Transgenic Sunflower - Genetics, Genomics and Breeding of Crop Plants - page 302

fast as other GM crops. Its seed uses per unit surface and the acreage, that in
the US and Argentina represents among one and two million hectares each
( http://faostat.fao.org/site/340/DesktopDefault.aspx?PageID=340 ) which are by
far lower than in corn or soybean crops.
Sunflower is an insect-pollinated outcrossing species, which hybridizes
with several Helianthus species (Heiser et al. 1969; OECD 2004). Although
first generation hybrids from crosses between sunflower and most annual
wild relatives are highly sterile, it is possible to obtain fertile progenies
through backcrosses to the cultivated sunflower. This technology is less
effective in crosses with perennial species, sexually detached by a number
of chromosome rearrangements and/or a different ploidy level (Jan 1997).
In spite of that, even without special techniques ( such as embryo rescue),
cultivated Helianthus annuus can easily hybridize with as least 16 wild
relatives naturally occurring in the US, and with some introduced in other
continents as well ( Table 9-5 ) .
Although horizontal gene flow between sunflower and other Asteraceae
including Thitonia spp., Vigiera spp. (Sossey-Alaoui et al. 1998) or Verbesina
spp. (Encheva and Christov 2005) seems unlikely, the probability and
implications of vertical gene flow from H. annuus , or diagonal with other
sexually compatible Helianthus species (Gressel and Al-Ahmad 2005) pose
additional constraints to transgenic sunflower release. Genetic exchange
with wild relatives set the risk of originating novel weeds, through dispersal
of transgenic pollen or seed escapes There are examples in wild radish
( Raphanus raphanistrum ) hybridized with R. sativus (Campbell et al. 2006)
and in Agrostis stolonifera (Reichman 2006).
At the center of origin in North America, introgression and persistence
of crop genes in wild Helianthus populations have been well known for
more than a decade (Heiser 1978; Whitton et al. 1997; Linder et al. 1998;
Rieseberg et al. 1999; Burke et al. 2002). Agro-ecological studies on transgenic
varieties demonstrated that some transgenes were expected to perform as
neutral, for example Sclerotinia resistance (Burke and Rieseberg 2003), while
others could increase fecundity in wild populations, such as Lepidopteran
resistance (Snow et al. 2003).
The high variability within H. annuus allowed Russian breeders to
develop a wide range of agricultural varieties starting from a few plants
introduced to Europe for ornamental purposes. This diversity would
comprise a high degree of endoferality, which would result in volunteer
plants able to interfere with the following crops and exoferality to give rise to
stable populations through crosses with genuine wild plants (Reagon and
Snow 2006). There are large naturalized populations of wild or wild-related
H. annuus in Spain and France (Müller et al. 2006), Serbia (Stankovic-Kalezic
et al. 2007), Italy and other European countries. They most likely originated
in seed contaminants (wild or wild-crop seed) imported from the US