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Morphometric Analysis of Sunfl

ower (Helianthus annuus

L.) Achenes from Mexico and Eastern North America 1

SOMAYEH S. T ARIGHAT2, D AVID L. LENTZ*,2, STEPHEN F. M ATTER

2, AND

R OBERT B YE3

2Department of Biological Sciences, University of Cincinnati, Cincinnati, OH 45221 USA 3 Jardín Botánico, Instituto de Biología, Universidad Nacional Autónoma de México, 04511 México,DF, México

*Corresponding author; e-mail: [email protected]

Morphometric Analysis of Sunflower (Helianthus annuus L.) Achenes from Mexico andEastern North America. Sunflower (Helianthus annuus L.) has played a major role in theevolution of agricultural systems in the Americas. The discovery of ancient domesticatedremains from archaeological deposits in pre-Columbian Mexico offers new dimensionsto widely accepted viewpoints on the domestication pattern of H. annuus. AlthoughAmerican sunflower populations north of Mexico have been examined extensively, Mexicanindigenous domesticated landraces have not been studied in any detail. In this study,we morphologically assessed wild and domesticated sunflower achenes from Mexicoand compared them to similar datasets from eastern North America. Additionally, weevaluated the utility of four computer-assisted shape measurements in discriminatingbetween wild and domesticated sunflower achenes (fruits) and compared variation inachene size among modern wild and cultivated populations from both Mexico and the U.S. We

found that, of the shape parameters tested, none were informative in distinguishing wild achenesfrom domesticated varieties. Subsequent size analysis, using conventional parameters of length,width, and thickness, showed that modern wild populations from Mexico had smaller achenescompared to modern populations from eastern North America. Domesticated achenes unearthedfrom Mexican archaeological sites, however, were significantly larger than the early domesticatedspecimens recovered from eastern North America. Our methodological results indicatethat variation in archaeological sunflower achenes is better described by conventionalsize parameters rather than computerized shape analysis.

Análisis Morfométrico de Aquenios de Girasol (Helianthus annuus L.) de México y Este delos Estados Unidos de América. El girasol (Helianthus annuus L.) ha jugado un parte importanteen la evolución de los sistemas agrícolas en las Américas. El descubrimiento de restos domésticosantiguos de yacimientos arqueológicos en el México precolombino ofrece nuevas dimensiones

a los puntos de vista ampliamente aceptados en el patrón de domesticación deH.

annuus. Aunque las poblaciones de girasol de México y Estados Unidos de América (EUA)han sido examinadas ampliamente, los restos arqueológicos domésticos de las razas indí-genas de México no han sido estudiados en detalle. En este estudio, se evaluaron morfo-lógicamente aquenios de girasol silvestre y domesticado de México y se compararon conconjuntos de datos similares del este de EUA. Además, se evaluó la utilidad de cuatromediciones de forma asistidas por computador en la discriminación entre los aquenios(frutos) de girasol silvestre y domesticado además la variación en comparación de tamaño entrelos aquenios modernos de poblaciones silvestres y cultivadas de México y los EUA. Se encontróque de los parámetros probados ninguno fue determinante para distinguir entre los aqueniossilvestres y las variedades domesticadas. El análisis de tamaño, utilizando por los parámetrosconvencionales de la longitud, anchura y espesor, mostró que las poblaciones modernasde México presentan aquenios más pequeños, en comparación con las poblaciones modernasdel Este de EUA. Sin embargo, los aquenios domesticados desenterrados de los sitios arqueoló-

1 Received 19 April 2010; accepted 21 July 2011;published online 13 August 2011.

Economic Botany , 65(3), 2011, pp. 260–270© 2011, by The New York Botanical Garden Press, Bronx, NY 10458-5126 U.S.A.



gicos de México fueron significativamente más grandes que los ejemplares domesticadosy recuperados del Este de EUA. Nuestros resultados indican que la variación metodo-lógica para aquenios de girasol arqueológicos está mejor descrita por los parámetros detamaño convencional en lugar del análisis de la forma informática.

