108 – II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES)
The levels of genetic variation and the differentiation (genetic structure) of
C. argyrosperma and C. moschata and their relatives have been described in various
studies. A close relationship between the populations of C. argyrosperma
ssp. argyrosperma (average D (Nei’s genetic distance) = 0.02 [range 0.00-0.06]) and
C. argyrosperma ssp. sororia (D = 0.01 [0.00-0.06]) was reported by Decker (1986).
Populations of C. moschata showed a greater genetic distance (D = 0.24 [0.16-0.32];
Wilson, 1989; Wilson, Doebley and Duvall, 1992). On the other hand, data on the genetic
diversity show a close kinship between C. argyrosperma ssp. argyrosperma and
C. argyrosperma ssp. sororia (average D = 0.03), and a large differentiation between
C. argyrosperma ssp. argyrosperma and C. moschata [average D = 0.22] (Wilson, 1989;
Merrick, 1991). Another study on the degrees of genetic diversity in Cucurbita revealed
that C. moschata has a greater genetic diversity (mean expected heterozygosity,
H = 0.052) than C. argyrosperma (0.039), although the number of individuals studied
was small in both species (Decker-Walters et al., 1990).
Montes-Hernández and Eguiarte (2002) studied cultivated populations of Cucurbita
argyrosperma ssp. argyrosperma and C. moschata, together with adjacent wild
populations of C. argyrosperma ssp. Sororia, and found similar high degrees of genetic
variations in the three taxa (P= 0.96, mean allelic diversity of 2.08, average expected
heterozygosity (He) = 0.407) and little genetic differentiation among conspecific
populations (D = 0.081: Fst = 0.087; Nm = 5.22). These findings indicate that Cucurbita
possesses a high pollen dispersal potential, and a UPGMA (unweighted pair group
method with arithmetic mean) analysis indicated the existence of at least two distinct
groups of populations: one consisting of both subspecies of C. argyrosperma and another
consisting of C. moschata. In C. moschata in Africa, Gwanama, Labuschagne and Botha
(2000) used 39 random amplified polymorphic DNA (RAPDs) markers, generating
144 fragments, 23% of which were polymorphic; 4 clusters were found to be associated
to the geographical origin of the samples. Ferriol et al. (2004), using 156 amplified
fragment length polymorphism (AFLP) fragments in C. moschata, found 86% to be
polymorphic; and using 148 repetitive fragments, found 66% to be polymorphic.
Ferriol, Picó and F. Nuez (2004) analysed genetic variability and differentiation
(genetic structure) of C. maxima with AFLP, where 55% were polymorphic, and, with
sequence-related amplified polymorphism (SRAP) markers where 57% were
polymorphic.
In the C. pepo complex, genetic diversity and its heterozygosity are moderately high
(D = 0.17 and H = 0.089; Decker and Wilson, 1987) and alleles typical of the cultivated
species have been found in wild populations (Kirkpatrick and Wilson, 1988). This has
been interpreted as evidence of gene flow between wild and cultivated populations
(Decker and Wilson, 1987; Kirkpatrick and Wilson, 1988; Wilson, 1990).
Decker-Walters et al. (2002) analysed with RAPDs, 37 wild populations and
16 cultivated varieties. Twenty-six primers yielded 70 scorable and variable markers.
Their data also suggested gene flow between wild and cultivated populations. The results
of Ferriol et al. (2003a), in a study with 69 cultivated variants of C. pepo, including the
2 subspecies ssp. pepo and ssp. ovifera, using AFLP markers comprising 476 fragments,
showed 53% were polymorphic, with an average genetic diversity of 0.18; and, with
SRAP markers and 88 fragments, found a polymorphism of 73%, with an average gene
diversity of 0.25. With the SRAP analyses, the percentage of polymorphic fragments and
the gene diversity were higher in ssp. pepo than in ssp. ovifera (0.19 and 0.16
respectively), and with the AFLP analyses were 0.12 for ssp. ovifera and 0.10 for
ssp. pepo. Kwon et al. (2004) analysed 16 varieties, including C. maxima, C. moschata
