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Traditionally in plant breeding there are 2 types of resistance to insects
and pathogens, vertical and horizontal. Vertical is typically a single
gene, while horizontal is quantitative in nature (several genes, probably
several different physiological processes being altered, also influenced by
the environment). Vertical is nice in the sense that it is very easy for a
breeder to work with and move from one genetic background to another. Bt
falls under the category of vertical resistance. Now for decades plant
breeders have released lines with vertical resistance knowing full well that
this resistance will be overcome. In the mean time they are continuing to
develop and release lines with horizontal resistance too.
This is similar to what we were talking about this morning, in that
traditionally plant breeders do not release material unless it merits
release (rule #1). This is another "rule" to add to the list, plant
breeders release single gene resistance all of the time knowing full well
that it will be overcome (rule #2).
>For the non-geneticists among us, would someone please post a
>lay-person's explanation of what an "incompletely dominant autosomal gene"
>is; along with an explanation of why this matters so much in designing a
>resistance management strategy. If someone would be willing to talk me
>through it, I'll write it up for the list.
>
Ok, I think that what they mean by "incomplete dominance" is that the
heterozygote (one functional copy of the resistance gene and one copy of the
non-functional resistance gene) will confer tolerance to Bt. By tolerance,
I mean that if the Bt protein level is not excessive then the insect will
survive. When there are insects homozygous for the resistance gene (2
functional copies of the gene) they should have more tolerance or resistance
to Bt protein.
"Autosomal" simply refers to the fact that the gene resides on the
"normal" set of chromosomes, rather than in the mitochondrial DNA.
This means that the trait can be passed both through the males and
females.
The reason that dominant resistance to Bt is alarming is that it can move
through the European Corn Borer population very rapidly. For example, lets
say that one male that is heterozygous for the resistance gene, survives on
Bt corn and finds a female moth that was from refugia corn. They mate, 50%
of their offspring will be resistant. If 2 heterozygous individuals mate,
75% of their offspring will be resistant, one third of which carry 2
functional copies of the resistance gene making them more resistant than
their parents.
There are 2 big differences between recessive and dominant resistance genes.
First for resistance to occur, 2 functional copies (homozygous) of the
recessive gene need to be present, while only 1 functional copy
(heterozygous) of the dominant gene needs to be present. The second
difference is how they arise and survive in the corn borer population. The
mutations always occur in the heterozygous condition (one functional copy
and one non-functional copy). Dominant ones will arise under selection
(i.e., on Bt corn) and those heterozygous individuals can actually survive.
Now I'm sure that recessive mutations will also arise on Bt corn, but those
individuals will not survive because you need to be homozygous for the
mutation. The only way that a recessive resistance gene could arise is in
individuals from the refugia that mated amongst themselves, to produce
individuals that were homozygous for the recessive resistance gene.
>Another relatively urgent request -- I am working on an
>assessment of the ECB Bt-corn resistance management plan that is
>under EPA review now. I, and others, need very quick help on a key
>issue. What insects spend some time in corn fields, and might
>develop resistance to Bt, that in other life-stages spend time in
>fruit and vegetable crops where growers are now largely reliant on
>foliar Bts for Lepidopteran control? If you know of people who have
>thought about this and could help create a list, and provide some
>sense of the potential for insect migration carrying resistance genes
>into other regions, please be in touch. I am willing to provide a
>modest level of $$ for anyone that can turn something out quickly
>(i.e. about 2 weeks). I will also eventually share what we learn
>with the list. So you bug-flow experts out there, please be in
>touch.
I'm not sure if I can be of any help with this. I've been under the
impression that each Bt protein has a fairly narrow range of insects that it
can actually work on. For example, the Bt protein that is now in corn, does
not affect corn root worm. However, I understand that other Bt proteins are
fairly effective against the corn rootworm.
ACLARK@plant.uoguelph.ca
Dr. E. Ann Clark
Associate Professor
Crop Science
University of Guelph
Guelph, ON N1G 2W1
Phone: 519-824-4120 Ext. 2508
FAX: 519 763-8933
http://www.oac.uoguelph.ca/www/CRSC/faculty/eac.htm
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