Key Words: San Andres, Cueva del Gallo, shape factors, sunflower, Helianthus annus.

IntroductionRecent reports of pre-Columbian sunflower

(Helianthus annuus L.) in Mexico (Bye et al.2009; Lentz et al. 2001, 2008a , b; Pope et al.2000) have focused attention on sunflower populations from that country. Unfortunately,

very little is understood about sunflower in

Mexico, either wild or domesticated. This study is an effort to provide comparable morphometricdata for assessment of modern and archaeologicalsunflower populations from Mexico and easternNorth America. Aside from its importance as oneof the world’s major oilseed crops (Steffansson2007), the study of sunflower bears an evolu-tionary significance to our understanding of ancient cultures of the pre-Columbian Americas.Moreover, the study of germplasm from warmer climes takes on an additional significance as

global warming and other aspects of climatechange become manifest. Archaeological records of H. annuus consist

mainly of achenes because these are the parts of the plant most likely to be preserved. Convention-ally, achene size has been the key indicator of domestication in H. annuus and, through thestudy of modern wild and domesticated popula-tions, achenes longer than 7 mm have beencategorized as structures derived from domesticatedplants (Heiser 1985). Determining whether anunearthed achene is domesticated or wild anddating these remains play a key role in reconstructing the domestication trajectory of H. annuus .

Seed identification and classification coupled withthe application of novel methods, which are faster and more reliable, have significant economic andtechnical importance in studies of seed populations(Granitto et al. 2000). In this context, moderncomputerized shape analysis methods have addedto the knowledge base developed through conven-tional size analysis. Image processing methodsapplied to plant morphology have been useful in

distinguishing wild from domesticated varieties as well as crop seeds from contaminants (Zayas et al.1989). Russ and Rovner (1989) utilized computer-assisted image analysis of plant phytoliths to

discriminate between wild Zea varieties and domes-ticated maize. The main measured shape factors,including formfactor, roundness, solidity, curl, andaspect ratio, proved to be informative in distin-guishing wild and domesticated maize populations.

Despite the growing application of computers inmorphometric approaches, there are not many

examples of computer-assisted studies involving seed populations (Rovner and Gyulai 2006). Travisand Draper (1985) used an image processing system to distinguish crop seeds from contaminant

weed species by measuring the size of the seeds andone shape parameter (formfactor). Their resultsindicated that fi ve of seven crop species could beseparated from weed species based on thesecharacteristics and that most of the weed seedscould be distinguished from each other using thissystem of measurements. Granitto and colleagues

(2000, 2002, 2005) also evaluated computer-assisted image processing methods examining mor-phometric (size and shape) characteristics of seedsand these techniques have been shown to be highly effective in automated weed seed identification.

Rovner and Gyulai (2006) developed andapplied new seed taxonomic approaches using computer-assisted image processing. These freshapproaches include measuring shape parameterssuch as formfactor, curl, convexity, and round-ness, which cannot be obtained through manualmethods. Heretofore, standard classification of seeds in sunflower has been based on sizeparameters usually represented by a composite

value called a “size index ” (length x width).Computer-analyzed shape features of seeds, whichmay be informative in distinguishing wild seedsfrom domesticated varieties, have not beenutilized in examining H. annuus . Developing new means of seed differentiation would enhanceclassification and identification endeavors inarchaeological contexts as well as in modern seedtaxonomy. Fast and reliable computer-facilitated

approaches could be of great benefit in this fieldof scientific research.

