II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES) – 119
General interactions with other organisms (ecology)
This section highlights several interesting interactions of note between the Cucurbita
and other organisms. It does not attempt to create an exhaustive list of interactions.
Cucurbitacin mediated interactions
Species in the family Cucurbitaceae are characterised by their biosynthesis of a group
of secondary compounds that are thought to function as chemical defense compounds
(Bar-Nun and Mayer, 1990; Tallamy et al., 1998a) against insects, fungi and herbivores.
These compounds are known as cucurbitacins (Rehm et al., 1957) and are responsible for
the bitter taste found most obviously in the wild Cucurbitaceae. The cucurbitacins
are highly oxygenated tetracyclic triterpene compounds (tetracyclid triterpenoids).
These non-volatile compounds possess cytotoxic properties. For example, one form of
cucurbitacin antagonises insect steroid responses (Dinan et al., 1997). There are
17 identified cucurbitacin compounds, generally named alphabetically, e.g. A, B, C, D, E,
F, I, J, K and L. These compounds are based on the unusual amino acid (-)-3-amino-3-
carboxypyrrolidine, and can occur both free and in glycosidic combination.
The 17 different members of the cucurbitacin class of natural toxicants can be found as
naturally occurring mixtures in species of the Cucurbita, primarily in the leaves and
seeds. Although originally isolated from species in the Cucurbitaceae, cucurbitacins occur
in a variety of plant families (e.g. Brassicaceae, Begoniaceae, Rosaceae) as well as
in some mushrooms (e.g. Russula and Hebeloma). The ability of the Cucurbita to
produce cucurbitacins influences several aspects of their ecology.
Animals
Humans find almost all the cucurbitacins contained within the fruit of the wild
Cucurbita to be extremely bitter and the compounds have been found to be toxic to
a number of animal species. The most toxic cucurbitacin has an LD 50 of 5 mg/kg body
weight in the mouse. The least toxic has an LD 50 of 650 mg/kg body weight in the mouse
(US Environmental Protection Agency, 1999). In spite of the bitter taste and toxicity
which appears to deter most animals, some animals can tolerate at least some of the pulp
of wild Cucurbita; e.g. coyotes (Canis latrans) and porcupines (Erethizontidae spp.)
eat seeds tainted by the pulp of xerophytic Cucurbita digitata (Sowls, 1997). Javelina
(Pecari tajacu) appears to have even greater tolerance as they have been reported to
dig up and eat the bitter tuberous roots of C. foetidissima and C. digitata (Sowls, 1997).
Phytophagous insects
In general, the cucurbitacins produced by the Cucurbita are thought to defend against
phytophagous insects (Tallamy et al., 1998a). However, for a group of Chrysomelidae
beetles of the tribe Luperini, cucurbitacins act as arrestants and feeding stimulants
(Metcalf et al., 1982). The beetles belong to the subtribes Diabroticina (about 900 species
distributed in the American continent) and Aulacophorina (about 480 species found in
Asia). Diabroticina beetles can detect these compounds in plant tissues and inert
substances like silica gel or filter paper at quantities as low as 0.1 ng (Metcalf, Metcalf
and Rhodes, 1980). When the beetles encounter bitter plant tissues they compulsively
ingest them. Furthermore, they sequester cucurbitacins in hemolymph and elytra as
chemical defense against natural enemies and transfer the compounds to their eggs
(Ferguson and Metcalf, 1985; Brust and Barbercheck, 1992; Tallamy et al., 1998b).
A paper by Nishida, Yokoyama and Fukami (1992) showed, for several members of the
