Dietary Preference: Hold the Pickles

posted in: Science | 25

DNA barcoding can be used to determine dietary preference, telling us how species can co-evolve when resources are limiting.

My son always orders his hamburger without pickles. He is six and this has been his preference ever since he could communicate. Meanwhile, his twin sister appears to live on a diet of chips and chocolate milk. While the boy avoids pickles, he consumes Korean, Japanese, Thai, and Texas cuisine with zeal. I am often left wondering what causes her to be a picky junk-food junkie while he experiments – except with pickles! Maybe part of the difference is permissive parenting (guilty!)– but is there also a genetic component to dietary preferences?

Pickle600
Pickles are a common food aversion. Credit: National Institute of Health. Wikimedia Commons

Some eating habits are genetic

frank-chips-small
“I’ll just have one I think…” Credit: KJHvM

Our species has set boundaries on eating habits, but we are remarkably varied within these boundaries, as my twins demonstrate. Research has shown that these variations are not entirely cultural or learned. Genes and culture appear to have co-evolved to produce variation in dietary habits (1). In such cases as lactose tolerance in adults, this co-evolution is well understood. It is less well understood in other cases, such as favism, where sufferers cannot make an enzyme that affects the circulation of sugar in their bodies. Very often, this condition is found in regions where there is also malaria. This may mean that the mutant red blood cells found in people with favism protect them from malaria.

Dietary preference and ecology

Differences in dietary preference appear to also allow otherwise similar species to coexist in the same environment. A guild (or ecological guild) is any group of species that utilize the same resources. Let’s imagine we live in an ice-cream store that has 31 flavors but only limited amounts of each flavor. If we all preferred chocolate-chip ice cream, life would be stressful. Life would be easier if we had different preferences. Even if we have different preferences, life gets complicated if we run out of our preferred flavor!

In the real-world, evolutionary biologists look at dietary differences between grazers and browsers. Grazing animals feed primarily on grass. Grass can be difficult to digest and is wearing on the teeth. A browsing animal, on the other hand, feeds on leaves, buds, and the like. Such food is easier to digest, so that the digestive system does not need to be quite so big. A browser and a grazer should be able to live in the same guild without much conflict. But what happens when multiple grazers occupy the same guild? This sounds complicated and confrontational!

DNA barcoding can tell us about dietary preference

A recent paper in the Proceeding of the National Academy of Sciences (PNAS) used DNA barcoding to investigate dietary preferences among large herbivores in Africa (2).

Zebra eating
Grevy’s Zebra eating, by bobosh_t. Wikimedia Commons

DNA barcoding compares short genetic markers in the specimen DNA with reference sequences. Like the barcodes on items in the supermarket, each DNA barcode indicates a particular species. The authors collected feces from large mammalian herbivores in the semi-arid African savanna and used barcoding to see what plant material the animals had consumed. They measured diet breadth, composition, and overlap for seven abundant herbivore species. These species ranged from almost-exclusive grazers to almost-exclusive browsers. For example, zebras get most of their calories from grasses (99%) while small antelopes called dik-diks get less than 1% of their calories from grasses. While this technology is new for dietary studies and ecology, some findings confirmed previous studies. For example, dietary overlap was greatest within species and between species that were similar in body size.

DNA barcoding also allowed more sophisticated data analysis. Diets were strongly divergent across species, irrespective of feeding guild. Grazers ate similar total amounts of grass but different suites of grass species. For example, two different zebra species each ate about 45 plant species, but 15 grasses differed significantly between their diets. DNA barcoding allowed the researches to identify nuances in the diets that could not be seen using ‘older technology’. These results support the theory that large mammalian herbivores partition food resources in order to coexist. This partitioning of resources may be due to dietary preference.

Unlike our fantasy ice cream shop, diets in the savanna can vary seasonally. These data from herbivore feces were gathered in a wet season when food was abundant. Such abundance likely masks some of the differences among species that occur in times of food insecurity.

Closing thoughts about dietary preference

This research shows that DNA barcoding can be used as a universal tool for dietary traceability. The technique has created the clearest picture yet of grazing habits of animals on the semi-arid African savanna.

i_060a
Jack Spratt and spouse, Mother Goose’s Nursery Rhymes

To go back to the ice cream shop analogy, this technology is like being able to tell retroactively which kind of ice cream a person prefers, including the specific flavors and quantities. It’s like being able discriminate between chocolate-chip ice cream lovers and chocolate-chip cookie dough lovers. For people in an ice cream shop, we can just ask or observe what types of ice cream they like, which is a good thing, because it’s unlikely there’s enough DNA in ice cream to detect in a person’s poop. But for herbivores on the savanna that would be difficult to observe all the time, this DNA barcoding technology could help us learn which plants the animals eat. [Paragraph edited for clarity on Sept 30 by Anastasia.]

Thankfully, I am no longer dealing with twins in diapers. While this does not diminish my curiosity about their differences in dietary preferences – it does negate my ability to readily research the issue.

Finally, have I overlooked a gender effect? After all, such a difference could greatly affect demand for scarce resources. Remember Jack Sprat? He could eat no fat. His wife, unnamed, could eat no lean. But between them both, they licked the platter clean.

References

  1. Krebs JR. The gourmet ape: evolution and human food preferences. Am J Clin Nutr. 2009 Sep;90:707S-11S.
  2. Kartzinel TR, Chen PA, Coverdale TC, Erickson DL, Kress WJ, Kuzmina ML, Rubenstein DI, Wang W, Pringle RM. DNA metabarcoding illuminates dietary niche partitioning by African large herbivores. Proc Natl Acad Sci U S A. 2015 Jun 30;112:8019-24.
Follow Kendal Hirschi:

Kendal D. Hirschi received his PhD in plant pathology from the University of Arizona, Tucson, Arizona. He is a professor of pediatrics and human molecular genetics at Baylor College of Medicine in Houston, Texas, and the associate director of research at the Vegetable and Fruit Improvement Center at Texas A&M University. Hirschi’s research works on understanding the basic mechanisms of plant nutrient transport and simultaneously leveraging this knowledge to impact nutrient content in agriculturally important crops.