We sought to test the utility of modern imageprocessing approaches by measuring and assessing

261TARIGHAT ET AL.: MORPHOMETRIC ANALYSIS OF SUNFLOWER 2011]



four shape parameters (i.e., formfactor, curl,convexity, and roundness) to discriminatebetween wild and domesticated sunflower achenes. We used a dataset that included

numerous samples of Mexican indigenous culti- vars and wild populations (Figs. 1 and 2)collected in Mexico during the course of a previous study (Lentz et al. 2008a , b). Further-more, we compared size information frommodern data to records of archaeological evi-dence from Mexico and eastern North America (ENA) in search of further insights into domes-tication patterns of this important crop plant.

Materials and Methods

Groups of seeds collected from Mexico andENA, both wild and domesticated, were used for this study. ENA indigenous domesticated land-races were obtained from the United StatesDepartment of Agriculture (USDA). Mexican

wild and indigenous domesticated landraces andENA wild populations were collected by Lentz

and Bye during field studies conducted from2001 through 2005. To minimize the potentialeffect of introgression between cultivated sun-flowers with conspecific wild H. annuus popula-

tions on achene size (Rieseberg et al. 1999),diligent sampling was conducted by collectors,

who selected only plants exhibiting wild pheno-type characteristics (i.e., multiple slender pediclesand small flower heads; Doebley et al. 2006).Overall, seeds from 27 wild and domesticatedpopulations including 14 wild (10 Mexican and 4eastern North American) and 13 domesticatedlines (8 Mexican and 5 eastern North American)

were examined in this study (Fig. 1). A completedescription of the Mexican sunflower domesti-

cates can be found in Tarighat (2010). For mostof the populations, 100 seeds were measured, butfor fi ve populations, only 40 seeds were availablefor measurement.

Three principal size dimensions, namely length, width, and thickness, were measuredusing a Vernier caliper (accurate to 0.01 mm)

Fig. 1. Map of North America showing modern distribution of wild sunflower (Lentz et al. 2008a ; Heiser et al.

1969). Closed circles show the collection areas of modern wild H.annuus in Mexico and ENA. Stars indicatearchaeological sites where domesticated sunflower achenes were discovered. Three well-preserved achenes (330–250 B.C.E.) were unearthed at the Cuevo del Gallo site and a seed and achene (2875–2482 B.C.E.) werediscovered at the San Andrés site. Two of the oldest ENA sites with clearly domesticated sunflower remains werethe Higgs site (1259–829 B.C.E., Brewer 1973), and Marble Bluff (1264–912 B.C.E., Fritz 1997).

262 ECONOMIC BOTANY [VOL 65



for each individual seed. Due to the irregular shape of sunflower seeds, the widest points for both length and width were recorded. A single

value, size index = length (mm) x width (mm), was calculated for each seed.

Shape factors were measured using an image-processing program (MATLAB version 7.4).Modern seeds were placed in groups of 50 seedsfor domesticated and 100 seeds for wild popula-

tions per image and randomly numbered by computer. Images of seeds of all populations,taken using a digital camera, were analyzed for four shape factors. Definitions of the shape

features follow Russ (2007) and are provided inTable 1.

Computer-assisted shape measurements wereobtained using digital images of more than oneobject in an image. Color digital images wereconverted to binary (black and white) images, whilean edge detection algorithm traced the exterior boundaries of each object using a threshold value.

We optimized the threshold value for each image toget the best boundary detection. Parameters werethen simultaneously measured in a two-dimensional

binary image for each individual object. TheMATLAB script developed for image analysis isavailable upon request from the corresponding author.

We initiated the computer-assisted measure-ment process using four selected shape parame-ters: formfactor, roundness, convexity, and curl.Formfactor is the configuration of the perimeter relative to the area of an object. A perfect circlehas the smallest ratio of perimeter to area and a formfactor value of 1. Departure from a true

circle will result in a smaller formfactor value(Rovner and Gyulai 2006). Roundness is similar to formfactor but it is measured using the length(longest chord) of the objects rather than their perimeters. This property of roundness makes itmore sensitive to how elongated the object is,

whereas formfactor is more responsive to irregu-larity of the outline of the object. There are many examples where formfactor and roundness of a single object vary significantly (Russ and Rovner 1989), suggesting that both parameters areimportant for a full evaluation of an object.Convexity is used to measure the degree of irregularity of an object, quantified by the ratio

Fig. 2. Sample digital images of domesticated and

wild Mexican sunfl

owers set for computer-assisted imageanalysis. Images of multiple achenes were uploadedto the MATLAB image analysis program to obtain shapemeasurements.

