a crop is a weed

If you’ve ever looked out along the roadsides of Portland, Oregon, you surely have noticed what looks like a tall dandelion in blue. This is chicory. Maybe you know chicory for its role as a coffee flavoring or coffee substitute, where the roasted root provides a certain nuttiness with a certain tang. The chicory variety that is forced from a root in the dark to produce a white bud is known as Belgian endive. Leafy varieties of chicory are eaten as greens in various countries, from Italy to Brazil. In the US leafy chicory varieties are sometimes marketed as dandelion greens, but perhaps more commonly the red varieties are known as radicchio. So why is chicory growing as a weed on the roadside? Was Portland once a hub of chicory production, blanketed with fields of bitter greens? Did seeds from a wild variety of chicory contaminate a batch of crop seeds and then go crazy in the new environment? Or did a tame plant from someone’s garden mutate into an invasive nuisance? Where would you turn to find out?

If you’ve been following this blog, you’ve probably already guessed that the answer is in the DNA. Just as DNA can be used to infer human ancestry, it is also indispensable for disentangling the past migration routes of other organisms, from salmon to yeast. Dr. Tomáš Závada used DNA to shed light on the origin of weedy chicory in the US. However, while the study of the human genome has been a colossal global endeavor, the global importance of the chicory genome ranks alongside that of Chinese red sage. Dr. Závada and his team had to rely on leftover gene sequence data from a study of the sunflower family tree, leveraging these data by using a curious property of DNA, the microsatellite.

Recall that DNA is a molecule made up of a long string of basic building blocks that come in four types. These building blocks can be thought of as letters in a code that contains the blueprint for all the structures and processes in an organism; scientists refer to them as A, C, G, and T. All the cells in the organism contain the same blueprint, which is faithfully copied with every cell division using a tool known as an enzyme. Well, mostly. Every so often the enzyme that copies DNA makes a mistake, about once for every ten million DNA letters copied. The cell has additional machinery for correcting errors after they are made, but with hundreds of millions of letters and countless numbers of copies, a few errors always escape correction, providing the basis for evolution.

One type of error is particularly easy to make during the process of copying, the type that takes place where the sequence of DNA letters gets repetitive. The replicating enzyme will come across a short sequence, say, “AAG”, that is repeated a few times in a row — AAGAAGAAGAAG — up to a couple dozen times. Here the enzyme basically loses its place and is prone to skip an AAG or add an extra AAG. This type of repetitive sequence is known as a Single Sequence Repeat or, more picturesquely, a microsatellite. Every time a mistake in copying it happens — or any other type of DNA copying mistake as well — that is a mutation, and a new lineage is created. But while other types of mutations are often used to tell different species apart, microsatellite mutations happen so frequently that they are useful to distinguish lineages within a species.

Dr. Závada searched the online sunflower family DNA database for published chicory DNA sequences and found a dozen of these microsatellite stretches. There is lab equipment that can detect small differences between the size of a microsatellite from one plant versus another version of the same microsatellite from another, indicating a different number of repeats, and by extension, different ancestors. In addition to microsatellites the team used a segment of DNA that indicates matrilineal descent. It turns out that the chlorophyll-containing bodies inside a plant cell — the chloroplasts — have their own DNA separate from the cell’s nucleus. Chloroplasts are not present in pollen, so chloroplast DNA is only inherited from the female parent, the plant that produces the seed. Thus chloroplast DNA sequence that is found in one plant and matches that of another plant on a separate continent indicates that seeds must have been carried from one continent to the other. With the microsatellite sizes and the chloroplast DNA sequences, the team was poised to connect North American chicory lineages to crop varieties and wild varieties in their native Eurasian range.

Weedy chicory is actually found in all 48 contiguous United States. The team, based out of UMass Boston, sampled chicory plants from around New England and also received samples from other states, from Virginia to California. From the Old World they sampled different edible varieties of chicory, some grown for the root and some for the leaves, and also more primitive varieties from all over Europe plus Iran. The analytical software churned through all the data, and what interesting results popped out.

That Portland roadside chicory? It seems to be descended from radicchio, those characteristic red streaks in the leaves having been lost as it adapted to its wild lifestyle. The Boston weedy chicory matches the genotype of Belgian endive. The Boulder, Colorado, roadside chicory shares genes mostly with the wild chicory from Iran. The chicory stands in Nevada and New Mexico seem to have developed new mutations as they adapted to the local conditions. And the weedy chicory growing around Thomas Jefferson’s Monticello may be partly descended from the radicchio seeds that Jefferson received from France in one of the first recorded introductions of chicory to North America.

So does weedy chicory descend from extensive fields planted in another century? Chicory is sometimes grown as a fodder crop for livestock, but the genetic data do not clearly show descendants of fodder crops, possibly due to inadequate sampling. Did weedy chicory find its way to some of its current locations as a contaminant of crop seed? This phenomenon was documented for grass and clover seed in 1920 and may have been the case in places like Boulder. And in Portland? Because radicchio seems more like a horticultural crop meant for human consumption, it was probably not originally a contaminant, and it seems unlikely it would be grown as a salad for livestock. That leaves the third conjecture, the garden mutant.

To be sure, chicory started out with certain advantages for living wild, such as high seed production, a system for avoiding inbreeding, toxicity to competitor plants, tolerance to a range of environments, and even an ability to grow differently under different conditions. And in general, domesticated plants thrive in disturbed soils. Add to that the many introductions of chicory into North America, giving it many more chances to take hold, as well as plenty of genetic diversity for adaptation. In addition, variegated radicchio turns out to be a hybrid between chicory and its close relative, endive, the extra genetic resources supplying it with even more wherewithal to face adversity.

So a nice little radicchio from someone’s plot in Portland dropped seeds that sprouted in disturbed areas nearby. Then, years of cross-fertilization and environmental selection got rid of useless traits like red streaks and amplified traits that increased overall hardiness, and a re-wilded chicory took its place as a roadside weed alongside sweet pea, St. John’s wort, the feral race of carrot known as Queen Anne’s Lace, and that bane of all Oregonians, the blackberry. And the moral of the story is — be careful of the plants you import, because today’s exotic import could become tomorrow’s weed.