Wild plant or food plant?

Fruit rinds provide new clues about crop domestication

Distinctly sculptured opaline phytoliths in soil and plant remains tell archaeologists which plants were present thousands of years ago. However, the production and purpose of these tiny glassy structures common in plant tissues is poorly understood. Dolores Piperno at the Smithsonian Tropical Research Institute (STRI) in Panama and colleagues predict that a single genetic locus controls both lignin and phytolith production in squash (Cucurbita spp.), making phytoliths even better evidence of plant domestication events.

Sometime after the last ice age, inhabitants of the western hemisphere began to select and cultivate food plants. Plant remains at archaeological sites may not be well preserved, but features often contain phytoliths, tiny silica dioxide deposits from plant tissues. These destinctive microfossils have been used increasingly over the last decade in studies of plant domestication, because they clearly identify a number of different crop plants and their wild progenitors.

However, little is known about how plants make phytoliths, and why.

A 1997 study showed that a single gene in maize controls phytolith production, lignification and silification, all characteristics modified when modern maize diverged from its wild ancester, teosinte.

On the hunch that the same might be true for squash, Piperno and Irene Holst from STRI with Linda Wessel-Beaver from the Univeristy of Puerto Rica and Thomas Andres of the Cucurbit Society set about to characterize the rinds of 148 fruits from wild and cultivated species of the squash genus, Cucurbita. They also crossed the plants and characterized the rinds of their offspring.

Thin sections of the soft rinds of domesticated species lacked lignification and big, scalloped phytoliths. All of the species with hard rinds (both wild and domesticated) were lignified and contained phytoliths.

One to one correnspondence between lignification and the presence of phytoliths plus identical segregation patterns for lignin and phytoliths in the fruits of first and second generations of hybridized specimens led the authors to present results in the Proceedings of the National Academy of Sciences postulating a single locus called “hard rind” (Hr) coding for this suite of plant defensive characters in Cucurbita.

They demonstrated that the distinctive shapes and surface sculptoring of the phytoliths are determined by the different types of cell configurations in Cucurbita rinds, as the phytoliths are formed in places in the rinds that are taxonomically useful for identification when rind specimens are analyzed by archaeobotanists.

Identification of a single suite of plant defensive characteristics determined by a single genetic locus will help archaeologists to determine whether plants in ancient samples were domesticated or wild varieties.