Move Over, Mendel (But Don’t Move Too Far)

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Today Gregor Mendel is a towering hero of biology, and yet during his own lifetime his ideas about heredity were greeted with deafening silence. In hindsight, it’s easy to blame his obscurity on his peers, and to say that they were simply unable to grasp his discoveries. But that’s not entirely true. Mendel got his ideas about heredity by experimenting on pea plants. If he crossed a plant with wrinkled peas with one with smooth peas, for example, the next generation produced only smooth peas. But when Mendel bred the hybrids, some of the following generation produced wrinkled peas again. Mendel argued that each parent must pass down factors to its offspring which didn’t merge with the factors from the other parent. For some reason, a plant only produced wrinkled peas if it inherited two wrinkle-factors.

Hoping to draw some attention to his research, Mendel wrote to Karl von Nageli, a prominent German botanist. Von Nageli was slow to respond, and when he did, he suggested that Mendel try to get the same results from hawkweed (Hieracium), the plant that von Nageli had studied for decades. Mendel tried and failed. It’s impossible to say whether von Nageli would have helped spread the word about Mendel’s work if the hawkweed experiments had worked out, but their failure couldn’t have helped.

After Mendel’s death, a new generation of biologists discovered his work and, with the insights they had gathered from their own work, they realized he had actually been onto something. Pea plants really do pass on factors–genes–to their offspring, and sometimes the genes affect the appearance of the plants and sometimes they don’t. Mendelian heredity, as it came to be known, was instrumental in the rise of the new science of genetics, and today practically every high school biology class features charts showing how dominant and recessive alleles are passed down from one generation to the next. Mendelian heredity also helped explain how new mutations could spread through a population–the first step in evolutionary change.

But what about that hawkweed? It turns out that usually Hieracium reproduces very differently than peas. A mature Hieracium does not need to mate with another plant. It does not even need to fertilize itself. Instead, it simply produces clones of itself. If Nageli had happened to have studied a plant that reproduced like peas, Mendel would have had more luck.

Hawkweed raises an important question–one that is particularly important this morning. Does it tells us that Mendel was wrong? Should teachers throw their Mendelian charts into the fire? No. Mendel found a pattern that is widespread in nature, but not a universal law. Most animals are pretty obedient to Mendel’s rule, as are many plants. Many algae and other protozoans also have Mendelian heredity, although many don’t. Many clone themselves. And among bacteria and archaea, which make up most of the diversity of life, Mendelian heredity is missing altogether. Bacteria and archaea often clone themselves, trade genes, and in some cases the microbes even merge together into a giant mass of DNA that then gives rise to spores.

Today in Nature, scientists found another exception to Mendelian heredity. They studied a plant called Arabidopsis (also known as cress) much as Mendel did, tracing genes from one generation to the next. They crossed two lines of cress, and then allowed the hybrids to self-fertilize for two more generations. Some of the versions of the genes disappeared over the generations from the genomes of the plants, as you’d expect. But then something weird happened: in a new generation of plants, some of the vanished genes reappeared. The authors think that the vanished genes must have been hiding somewhere–perhaps encoded as RNA–and were then tranformed back into DNA.

Is cress the tip of a genetic iceberg (to mix my metaphors hideously)? Only more experiments will tell. If it is more than just a fluke, it may turn out to play an important part in evolution, joining some other weird mechanisms, such as "adaptive mutation," in which bacteria crank up their mutation rate when they undergo stress. But hold onto those Mendelian charts. These cress plants are wonderfully weird–but no more wonderfully weird than hawkweed.