T ABLE 1. DESCRIPTION AND FORMULA OF SHAPE FACTORS ASSESSED IN THIS STUDY (RUSS 2007).

Variable Description Formula

Formfactor Departure of an object’s perimeter from a smooth circle

4π area/perimeter 2; it is 1.0 for a perfectcircle and diminishes for irregular shapes.

Convexity The ratio of a convex perimeter to perimeter

Convex perimeter/perimeter. It diminishesif there are surface indentions.

Curl Degree of departure of anobject from a straight line Length/*fi

ber length

Roundness Departure of an object from a circle 4 area/ π x length2

*Fiber or skeleton of a shape is obtained by erosion (sequential removing) of edge pixels of that shape.




of the true perimeter of the object to its fittedconvex hull. Curl is used to quantify thedeparture of an object from a straight line. Dueto the significantly high systematic error of

measurement in sunflower achenes when using the curl parameter and confident in the fact thatcurl provides the least discriminating power, wecompleted the analysis using only the remaining three feasible shape parameters: formfactor, con-

vexity, and roundness. Assessing the utility of these three factors in discriminating between wildand domesticated seeds was one of our principalobjectives in this study.

Statistical analyses of the data were performedusing JMP analysis software (version 7). Data

points were inspected for normality and homo-geneity of variance. For non-normal data we useda log10 transformation. To determine whichcontinuous variables (shape or size features) coulddiscriminate between the main sunflower catego-ries, our data sets were subdivided into four groups: ENA wild, Mexican wild, ENA indige-nous domesticated, and Mexican indigenousdomesticated. To distinguish among the achenecategories we conducted a discriminant function

analysis which generates a set of discriminantfunctions based on linear combinations of con-tinuous predictor variables. These variables pro-

vide the most effective ways to distinguish among

the seed categories and thereby render a measureof the overall similarity among them (Rashed etal. 2008). Six measured characters includedachene length, width, thickness, achene form-factor, roundness, and convexity, all of which

were used in the analysis as predictors for membership in one of the two wild or one of the domesticated groups.

To determine if a significant difference existedamong sunflower populations of Mexico and easternNorth America, discriminant function analysis

(DFA) and analysis of variance (ANOVA) wereperformed on size measurements. Separate one-way

ANOVA tests comparing the size index variable(length x width) within wild populations anddomesticated populations were carried out. Theresults were compared to size differences betweenarchaeological specimens unearthed from Mexicoand ENA (Tables 2 and 3). In addition, weanalyzed climatic data (average annual temperature,average annual rainfall, and elevation) of wild

T ABLE 2. AVERAGE SIZE INDEX (LENGTH X WIDTH) OF ARCHAEOLOGICAL DOMESTICATED REMAINS RECOVEREDFROM MEXICO AND EASTERN NORTH AMERICA. BOTH HIGGS (BREWER 1973) AND HAYES (CRITES 1993) ARE EARLY

ENA SITES WITH SUNFLOWER EVIDENCE, BUT HAVE NO ACHENES.

