Polyembryony in citrus: does the largest embryo in the seed develop a nucellar seedling?

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.