In 2007, the first case of field resistance to Cry1Ab was reported in South
Africa, which is a concern as it negates the benefit of this technology. It has
been suggested that a major contributing factor to the development of
resistance in the target insect was the lack of compliance by commercial
farmers to plant refugia. However, another possible mechanism of resistance
development is the production of sub-lethal doses of Cry1Ab that could have
resulted in a high selective pressure for resistance alleles.
Although there are studies that have determined levels of Cry1Ab in different
tissue in MON810 maize, the available data is not complete, especially for
important feeding tissue of B. fusca larvae, such as silk and cob sheath. In
this study, a comprehensive analysis of levels of Cry1Ab within and between
different tissue over the growing season was conducted, taking the effect of
gene flow also into account. Field trials were performed over the 2008/2009
and 2009/2010 growing seasons under conventional farming practice. Gene
flow was allowed to occur between IR and non-IR maize in the 2008/2009
growing season, and the F1 seed was planted in the 2009/2010 growing
season. The levels of Cry1Ab were monitored over both growing seasons,
including the F1 plants in the second season. Notably, this study was the first
to determine levels of Cry1Ab in cob sheath, which is considered one of the
primary food sources for B. fusca larvae. It was found that there was considerable variation in levels of Cry1Ab within
and between different tissue over the growing season. The data for the
majority of the sampling points was moderately to highly skewed, indicating
the non-parametric range in variation of Cry1Ab levels. There was a
significant difference in Cry1Ab production between the two growing seasons,
which was attributed to the lower than average rainfall in the 2008/2009
growing season and a higher than average rainfall in the 2009/2010 growing
season. The overall trend in Cry1Ab production was congruent with the
pattern of target insect larval survival after feeding on different tissue as
reported by Van Rensburg (2009). Based on these data we suggest that
important insect feeding tissue, namely silk, cob sheath and cob, could be
producing sub-lethal doses of Cry1Ab that may result in ineffective control of
insect pests. It appears that the decline in Cry1Ab production at late growth
stages, in conjunction with variable levels of Cry1Ab between different tissue,
may compromise the high dose/refugia strategy, resulting in selective
pressure for the evolution of resistance.
The gene flow study determined that outcrossing between IR and non-IR
maize adversely affects the level of Cry1Ab in F1 plants. The levels of
Cry1Ab were significantly lower in F1 maize when compared to a commercial
MON810 maize hybrid, possibly as a result of reduced fitness. These data
support the observation of increased insect larvae damage to F1 plants,
suggesting that F1 maize may produce sub-lethal doses of endotoxin, and
consequently will not effectively control insect pests. The considerably lower
expression of Cry1Ab in F1 plants is a consideration in respect to subsistence farming practice in Africa, where seed is saved or exchanged among farmers.
We postulate that the introduction of IR maize in subsistence farming could
promote the development of insect resistance if not managed correctly.
In conclusion, the current study has determined that there is a wide range of
level of Cry1Ab within and between different tissue over the growing season.
Gene flow adversely affects Cry1Ab production, potentially due to reduced
fitness of the F1 plants. These data support the observation of differential
rates of larvae survival when feeding on different IR maize tissue. Finally, the
study provides an important basis for understanding the potential role that
variable levels of Cry1Ab may have had on the development of resistance in
B. fusca in South Africa.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-06192013-154504 |
| Date | 19 June 2013 |
| Creators | Richardson, Grant Anthony |
| Contributors | Prof CD Viljoen |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-06192013-154504/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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