Site Time PeriodNo. of

Achenes Index (LxW)† References‡

Patrick, TN Early, Middle Woodland(318 B.C.E.–C.E. 287)*

3 21.2(30.8) 1

Rose Island, TN Early, Middle Woodland(318 B.C.E.–C.E. 287)*

4 17.8(25.3) 1

Newt Kash Hollow, KY Late Archaic, Early Woodland(1162–369 B.C.E.)*

14 29.2 2, 3

Marble Bluff, AR (34-23-345) Late Archaic (1264–912 B.C.E.) 19 27.2(39.2) 4Marble Bluff, AR (34-23-327) Late Archaic (1032–920 B.C.E.) 14 24.5(35.2) 4Eden’s Bluff, AR Early, Middle Woodland

(170 B.C.E.–C.E. 50)4 25.9 5

Salts Cave, KY Early Woodland (654–416 B.C.E.)*57 17.4(24.4) 6,7

Salts Cave, KY (feces) Early Woodland (970–660 B.C.E.)* 1,000 16.8(23.7) 6,7Mammoth Cave, KY Early Woodland (539–239 B.C.E.)* 80 15.1(21.7) 2, 6Cueva del Gallo, Mex. Formative (330–250 B.C.E.) 3 57.5 5San Andrés, Mex. Late Archaic (2875–2575 B.C.E.) 1 36.9(51.9) 8

* Converted from conventional radiocarbon dates to calibrated dates using Fairbank ’s calibration curve (Fairbanks et al.2005).

† Numbers in parentheses represent values that were enhanced by correction factors (see Yarnell 1978). These are oftenused to compensate for achene shrinkage during carbonization.

‡ References: 1 = Chapman and Shea 1981; 2 = Watson and Yarnell 1966; 3 = Yarnell 1978; 4 = Fritz 1997; 5 = Lentz etal. 2008b; 6 = Yarnell 1981; 7 = Gardner 1987; 8 = Lentz et al. 2001.




sunflower locations using three single linear regres-sion tests to investigate the relationship betweenclimatic factors and achene size index. Climatic data at each collection site, including temperature, rain-fall, and elevation, for 14 wild populations in thespecific year of seed collection, were derived fromrecords of the Servicio Meteorological Naciónal deMéxico (SMN) and the National Climatic Data Center (NCDC) of the U.S. National Oceanic and

Atmospheric Administration (NOAA).Results

Discriminant function analysis (DFA) indi-cated that size factors (length and width) con-tributed significantly to the discriminant functionand consistently maximized the differencebetween the two wild and domesticated groups(Wilks’ Lambda= 0.06, F3, 2532=11417.8, P<

0.0001) (Fig. 3). According to the DFA of shapefactors, the morphological differences among populations were significant (Wilks’ Lambda=0.81, F3, 2532=195.5, P= 0.001); however, the

difference between wild and domesticatedachenes was much less than that seen using sizefactors (Fig. 4). Size measurements of modern

wild and domesticated populations are presentedin Table 4.

Analysis of variance (ANOVA) results (Table 5)showed that the achenes of modern wild sun-flower from ENA (average size index= 12.61)

were significantly larger than modern Mexican wild achenes (index size=8.6) (F1, 1338=1258.8,P<0.0001). The average index size of indigenous

domesticated seeds from Mexico ( X ¼ 64:

06) was smaller than the size of those from ENA ( X ¼ 71:62) (F1, 1190=21.4, P<0.0001). Addi-tionally, our sunflower populations showed high

within-population variation in their achene sizeindex (ANOVA; ENA wild F3, 396 =192.4, P<0.0001; Mexican wild F9, 930 =138.7, P<0.0001).

ANOVA results of archaeological sunflower speci-mens, however, showed that Mexican domesti-cated achenes were significantly larger than

domesticated samples from ENA (F1, 9 =22.53,P =0.001) (Table 5).Linear regression analysis testing the associa-

tion between climatic factors and achene sizeyielded nonsignificant results for rainfall (R 2 =0.071, P= 0.35) and elevation (R 2 =0.013, P=0.69). Average annual temperature, on the other hand, was significantly associated with size index (R 2 =0.543, P= 0.0026) in an inverse manner

T ABLE 3. ACHENE SIZE COMPARISON OF ANCIENT AND

MODERN POPULATIONS FROM MEXICO AND EASTERN

NORTH AMERICA. MODERN SAMPLES WERE DERIVED

FROM BOTH WILD AND DOMESTICATED POPULATIONS

COLLECTED FROM TWO REGIONS OF STUDY .

