The liver’s carbon fixation
The possibility that insects can harness solar energy (SN: 1/15/11, p. 8) is no less fascinating than the ability of the mammalian liver to do the light-independent part of photosynthesis: carbon fixation. When concentrations of the amino acid methionine rise after a high-protein meal, the liver shifts gears to get rid of the excess via activation of a specific transmethylation pathway requiring the amino acid glycine as a methyl acceptor. This also sets in motion what I call the “glycine generator” — a short cycle involving two reversible folate-requiring enzymes cranking out two moles of glycine for each mole of serine, ammonia and carbon dioxide. Copying this aspect of nature on an industrial scale might enable the re-cycling of substantial amounts of carbon dioxide.
Joel Brind, New York, N.Y.
Brind is a professor of biology at Baruch College, the City University of New York.

Light on genetic dark matter
“Genetic dark matter” (SN: 12/18/10, p. 18) might be hiding in plain sight. For over two decades, abundant variation in the number of tandemly repeated units in microsatellite and minisatellite DNA has been used for genetic fingerprinting. For years, this variation has been widely regarded as functionally meaningless. For much of that time, several biologists (including myself) have hypothesized that such repeat-number variation might help account for heritable variation in certain traits.

Most such repeats are indeed found in genomic regions that lack known function. But among the hundreds of thousands of repeats scattered throughout the human genome are many that are closely associated with genes. Lots of genes, perhaps most, include at least one variable tandem repeat sequence at sites where the number of repeat units can influence gene function. Nevertheless, most attention (and most research investment) remains focused on single nucleotide polymorphisms (SNPs) for the very simple reason that collecting vast data sets on SNPs has become cheap and easy. Although surveying repeat number variation and linking this variation with phenotypic differences are technically challenging, biologists should not lose sight of this very visible source of heritable variation.

Science News has covered some of the relevant studies in past articles (SN: 12/18/04, p. 387; SN: 1/31/09, p. 26). The subject of repeat number variation might be suitable for an article that could shed light on many inter-related topics — triplet repeat diseases, evolutionary facilitation, molecular genetics, genomic diversity, etc.
David G. King, Carbondale, Ill.
King is an associate professor of zoology at Southern Illinois University.

Heartburn drugs’ pros and cons
Nathan Seppa’s article (“It’s enough to give you heartburn,” SN: 12/4/10, p. 30) edges toward tabloid in tone. Proton pump inhibitors and the like have saved many lives and reduced nearly to vanishing the need for peptic-ulcer surgery, and, as Seppa points out, are important in the control of common and dangerous gastroesophageal reflux disease. PPIs have proved remarkably well tolerated in general. Obviously, like all drugs, they should be used prudently.

That said, in addition to the unusual untoward effects of PPIs listed in the article, two more might be mentioned. Because hydrochloric acid is a first-line defender against gastrointestinal infection, the risks of other gastro-intestinal infections besides C. difficile are likely increased. PPIs may also cause tubulointerstitial nephritis.
Harvey E. Finkel, Brookline, Mass.
Finkel is a physician.

Showing that overuse of drugs can be injurious to your health is not only important from the standpoint of an individual’s risk, but also for the overwhelming bill for these drugs, paid sometimes by the individual and at other times by the insurer. There is always a risk-benefit ratio for drugs. Added to the equation should be the cost of profligate use of expensive drugs.
Nelson Marans, Silver Spring, Md.

One cat not lapping
One of my cats does not lap. She puts her paw in the water dish and then licks her paw. I have left your December 4 issue (“Cats lap liquids with a flick of the tongue and fluid dynamics,” SN: 12/4/10, p. 5) open to the pictures of the cat drinking, so perhaps she will get the idea.
Emily Johnston, Westminster, Md.

Shell-less snails
In “Snails shed shells in one fell swoop” (SN: 11/6/10, p. 9), you describe an experiment in which baby snails exposed to the metal platinum develop without external shells. This is hypothesized as a rapid evolutionary mechanism that could explain such events as the transition from snails to slugs.

However, one must inquire whether the creatures’ DNA was modified. If these newly shell-less snails were allowed to mature and procreate, would their progeny (in the absence of platinum) still develop shells? If so, then what we are observing is more akin to a birth defect, not evolution.
Irwin F. Kraus, Attleboro, Mass.

The scientists didn’t demonstrate that the changes in snail body plan after exposure to platinum were heritable, and we don’t know if the next generation would have had shells or not. What the study did show is that a drastic change in body plan doesn’t necessarily result from slow, incremental changesbig shifts in development, however induced, can happen quickly. The work suggests that it’s possible that some ancient snail could have acquired genetic mutations similar to those induced by the platinum that may have had a large effect on body plan and perhaps on snail and slug evolution.Rachel Ehrenberg

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