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RISK ASSESSMENT: Pollen-mediated gene flow from GE sorghum expectedto be extensive



------------------------------- GENET-news -------------------------------
TITLE:  Biosafety of transgenic sorghum - A comment on Visarada and
        Kishore (2007)
SOURCE: Information Systems for Biotechnology News Report, USA
AUTHOR: Allison Snow & Gebisa Ejeta
URL:    http://www.isb.vt.edu/news/2007/news07.may.htm#may0701
DATE:   01.05.2007
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Biosafety of transgenic sorghum - A comment on Visarada and Kishore (2007)
[Improvement of Sorghum through transgenic technology
http://www.isb.vt.edu/news/2007/news07.mar.htm#mar0701]

In their recent ISB News Report article about transgenic sorghum,
Visarada and Kishore (2007) provide incomplete information about the
likelihood that transgenes would spread to wild relatives of the crop.
Introgression of transgenes that enhance the fitness of weedy relatives
could possibly make them more difficult to manage (e.g., Snow et al.
2005). Visarada and Kishore note that, "The most important issue related
to biosafety concerns in sorghum is pollen-mediated gene flow to the
wild species Sorghum halepense (Johnsongrass), a wild weedy relative ...
" However, by citing only a single reference - Godwin (2005) - they give
the impression that crop-to-wild gene flow will not occur because Godwin
reported that "hybridization of S. halepense (2n=40) and cultivated
sorghum, S. bicolor (2n=20), would produce unviable hybrids." Contrary
to this citation, many other studies show that pollen- and seed-mediated
gene flow to wild and weedy relatives of sorghum is expected to be
extensive (e.g., De Wet and Harlan 1971; Holm et al. 1977; Arriola and
Ellstrand 1996; Ejeta and Grenier 2005; and references therein).

Despite having different ploidy numbers, sorghum and Johnsongrass easily
hybridize and produce fertile offspring in the USA (Arriola and
Ellstrand, 1996, 1997). Johnsongrass is self-incompatible, and crop-wild
hybrids were detected 100 m away from the crop, which was the greatest
distance examined. Crop genes appear to accumulate and persist in
Johnsongrass populations (Morrell et al. 2005). These authors found 32%
frequencies of putative crop-specific alleles in Johnsongrass
populations that were adjacent to sorghum fields in Texas and Nebraska,
USA. Surprisingly, they also found low frequencies of crop-specific
markers in Johnsongrass populations with no recent exposure to the crop.
Therefore, they concluded that gene flow among Johnsongrass populations
could provide a bridge for the wide dissemination of transgenes
following initial hybridization with the crop.

Sorghum also is expected to hybridize with its conspecific wild
relatives with 2n=20 chromosomes, which are widespread in Africa and
elsewhere (De Wet and Harlan 1971, Schmidt and Bothma 2006, Ejeta and
Grenier 2005). "Shattercane" is a common name that is often applied to
weedy conspecifics of Sorghum bicolor, some of which may represent feral
crop plants (e.g., Dahlberg 2000, Ejeta and Grenier 2005). Research on
genetic transformation of sorghum should and will continue as it may
render solutions to the more intractable problems of sorghum production
and nutrition. However, it is imperative that parallel research is also
underway to 1) further understand the nature and extent of gene flow
between sorghum and its wild relatives, 2) assess the effects of gene
flow from proposed transgenic lines with new traits, and 3) mitigate
potential problems that may arise as a result of gene flow between and
among sorghum and its wild and weedy relatives. In general, transgenes
that are incorporated into sorghum crops are expected to disperse via
seed transport, seed mixing, as well as hybridization with cultivated
varieties and weedy relatives. It will, therefore, be important to
consider all of these processes when new transgenic sorghums are
deployed for large scale cultivation or are proposed even for limited
environmental release.


References
Arriola, PE & Ellstrand NC (1997) Fitness of interspecific hybrids in
the genus Sorghum: persistence of crop genes in wild populations.
Ecological Applications 7, 512-518
Arriola, PE & Ellstrand NC (1996) Crop-to-weed gene flow in the genus
Sorghum (Poaceae): spontaneous interspecific hybridization between
johnsongrass, Sorghum halepense, and crop sorghum, S. bicolor. American
Journal of Botany 83, 1153-1160
Dahlberg DJ (2000) Classification and characterization of Sorghum. Weeds
and their control in grain sorghum. in: Sorghum: Origin, History,
Technology, and Production. CW Smith and RA Frederiksen, eds. John Wiley
and Sons Inc. NY, USA.
De Wet, JM & Harlan, JR (1971) The origin and domestication of Sorghum
bicolor. Economic Botany 29, 99-107
Ejeta, G, Grenier, C (2005) Sorghum and its weedy hybrids. Pages 123-135
in Crop Ferality and Volunteerism, Gressel, J, ed CRC Press, Taylor and
Francis Group, USA
Godwin ID (2005) Sorghum genetic engineering: Current status and
prospectus. Pages 1-8 in Sorghum tissue culture and transformation. M.
Seetharama and I. Godwin, eds. Oxford & IBH Publishing Co. Pvt. Ltd, New Delhi
Holm, LG et al. (1977) The world's worst weeds. University Press of
Hawaii, Honolulu, HI, USA
Morrell, PL et al. (2005) Crop-to-weed introgression has impacted
allelic composition of johnsongrass populations with and without recent
exposure to cultivated sorghum. Molecular Ecology 14, 2143-2154
Schmidt, MR & Bothma GC (2006) Risk asssessment for transgenic sorghum
in Africa: Crop-to-crop gene flow in Sorghum bicolor (L.) Moench. Crop
Science 46, 790-798
Snow, AA et al. (2005) Genetically engineered organisms and the
environment: current status and recommendations. Ecological Applications
15, 377-404
Visarada KBRS & Kishore NS (2007) Improvement of Sorghum through
transgenic technology. ISB News Report March 2007, 1-3

Allison A. Snow
Department of Evolution, Ecology, and Organismal Biology
The Ohio State University
Columbus, OH 43210, USA
Snow.1@osu.edu

Gebisa Ejeta
Department of Agronomy
Purdue University
West Lafayette, IN 47907, USA
gejeta@purdue.edu


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