CategoriesMean Size

Index SD

ArchaeologicalENA Domesticated 28.38 5.79Mexico Domesticated 54.7 3.96

ModernENA Domesticated 71.62 0.24Mexico Domesticated 64.06 0.16ENA Wild 12.61 0.018Mexico Wild 8.6 0.018

Wild Domesticated

Fig. 3. Discriminant function analysis (DFA) of size measurements. The left cluster represents the wild achenesgrouped together, while domesticated achene populations formed a distinct group.




Fig. 4. Discriminant function analysis (DFA) of shape parameters. No separation can be detected between the wild and domesticated achene types.

T ABLE 4. SIZE MEASUREMENTS OF MODERN WILD AND DOMESTICATED SUNFLOWER ACHENE POPULATIONS FROM

MEXICO AND EASTERN NORTH AMERICA .

Length (mm) Width (mm) Thickness (mm)

CategoriesName/ID

No. No. Max Mean Min Max Mean Min Max Mean Min

MexicoIndigenousDomesticated

34110 100 10.4 9.4 8.1 5.0 4.3 3.2 3.3 2.7 1.734116 100 12.4 11.4 10.4 7.6 6.5 5.5 5.6 4.7 3.534111 100 12.2 11.3 10.0 6.1 5.3 4.2 4.0 3.2 2.034055 100 12.3 11.1 9.1 8.0 5.8 3.7 5.2 3.7 2.234109 100 12.5 10.8 8.6 5.5 4.8 3.4 3.8 2.8 2.033656 40 11.9 11.3 10.0 7.7 6.7 5.2 5.8 4.4 2.833655 52 15.4 11.8 8.9 8.6 6.8 5.4 5.7 4.2 2.334175 100 13.3 12.8 11.9 7.2 6.4 5.3 4.8 3.9 3.0

ENA IndigenousDomesticated

Mandan 100 13.1 11.5 9.1 10.3 8.3 5.7 6.6 5.3 3.6Seneca 100 14.8 11.3 8.0 9.0 5.7 4.1 5.3 3.4 1.9Hopi Dye 100 12.2 9.9 8.4 5.2 4.2 3.3 3.7 2.8 1.9 Arikara 100 15.1 11.0 7.1 9.1 5.9 3.8 5.2 3.4 2.4

Hidatsa 100 15.6 12.0 10.0 11.3 7.5 5.5 6.4 4.2 3.2Mexico Wild San Luis

Potosi100 4.5 3.8 2.7 2.8 2.0 1.4 1.6 1.1 0.5

Nuevo Leon 100 4.6 3.9 3.1 2.5 1.9 1.4 1.5 1.2 0.8Coahuila 100 4.7 4.0 3.3 2.3 1.8 1.4 1.6 1.1 0.8Tamaulipas 56 4.3 4.0 3.6 2.5 2.3 1.8 1.5 1.3 0.8Chihuahua 100 5.9 4.9 4.2 2.9 2.4 1.4 2.0 1.4 0.9Zacatecas 100 4.9 4.0 2.9 2.5 2.2 1.7 1.5 1.2 0.7Sonora 84 5.3 4.4 3.3 2.4 2.0 1.5 2.0 1.2 0.7Durango 100 4.9 4.5 3.9 2.5 2.2 1.7 1.5 1.2 0.8Guanajuato 100 4.4 4.1 3.7 2.1 1.9 1.7 1.4 1.1 0.9 Veracruz 100 4.3 4.1 3.8 2.1 1.9 1.7 1.2 1.1 0.9

ENA Wild Missouri 100 5.3 4.8 4.1 2.7 2.3 1.8 1.8 1.3 0.9Tennessee 100 5.4 4.8 4.3 2.8 2.4 1.8 2.2 1.3 0.9 Arkansas 100 6.1 5.3 4.5 3.1 2.8 2.3 1.9 1.6 1.3Illinois 100 5.6 5.1 4.3 3.0 2.6 2.2 1.7 1.4 1.1




where increasing temperature was associated withsmaller size index (Fig. 5). In the Mexicanpopulations, higher values of average annual

temperature were recorded for eastern andcentral Mexico where smaller achene size indices were observed (Table 6).

