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October 5, 2005

Chimeras

A while back, as part of a post on the pedigree of the grapefruit, I posted a photo of a half-red chimeric grapefruit (see it here, too). Today, I stumbled across another interesting citrus chimera, a multi-sectored green and yellow 'Valencia' orange, at right.

It occurred to me that many people probably have no idea what a chimera is. For those with a little knowledge of Greek mythology, the name may provide a little hint. The Chimera was a monster comprised of a number of different animals: a lion, a goat, and a snake. It also breathed fire, but that's not relevant to the genetic term. The important idea here is the idea of several different things of different genetic makeup all lumped into one. Technically, any one who has received a transplant is a chimera, as is any kind of a grafted plant, though the term is rarely applied in either such situation. (There are even human chimeras, often the result of people who developed partially from the tissue of an unborn fraternal twin...one such woman apparently gave birth to two children who appeared not to be her own!)

Usually, in plants, chimeras are the result of a mutation which is present in some cells and not in others. This mutation occurs in a meristem (an area of active growth and division), and is replicated by normal cell division, creating parts of the plant with different genetic constitutions. Plant tissues are produced in distinct layers, and some of the most common plant chimeras are the result of a mutation in one of these layers.

The usual model consists of three layers designated L.I, L.II, and L.III. The cells of L.I form the epidermis, or outer layer of the plant, L.II forms the cortex, part of the vascular tissues and much of the leaf blade, and L.III forms most of the internal tissues, such as pith and some vascular tissues, as well as the roots. Reproductive tissues usually form from L.II. This means that a gene present in the outer layer of a chimeric plant (the L.I), for example, might not be passed onto its progeny, as pollen would originate from the L.II, which lacks the mutant gene.

There are basically three types of chimeras in plants: Periclinal, or "hand-in-glove" chimeras, where a single cell layer contains the mutation, sectorial, where mutant cells and normal cells are side by side in different sectors, and mericlinal, sort of combination of the two, where a sector of a single layer contains the mutation. The two citrus fruits I've shown were sectorial chimeras. Some of the more commonly propagated chimeras are periclinal.

Because they do not breed true, they must be reproduced by vegetative propagation. 'Evergreen Thornless', a blackberry, and 'Thornless Loganberry' (see this recent post) are both good examples of periclinal chimera. As previoiusly mentioned, these two thornless brambles do not breed true, but using tissue culture truly thornless variants have been created. These new cultivars, 'Everthornless' and 'Lincoln Logan', do breed true, each passing on a different, dominant, gene for thornlessness.

Many ornamental plants with variegated leaves are also chimeras, with the outer layer lacking chlorophyll. The green inner layer shows through, giving the "green center, white margins" look. 'Pinot Meunier', an ancient grape cultivar, has a mutation in its L.I cell layer which gives it hairy leaves (though occasionally L.II cells wind up taking the place of L.I cells in the leaves, resulting in leaves with hairless sectors). Interestingly, if the L.I layer is separated through tissue culture, the resulting plant has hairy leaves, as expected, but it's also a dwarf, with extremely short internodes. Carole Meredith, from UC Davis, believes the inner, non-mutant portion of 'Pinot Meunier' is probably a pretty typical 'Pinot Noir'. The Pinot group of cultivars is particularly prone to mutation, and a recent study turned up chimeric clones of 'Pinot gris', 'Pinot blanc', 'Pinot moure', and 'Pinot noir' as well as 'Pinot Meunier'. A few other grapes, 'Chardonnay' clone 96, 'Primitivo di Gioia', for example, have also been shown to be chimeras, though none quite so striking.

Probably the weirdest of all chimeras is the graft chimera. In rare cases, a shoot originating at the graft union forms with cells from the rootstock making up one layer, while cells from the scion make up another. In some cases, this can even result in plants in which layers are completely different genera. Examples in the fruit world include Pyrocydonia danielii, a union of pear and quince, Crataegomespilus dardari and Crataegomespilus asnieresi, formed from a medlar grafted to a hawthorn stock, Ortegopuntia, a graft chimera of Ortegocactus and Opuntia.

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