II.2. SQUASHES, PUMPKINS, ZUCCHINIS, GOURDS (CURCURBITA SPECIES) – 105
In addition to Peponapis and Xenoglossa genera, other species of bees can pollinate
plants of the genus Cucurbita. One of the most frequent pollinators in cultivation systems
around the world is the domestic honeybee, Apis mellifera (Canto-Aguilar and
Parra-Tabla, 2000; Mariano and Dirzo, 2002). While the bees of the genera Peponapis
and Xenoglossa make more contact with the reproductive parts of the flower, work faster
and work earlier in the morning, they are no more efficient than honey bees
(Apis mellifera) at setting fruit (Tepedino, 1981). However, honey bees, in contrast to the
solitary bees of the genera Peponapis and Xenoglossa, are generalists and readily move to
any competing bloom that offers richer rewards. Also, early bee activity can be important
on hot days when the flowers close early, and bees of the genera Peponapis and
Xenoglossa are adapted to working early in the morning. In general, where Peponapis
and/or Xenoglossa populations are strong, the use of honey bee colonies is superfluous as
most pollination is accomplished by Peponapis and Xenoglossa bees before the honey
bees arrive (Tepedino, 1981).
In addition to the bees of the Peponapis and Xenoglossa genera, bees of the Bombus
genus, e.g. Bombus impatiens, have been identified in the United States as pollinators of
Cucurbita pepo (Julier and Roulston, 2009).
Table 2.5 indicates the known species of pollinators that have been identified in wild
and cultivated Cucurbita populations in Mexico and the species they are known to
pollinate. Peponapis species are found from Ontario, Canada through the United States
through Mexico to northern Argentina in South America (Michener, 2007). Xenoglossa
species are found through the United States through Mesoamerica. The ranges of
Peponapis and Xenoglossa species are similar in tropical regions of the Americas. Some
species of Peponapis are also capable of surviving in temperate regions (thus their
distribution extends from Canada to Argentina). In contrast, species of Xenoglossa have a
more restricted distribution toward lower latitudes, but are capable of establishing in more
desertic environments. Megalopta species are found in Mexico through the mid section of
South America (Discover Life, 2011). Melitoma species are found in the central and
eastern United States through Mexico to northern Chile and Argentina (Discover Life,
2011). Although most common in the northern hemisphere, Bombus species can be found
around the world (Discover Life, 2011).
Hurd (1966) identified several insects other than bees that might play a minor role in
pollination of Cucurbita: cucumber, scarab and meloid beetles, as well as flies and moths.
Asexual reproduction
The Cucurbita species propagate in nature through sexual reproduction as there is
essentially no asexual propagation by means of runners or stolons, or apomixes in nature.
Genetics
The basic chromosome number of the Cucurbita is 2n = 2x = 40. Karyotypes suggest
that these species are of allopolyploid origins. Results from electrophoretic analyses also
helped confirm this genus’ polyploidy (Kirkpatrick, Decker and Wilson, 1985), or more
specifically, allotetraploid origin (Weeden, 1984). Weeden (1984) and Singh (1990)
suggested that the Cucurbita are ancestral tetraploid, derived from an ancestor with a
haploid chromosome number of ten. Although these authors suggest an apparent
homogeneity, Weiling (1959) suggested that the genome in Cucurbita ficifolia is AACC
(each letter refers to a different ancestral plant genome), whereas in the four remaining