DiscussionMorphometric analysis of sunflower achene

populations provided a number of importantinsights into both modern populations and historicalinterpretations of this valuable crop plant. Thecomputer-assisted evaluations of sunflower achenesconducted using discriminant function analysis(DFA) revealed that shape analysis techniques wereless effective than techniques commonly utilized inconventional methods to distinguish between wildand domesticated populations of sunflower. Thefour shape parameters measured in this study (curl,roundness, convexity, and formfactor) did not bear suf ficient recognition robustness to recommend

their use in establishing quantifiable differencesbetween wild and domesticated achenes. In sun-flower populations, the transition from wild to

domesticated form is accompanied by dramatically altered achene size but, evidently, not the shapefeatures evaluated in this study. Computer-assistedapproaches may continue to offer cost-effective,rapid, and accurate methods for discriminating between seed populations if appropriate parametersare selected for the species or structures being evaluated, but the parameters tested in this study could not be successfully applied to sunflower achenes. Technical enhancements in differentiationand classification of sunflower achene populationsshould focus on comparative aspects that have beendemonstrated to be effective at discriminating between wild and domesticated types (i.e., achenelength or size index).

Analysis of variance of achene size indicatesthat modern wild sunflower populations fromeastern North America (ENA) have significantly

T ABLE 5. ANOVA RESULTS OF ACHENE SIZES COMPARING MODERN WILD AND DOMESTICATED ACHENES COLLECTED

IN MEXICO AND EASTERN NORTH AMERICA (ENA). THE LAST ROW SHOWS THE RESULTS COMPARING THE

MEAN SIZE INDICES OF PRE-COLUMBIAN DOMESTICATED SPECIMENS DISCOVERED FROM MEXICO AND ARCHAEO-

LOGICAL DOMESTICATED SAMPLES COLLECTED FROM ENA .

Evaluations Error DF MSE F P-value

Modern ENA Wild vs. Mexican Wild 1338 3.31 1361.84 <0.0001Modern ENA Dom. vs. Mexican Domesticated 1194 392.6 42.67 <0.0001 Ancient ENA Dom. vs. Mexican Domesticated 9 47.3 22.53 0.001

7

8

9

10

11

12

13

14

15

A c h e n e S i z e I n d e x

10 12.5 15 17.5 20 22.5 25Temperature

R2 = 0.54,P = 0.0026

Fig. 5. Graph showing the inverse relationship between temperature and average size index in modern wildpopulations sampled from Mexico and eastern North America.




larger size indices than modern wild populationsfrom Mexico. Wild sunflower populations examinedin this study showed high variability in their sizeindices both within and between populations. Our

expectation was that this variation would be highly influenced by climatic factors, but our results suggesta more complicated relationship. Regression analysisrevealed no significant association between size index and two climatic factors, rainfall and elevation, butindicated an inverse relationship between size andtemperature. The largest wild achenes in Mexico

were recorded for populations collected in thenorthern state of Chihuahua, where we found thecoolest temperatures in the corresponding year.Temperature seems to be the important climatic

factor affecting sunfl

ower achene size in North America. This observation makes sense in light of the fact that sunflower is essentially a temperateplant and is well adapted to the climate of the middlelatitudes. Accordingly, our results reflect existing

variation of achenes that largely can be attributed tothe wild sunflower genome and environmentalfactors.

