110 – II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES)
column can yield at least partially fertile F1 on hybridisation. Although genes can be
moved in breeding between the cultivated species and plants in their secondary gene pool,
the F1 are usually sterile or sparingly fertile. Species listed as being in the tertiary gene
pool of the cultivated species represent the outer limit of potential genetic resources for
breeding: Pre-zygotic and post-zygotic barriers can cause partial or complete
hybridisation failure, inhibiting introgression between the cultivated species and plants in
the tertiary gene pool (Lebeda et al., 2006). Crosses between a cultivated Cucurbita
species and other cultivated species in the secondary or tertiary gene pools (Table 2.6),
present a more complicated picture; the use of techniques such as embryo culture, which
are used to bypass hybrid sterility barriers, may be required. Hybrids obtained from such
crosses are frequently sterile or exhibit reduced fertility (Whitaker and Robinson, 1986).
Among the Cucurbita, success in crossing frequently depends on the genotypes used as
parentals.
C. ficifolia is the least compatible species, not only with the other cultivated species,
but with all the remaining species of the genus (Whitaker, 1951; Whitaker and Davis,
1962; Whitaker and Bemis, 1965; Merrick, 1990; Lira, Andres and Nee, 1995; Robinson
and Decker-Walters, 1997). Some interspecific hybrids have been obtained from crosses
between C. ficifolia and C. pedantifolia, C. foetidissima, or C. lundelliana, but they often
lack the capacity to produce an F2 generation (Lira, Andres and Nee, 1995).
Among the cultivated species, C. moschata has the best crossability. Among the
cultivated species, it is easiest to cross C. moschata with C. argyrosperma. Hybridisation
experiments (and some field observations) have revealed that C. moschata has the highest
degree of compatibility with C. argyrosperma, placing C. argyrosperma into the
C. moschata secondary gene pool (Table 2.6; see Lebeda et al., 2006). The C. moschata
tertiary gene pool is formed by C. lundelliana and some taxa of the groups Maxima and
Pepo (Lira, Andres and Nee, 1995).
Conversely, hybridisation experiments (and some field observations) place
C. moschata into the C. argyrosperma secondary gene pool. The next level of
C. argyrosperma cross-compatibility involves the wild and cultivated species of C. pepo,
some cultivars of C. maxima, and the wild perennial species C. foetidessima, which
collectively represent the C. argyrosperma tertiary gene pool (Lebeda et al., 2006).
The primary gene pool of C. maxima includes C. andreana, which some authors
classify as a C. maxima subspecies (Systax Database, 2011; see also Annex 2.A1).
The secondary gene pool of C. maxima is represented by C. ecuadorensis; and its tertiary
gene pool includes C. lundelliana, C. argyrosperma, C. ficifolia and C. pepo (Lira,
Andres and Nee, 1995; Lebeda et al., 2006).
The primary gene pool of C. pepo is formed by its various edible and ornamental
cultivars, as well as populations of the wild taxa, ssp. fraterna, and ssp. ovifera
var. texana and var. ozarkana; until recently these wild taxa were identified as distinct
species (Singh, 1990). There are a great many C. pepo cultivars with particular
characteristics that, together with local landraces (grown mostly in Mexico), constitute an
extraordinary genetic stock. Populations that could be considered as part of the C. pepo
secondary gene pool are scarce; most attempts at hybridising C. pepo with other wild or
cultivated Cucurbita species have required the use of special techniques such as embryo
culture (Lebeda et al., 2006).
The wild mesophytic annual taxa C. lundelliana, C. okeechobeensis and
C. ecuadorensis have shown some possibilities of introgression through breeding
hybridisation with cultivated species and/or with one or more of these species’ ancestors.
