, CCSM4, F2, F4, F6, GT03, TAA41, TAA45, TAA1, F7, F11 and TAA52) identified nucellar plants identical to the female parent (genetic similarity index [GSI] value ≥ 0.95). This study establishes for the first time the relationship between the sexual or asexual origin of seedlings derived from LE embryos isolated from seeds and the SSR primers described above. While the five citrus cultivars had high polyembryony levels, 30 % of the resulting plants differed from the female parent in C35 Citrange, 45 % in Volkamer lemon, 15 % in Amblycarpa mandarin, 15 % in Valencia orange, and 45 % in Minneola tangelo. The largest seedling is not always nucellar: in the five citrus species studied, the LE produces 55 to 85 % of the nucellar embryos.
Highlights
Polyembryony is a type of sporophytic apomixis commonly found in citrus species, except in Citrus medica L., Citrus grandis Tanaka Citrus clementinaTanaka, and in a number of mandarin hybrids (Aleza et al, 2010; Ribeiro et al, 1999)
This study aimed to identify nucellar seedlings produced by female parents in seedlings obtained from the largest embryo in seeds for two citrus cultivars (Valencia orange and Minneola tangelo) and three rootstocks (C-35 citrange, Volkamer lemon, and Amblycarpa mandarin) by using 30 microsatellites as SSR markers
The percentage of polyembryonic seeds differed in the two cycles for C35, Amblycarpa, and Minneola, while Volkamer and Valencia showed no statistical differences (Table 2)
Summary
Polyembryony is a type of sporophytic apomixis commonly found in citrus species, except in Citrus medica L., Citrus grandis Tanaka Citrus clementinaTanaka, and in a number of mandarin hybrids (Aleza et al, 2010; Ribeiro et al, 1999). Ex Tanaka seed, the smallest embryos developing at the micropyle end are usually considered zygotic embryos (Andrade-Rodríguez et al, 2005), while the nucellar embryos may develop near or away from the micropyle (Kishore et al, 2012). In apomictic citrus, developing nucellar embryos were found to promote early embryogenesis and competition for endosperm nutrients among embryos (Wakana and Uemoto, 1988). The initial development of both zygotic and nucellar embryos is inhibited in the chalazal region (Koltunow et al, 1995; Wakana and Uemoto, 1988), and both embryos grow preferentially near the micropyle end. Different researchers claim that cultivars showing high polyembryonic percentages are less likely to develop hybrid seedlings since their zygotic embryos will not survive under field conditions. The occurrence and germination of the two embryo types depend on multiple factors like genotype, climate, pollinators, fertilization, seed development stage, and other plant physiological conditions (Kishore et al, 2012; Rao et al, 2008; Yildiz et al, 2013)
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