The observed pattern of size variation betweenmodern wild populations becomes more intriguing

when we consider them in relationship to theachene sizes from archaeological contexts. Based onavailable archaeological records of ancient domes-ticated H. annuus from both regions, ancientcultivated sunflowers from Mexico producedachenes larger than the oldest H. annuus discoveredin ENA (Lentz et al. 2008b). These observationsraise provocative questions about the trajectory of crop development in the two regions and thecultural exchanges between ENA and Mesoamerica during pre-Columbian times.

Pre-Columbian Mexico has been identified as a cradle of domestication for other important crop

plants such as squash (Cucurbita pepo), which wasgrown in the highlands of Mexico in 10,000 B.P.(Smith 2006; Sanjur et al. 2002), and maize ( Zea mays ), which is believed to have been domes-

ticated in southern Mexico more than 6,300 yearsago (Burger et al. 2008; Matsuoka et al. 2002;Piperno and Flannery 2001). It is reasonable tohypothesize, given the backdrop and long history

of previous domestication processes in Mexico,that the occupants of early Mexico had theintellectual understanding, adequate chronologi-cal space, and the necessary wild germplasmavailable to complete the domestication processby 4600 B.P., the time of the first domesticatedsunflower evidence from Mexico at the San

Andres site. This position, however, has not been without controversy. Two domesticated sunflower disseminules were recovered from San Andres: anachene and a seed. These disseminules came from

different test pits, but dated to exactly the same timeperiod. Heiser verified the identification of the San

Andres achene soon after its discovery in 2000, butrecanted his identification eight years later, long after the specimen had been consumed by theradiocarbon dating process (Heiser 2008a ). TheSan Andres seed has attracted far less discussion.Our publication of the Cueva del Gallo sunflower finds from Morelos, Mexico (Lentz et al. 2008a ,b), engendered a spirited response (Brown 2008;Heiser 2008b; Rieseberg and Burke 2008; Smith

2008), but none of these commentaries seriously questioned the basic premise of the article, that thefinds from central Mexico represented fully domesticated H. annuus achenes. These new data,therefore, reinforced our earlier contention thatsunflower was indeed a crop of the pre-ColumbianMesoamericans. It is our hope and expectation thatfuture archaeological, paleoethnobotanical, andmolecular studies in Mexico and ENA will helpto clarify the relationship of the ancient sunflower populations in these two regions.

ConclusionIn conclusion, the four shape parameters we

evaluated (formfactor, roundness, convexity, andcurl) were less effective than conventional methodsin separating wild and domesticated sunflower achenes despite their utility in other seed popula-tions. Our assessment of achene size variation inmodern wild sunflowers showed that populationsfrom Mexico had significantly smaller achenes thanpopulations from eastern North America. Observedsize diversity in modern populations probably is a

result of both genetic and environmental variation.Conversely, domesticated archaeological achenesdiscovered in Mexico were significantly larger thanearly specimens recovered from eastern North

T ABLE 6. RESULTS OF THREE SINGLE REGRESSION

ANALYSES ASSESSING ASSOCIATION OF MEAN ACHENE

SIZE INDEX WITH THREE CLIMATIC FACTORS IN MODERN

WILD POPULATIONS.

Climatic Factor R 2 P-value Relationship

Temperature 0.543 0.0026* InverseRainfall 0.071 0.35 NoneElevation 0.013 0.69 None




America. Collectively these data will provide usefulguideposts as more archaeological specimens of H.annuus from ENA and Mexico are unearthed.

AcknowledgmentsThis work was supported by the National

Science Foundation (Grant Number BCS-0228049), the National Geographic Society (Grant Number 7030-01), and a Faculty Research Support Grant from the University of Cincinnati. We thank Abbas Shirinifard from thePhysics Department, Indiana University, for developing the MATLAB image processing script,an invaluable contribution to this project. Wealso thank Dr. Theresa Culley for her instrumen-

tal guidance. Kim Thompson and Edelmira Linares kindly offered editorial help.

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