Four Potatoes Species NOT Seven

"One potato, two potato, three potato, four" turns out to be exactly right--when classifying cultivated potatoes, that is.

Scientists at the United States Agricultural Research Service (ARS) and the International Potato Center (CIP) in Peru [the country considered the centre of origin of potato species] have used morphology--the outward appearance of a plant--in combination with molecular markers to revise the number of potato species from seven to four.

Until recently, potato species designations have been based primarily on morphological characteristics and estimates--often incorrect--of how many chromosome sets they possessed.
Botanist David Spooner works in the ARS Vegetable Crops Research Unit, Madison, Wis. His initial research with CIP colleagues indicated that morphological variations [the commonly used botanical tool] among cultivated potatoes were not reliable indicators of a particular species.

They then examined DNA molecular markers from 742 cultivated potato varieties and eight wild relatives of potatoes. Based on results from this study and previous studies, Spooner and CIP lead scientist Marc Ghislain concluded that cultivated potato varieties could most accurately be assigned to one of four species - not the seven currently used.

They refined the species designations by checking each potato variety for the presence of one particular DNA mutation. This characteristic mutation distinguishes between potatoes from the Chilean lowlands and potatoes from the high Andes.

The domestic potato, Solanum tuberosum--the type eaten around most of the world--is one of the four recognized species. This is by far the most common potato species and has from two to four sets of chromosomes. The less common potato species--S. ajanhuiri, S. juzepczukii and S. curtilobum--have two, three and five sets of chromosomes, respectively. These can often be distinguished from each other by morphological data.

This new system of species classification eliminates much of the guesswork that previously served as the foundation for the potato classification system. Potato breeders will benefit greatly from a classification system that groups related collections by combining traditional morphological with modern molecular methods.

A paper reporting the results of this study was published this week in the Proceedings of the National Academy of Sciences of the United States of America.

While it may not seem much, successful breeding of new varieties does rely significnantly on understanding the genetics of the material being used. It does become more complex with widely used cultivated plants, due to the enormous influence of man [and sometimes serendipity] on developing varieties over the centuries. Knowing what you are working with allows better planned breeding programs.