Natural Cloning, Mandarin Style

Mandarin, Citrus reticulata 'Parramatta Sweets'

No prizes for guessing where heritage ‘Parramatta Sweets’ mandarins (Citrus reticulata) originated.

In autumn I sowed 100 mandarin seed. I’ve just potted up the resultant seedlings – all 170 of them! How is this possible? Most flowering plants (angiosperms) generate a single embryo per seed following pollination. Seed results from sexual reproduction and the offspring have some genetic variation. But some cultivars of citrus generate multiple embryos in an individual seed without pollination – they are polyembryonic seed and they’re a result of asexual reproduction. Polyembryonic seed go on to produce multiple seedlings, and you can carefully separate and pot these seedlings individually. My most prolific seed generated five seedlings, but quite a few produced two to three seedlings each. Back in 1719, Linnaeus observed and defined apomixis, a natural cloning process involving polyembryony. Apart from sheer numbers, the other advantage of  apomixis to the gardener is the offspring are genetically identical – true clones – of their mother. I know that the seedlings of my heritage Australian Parramatta Sweets‘ mandarin will mature – with less fuss and effort than modern mandarins – to produce an equally heavy crop of sweet, juicy fruit enclosed by a rind that separates perfectly from the fruit. And I have enough offspring to create a commercial orchard. If you’re interested in delving deeper, read on…

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Polyembryonic seed formation in citrus is one of many apomictic processes that have been described to occur in the ovules of angiosperm species (Koltunow, 1993). In polyembryonic seed formation, many non-zygotic, nucellar embryos are initiated directly from the maternal, nucellar cells surrounding the embryo sac containing a developing zygotic embryo. During embryo sac expansion, embryogenic nucellar cells obtain access to endosperm and develop into embryos alongside the zygotic embryo that may or may not complete development. Nucellar embryos give rise to seedlings that are of the same genotype as the female parent – they are natural clones. Reference, see: Oxford Journals, Annals of Botany Polyembryony in non-apomictic citrus genotypes Pablo Aleza, José Juárez, Patrick Ollitrault and Luis Navarro – Jerry Coleby-Williams 25th August 2012 (while waiting for a radio interviewer to call)