94 – II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES)
C. pepo domestication appears to have predated that of other American crops such as
maize (Zea maize) and common beans (Phaseolus vulgaris) by some 2 000 to 5 000 years
(Smith, 2006). The archeological record shows that C. pepo was cultivated in different
ecological niches in the Americas from the high plains to fluvial systems.
The archaeological record suggests that C. pepo was domesticated on at least
two occasions and in two different regions of North America: in Mexico and in the
United States (Decker, 1988, 1986; Andres, 1987a). The hypothesis of two domestication
events is supported by the grouping of allozymic patterns of the cultivated forms (Decker,
1985). The hypothesis is also supported by additional allozymic assays, comparisons of
chloroplastic and mitochondrial DNA, as well as random amplified polymorphic DNA
markers (Kirkpatrick and Wilson, 1988; Wilson, Doebley and Duvall, 1992;
Decker-Walters et al., 1993, 2002; Sanjur et al., 2002). As evidence has accumulated, it
has become clear that C. pepo is comprised of two molecularly divergent groups that had
already differentiated through geographical isolation long before humans domesticated
them (Decker-Walters et al., 1993, 2002; Smith, 2006). These two divergent groups are
classified as two subspecies: C. pepo ssp. pepo and C. pepo ssp. ovifera. C. pepo ssp.
pepo includes pumpkin, zucchini and other marrow squashes, Mexican landraces and a
few ornamental gourds. C. pepo ssp. ovifera comprises both domesticated and free-living
populations, and is further divided into three taxonomic varieties: C. pepo ssp. ovifera
var. ovifera which includes some cultivated cultivars (e.g. acorn, crookneck and scallop
squash) and most ornamental gourds, and the free-living populations in the United States,
which represent two molecularly distinct populations: C. pepo ssp. ovifera var. texana
and C. pepo ssp. ovifera var. ozarkana (Decker-Walters et al., 1993, 2002). C. pepo ssp.
ovifera appears to have been domesticated in what is now the United States, while
C. pepo ssp. pepo appears to have been domesticated in Mexico (Sanjur et al., 2002).
With regard to the likely ancestor(s) of the cultivated C. pepo, the three most closely
related wild relatives are: C. pepo ssp. fraterna which is found in Mexico, and C. pepo
ssp. ovifera var. texana and var. ozarkana which are found in the United States. C. pepo
ssp. fraterna is known only from a few localities in the states of Tamaulipas and
Nuevo León in the northeastern region of Mexico (Bailey, 1943; Andres 1987a; Nee,
1990; Rodríguez and Lira, 1992; Wilson, Lira and Rodríguez, 1994). C. pepo var. texana
and var. ozarkana have distribution patterns associated with drainage patterns of the river
systems that lead from the central United States to the Gulf of Mexico (Smith, 1992;
Asch and Sidell, 1992; Wilson, 1998; US Department of Agriculture, 2011a).
With regard to the probable ancestor of C. pepo ssp. ovifera, Sanjur et al. (2002), on
the basis of sequence analysis of and intron of the mitochondrial gene nad1, suggest that
C. pepo ssp. fraterna is the most probable ancestor of C. pepo ssp. ovifera. In the
Sanjur et al. studies (2002), C. pepo ssp. fraterna, C. pepo ssp ovifera, C. pepo
ssp. ovifera var. texana and var. ozarkana form a group and share the same haplotype.
Smith (2006), however, posits that although the analysis of the nad1 mitochondrial gene
in the studies by Sanjur et al. (2002) could not distinguish between these taxa, random
amplified polymorphic DNA data (Decker-Walters et al., 2002; 1993) was able to
separate ssp. fraterna from ssp ovifera var. texana and var. ozarkana and effectively
excluded ssp. fraterna from the cluster that includes all cultivated and wild varieties of
ssp. ovifera. Further support for excluding ssp. fraterna from potential contention as an
ancestor of ssp. ovifera is provided by a recent genetic study employing amplified
fragment length polymorphism, inter simple sequence repeat and simple sequence repeat
markers, in which ssp. fraterna was placed at a greater genetic distance from the
domesticates of ssp. ovifera than the eastern North American wild gourds (Paris et al.,
