Stay Right There, Mendel

Back in March I described a provocative paper that suggested that plants might be able to get around Mendel's laws of heredity. Reed Cartwright, the grad student behind De Rerum Natura, left a comment expressing some deep skepticism. Now he reports that he and Luca Comai of the University of Washington have published a letter in the journal Plant Cell. You can read the letter for free. (There's another paper commenting on it in the journal, but it requires a subscription.)

In the original experiment, scientists bred plants, noting which version of a gene called hothead got passed down to new generations and which did not. Sometimes plants were born with a version of hothead that appeared to have been lost in previous generations. The scientists suggested that somehow the plants were storing a back-up copy of the hothead allele somewhere.

Comai and Cartwright argue that something more conventional was actually happening. Thanks to how the scientists carried out the experiment, they inadvertently caused their plants to mutate much more often than normal plants would. In all those mutations, some happened to alter the hothead gene, changing it back to its ancestral form. Comai and Cartwright propose that pollen grains containing the newly mutated hothead gene could do a better job of fertilizing eggs than the other version. The combined effect of a higher mutation rate and selection produced the strange results that seemed to violate Mendel's laws.

This is turning into a fascinating debate--and one that seems to have some parallels with another debate that Cartwright doesn't appear to have mentioned.

In the 1980s, some scientists claimed to have found evidence of what they called "adaptive mutation." Conventionally, mutations were seen as occurring pretty much randomly, with no influence from the environmental challenges organisms face. It just so happens that some of those mutations help some individuals reproduce more than others. But scientists did experiments that suggested that bacteria could rapidly acquire the mutations they "needed" when faced with a challenge. The classic example of adaptive mutation involved E. coli that was given lactose to eat. But before the bacteria got a chance to enjoy this meal, the scientists inserted mutations in the gene that produced an enzyme that's essential for digesting lactose. Remarkably, the bacteria did not starve. Instead, they rapidly acquired mutations to the lactose-digesting gene that let it function again.

Almost 20 years later, some scientists still argue that this represents a weird and wonderful exception to the conventional picture of evolution. But others have expressed serioius skepticism. It's likely, they argue, that a pretty ordinary series of events produces a seemingly strange result.

Introducing a mutation into the lactose-digesting enzyme cripples it, they argue, but doesn't completely destroy it. On its own, this crippled enzyme can't provide enough food for E. coli to stay alive. But every now and then genes get accidentally duplicated. Extra crippled genes boost a microbe's ability to digest lactose, allowing E. coli to get just enough energy to reproduce. Microbes with extra copies of the gene are strongly favored by natural selection, so that more and more copies spread through the population. And with all these extra copies of the crippled gene floating around the population, the odds are raised that a random mutation will restore it to its normal function. You can read the latest version of this attack on adaptive mutation here.

This kind of evolution may actually matter a lot to E. coli and other microbes in the wild, giving them the ability to adapt to new challenges. The mechanism that Comai and Cartwright propose for plants, on the other hand, may only have bearing on the particular experiment in question. But it's still intriguing in both cases to see how conventional evolutionary biology may be able produce some results that look anything but conventional.