Harmonisation of Regulatory Oversight in Biotechnology Safety Assessment of Transgenic Organisms in the Environment, Volume 5..

II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES) – 111

Of the wild species, C. lundelliana is generally the most crossable with the other

mesophytic species, being in the tertiary gene pool of C. ficifolia, C. maxima,

C. moschata and C. pepo.

Table 2.6. Cross-compatibility of cultivated Cucurbita species: Gene pools

Species Primary gene pool Secondary gene pool Tertiary gene pool

C. argyrosperma C. argyrosperma ssp. soraria

C. argyrosperma ssp. argyrosperma

C. moschata C. pepo

C. maxima

C. foetidessima

C. ficifolia C. ficifolia C. pedatifolia

C. foetidissima

C. lundelliana

C. maxima

C. pepo

C. maxima C. maxima ssp. maxima

C.maxima ssp. andreana

C. ecuadorensis C. lundelliana

C. argyrosperma

C. ficifolia

C. moschata C. moschata C. argyrosperma C. lundelliana

C. maxima

C. pepo

C. pepo C. pepo ssp. pepo

C. pepo ssp. ovifera

C. pepo ssp. ovifera var. texana

C. pepo ssp. ovifera var. ozarkana

C. pepo ssp. fraterna

C. argyrosperma

C. okeechobeensis

C. moschata

C. ecuadorensis

C. lundelliana

C. ficifolia

C. maxima

Sources: Adapted from Lira, Andres and Nee (1995); and Lebeda et al. (2006).

Examples of breeding crosses performed to obtain specific introgressions

Interspecific crosses are an important mechanism for the introduction of valuable

traits that are not available, or cannot be found, within the gene pool of a crop species.

However, such crosses are often only achieved with difficulty as there are many natural

barriers, both pre- and post-fertilisation, that protect the integrity of a species. Even if

a cross between the parental plants produces hybrid offspring, the alien gene must

introgress into the genome, including successful chromosome pairing in the target

species.

Diverse studies have analysed hybridisation in Cucurbita (Whitaker and Bohn, 1950;

Whitaker and Bemis, 1965; Merrick, 1991, 1990). In spite of hybridisation barriers,

desirable traits have been successfully introgressed among species of the Cucurbita.

In most cases, success in crossing between cultivated species depends on the genotypes

used, with some attempts more successful than others (Whitaker and Davis, 1962;

Robinson and Decker-Walters, 1997; Lebeda et al., 2006). For example, although

C. moschata is in the secondary gene pool of C. argyrosperma and the hybridisation

possibilities between the members of the subspecies of C. argyrosperma

(i.e. C. argyrosperma ssp. argyrosperma and ssp. sororia) and C. moschata are good

(Wilson, 1990; Wilson, Doebley and Duvall, 1992), there are reports of a decrease in the

level of compatibility when C. moschata is used as the female parent (Merrick, 1991,

1990; Wessel-Beaver, 2000a).

Information related to hybridisation among Cucurbita species and techniques to

overcome crossing barriers and hybrid sterility has been summarised by Lira, Andres and

Nee (1995; see also Sisko, Ivancic and Bohanec, 2003). The breeding of Cucurbita has

primarily focused on improving the production and quality of the fruits by attempting to

increase resistance to pathogens and diseases, and by modifying plant architecture and

sex expression (Lebeda et al., 2006). Interspecific hybrids have been made to identify