Peanuts: more genetically diverse than expected


Virginia-type peanuts, the big ones sold in the shell or used in cocktail nut mixes, are more genetically variable than previously assumed, according to a new study from North Carolina State University. Before now, cultivated peanuts showed very little variability for molecular markers, leading some to conclude that there was virtually no genetic variation in the species. However, anyone who has strolled through a seed nursery of diverse peanut lines or compared elite with unimproved varieties would know that this cannot be true.

Crop scientist Dr. Susana Milla-Lewis used microsatellite markers, or “simple sequence repeats,” that had been previously reported to vary across genetically diverse types of peanut, and found that they also varied within the commercially available Virginia-type varieties. The results were reported in the July-August 2010 edition of the peer-reviewed journal Crop Science, published by the Crop Science Society of America.
While plant breeders want their variety to succeed and dominate the seed market, widespread use of a single variety can lead to genetic vulnerability to diseases and pests. There are examples of disastrous results from genetic vulnerability, including the southern corn leaf blight epidemic that occurred in the United States in 1970. And it isn’t always caused by the use of a single variety: it can be because of a high degree of genetic relatedness of the varieties that occupy most of a crop’s acreage.
Milla-Lewis extracted DNA from varieties of Virginia-type peanut released since 1940. The study revealed that, instead of the expected steady, gradual narrowing of genetic variation across time, variation actually increased from the 1940s to the 1970s, then declined a bit in the subsequent two decades and went back up in the 2000s.
Variation declined after the 1970s because of the frequent use of the wildly successful variety Florigiant as a parent in crosses made while that variety enjoyed 15 years of dominating the Virginia-type seed market. More recently, exotic parents used to introduce greater levels of disease resistance have brought greater genetic variation into the breeding population. However, the level of genetic richness has not yet returned to the earlier diversity.
“Results of this study indicate that peanut breeders have managed to achieve steady genetic gains for yield while maintaining levels of genetic diversity in a crop that has a narrow germplasm base to being with; that is quite an achievement,” says Milla-Lewis.
Research is ongoing at North Carolina University to use molecular markers to investigate genetic diversity in the runner-market type, the smaller peanuts that are used to make peanut butter and candies. Studies are also underway to associate specific markers with useful characteristics such as early maturity and disease resistance.
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