Contributed by Vic High:
I've
been hearing a fair bit of confusion from many on how to
create a true breeding strain and so I'm writing this
page to try and help shed some light on the subject.
There are a few situations where a plant breeder would
want to create a true breeding strain (IBL) and a few
ways of accomplishing the task. But understanding the
subtle differences of the various techniques is not so
easy. This paper will attempt to give a basic
understanding of what is actually happening with each
technique and then apply what is learned to actual
projetcs. As a friend worked overtime making sure I
didn't forget, breeding is not a black and white subject
and as a whole, it would be too complex to put on paper
in an easily understood form. Therefore, I will create
small fictional examples to reinforce various concepts
and then we will take those examples and concepts and
apply some reality to them. Try not to get hung up on
the erroneous assumptions used here such as flavour
being monogenic, the assumption is simply used to make
it easier to learn a certain concept.
Just What Is It That We Are Doing?
Before we dive in, maybe we should take the time
to understand what we are trying to accomplish when we
set out to create a true breeding strain. There are
hundreds of possible phenotypic traits that we could
observe within a cannabis population. Are we trying to
make all of them the same and remove ALL variation? Not
likely, the genetic code is just too complex to try.
Plus, since phenotype (what we see) is 1/2 genotype +
1/2 environment, everytime the population was grown
under new conditions, new heterozygous traits would be
observed. Basically, all we are trying to create is an
overall uniformity while not worrying about the minor
individual variations. No different than a dog breed.
You can look at a german shepard and recognise it as
belonging to a discrete breed. But if you look closer at
several german shepards all at the same time, you will
find variations with each and every one of them. Some
will be a little taller, some a little wider, some more
agressive, some a little fatter, some darker, etc. But
they would all fall within an acceptable range for the
various traits. Generally speaking, this is what a plant
breeder is trying to accomplish when creating a true
breeding strain, or IBL.
However this isn't
always the case. Sometimes a breeder will just
concentrate on a specific trait, like say outdoor
harvest date, or mite resistance. You could still have a
population where some are 2' bushes and some 10' trees.
In this case, you would say that the strain was true
breeding for the particular trait, but you wouldn't
consider it true breeding strain per se. In genetics,
wording plays a big part in meaning and understanding.
As does point of reference as my F1 vs F2 comparison
page illustrates.
Ok, so we want to make a
cannabis population fairly uniform over a few
phenotypically important traits, like say flavour for
instance. For simplicity sake, we'll just deal with the
single trait flavour, it's complex enough. And although
flavour is controlled by several gene pairs (polygenic),
we'll make the simplistic assumption that it's
controlled by a single gene pair (monogenic) for many of
the models and examples in this paper. There are many
flavours such as chocolate, vanilla, musky, skunky,
blueberry, etc, but in this paper we'll just deal with
two flavours, pine and pineapple. Either gene in the
gene pair can code for either of the flavours. If both
genes code for pineapple or both genes code for pine
flavour, we say that the gene pair (and individual
plant) is homozygous for flavour. If the one gene codes
for pine and the other codes for pineapple, we say that
the gene pair (and individual plant) is heterozyous with
respect to flavour. The heterozygous individual can
create gametes (pollen or ovules) that can code for
either pine flavour or pineapple flavour, the homozygous
individuals can only create gametes that code for one OR
the other. A homozygous individual is considered true
breeding and a heterozygous individual is not.
However, as the words imply, when we are
creating a true breeding strain, we are looking at a
population, not individuals. We are trying to make all
the individuals in the population homozygous for a
particular trait or group of traits. Lets say we have a
population of 50 individual plants, and each plant has
has a gene pair coding for flavour. That means that 100
flavour genes make up the flavour genepool (reality is
much more complex). When trying to create a true
breeding strain, we are in fact trying to make all 100
of those genes code for the same trait ( pineapple
flavour in our case). The closer our population comes
getting all 100 genes the same, the more homozygous or
true breeding it becomes. We use the terminology gene
frequency to measure and describe this concept, where
gene frequency is simply the ratio or percentage of the
population that actually contains a specific gene. The
higher the gene frequency, the more true breeding the
population is. A fixed trait is where the gene frequency
of the trait reaches 100%.
And folks, this is
the basic backbone of what breeding is all about,
manipulating gene frequencies. It doesn't matter if your
making IBL, F1s, F2s, selecting for this or selecting
for that, all you are really doing is manipulating gene
frequencies. Therefore, to ever really understand what
is happening in any breeding project, the breeder must
pay attention to gene frequencies and assess how his
selective pressures and models are influencing them.
They are his measure of success.
What are
we trying to create a true breeding strain from?
This a good question. Sometimes a gardener will
notice a sport or unique individual in an F2 population,
like say it has pineapple flavour when the rest have
pine flavour. For one reason or another he decides he
wants to preserve this new trait or combination of
traits from that single individual. For the sake of ease
of comprehension, we tend to call this special unique
individual the P1 mom. He could start by selfing the
individual OR breeding that individual with another and
create what can be described as F1 offspring. If the F1
route was chosen, then breeders can diverge down two new
paths. Some breeders will take the progeny of the F1
crossing and breed it back to the P1 mom, and then
repeat for a couple more generations. This is referred
to as backcrossing or cubing by cannabis breeders.
Another common strategy is to make F2 progeny from the
F1 population and then look for individuals that match
the P1 mom. They would repeat the process for a few
generations. We can call this filial or generational
inbreeding since the parents from each cross belong to
the same generation.
In another situation,
sometimes a farmer will notice a few individuals in his
fields that stand out from the crowd in a possitive
manner. Like say the are resistant to a problem pest
like powdery mildew. In this case, he will collect the
best of the individuals and his starting population will
contain several similar individuals and not a unique
single individual as in the previous example. He would
skip the hybridizing step (making the F1s) and go
straight to the generational inbreeding step. Links to
pages going into detail of each of these basic
techniques and their impact on influencing gene
frequencies are at:
A)
Selfing
the individual B)
Backcrossing
and Cubing C) Filial or Generational
Inbreeding from an individual
D) Filial or
Generational Inbreeding from a group
Applying the Pressure
Another excellent
method to influence gene frequencies is to apply
selective pressure. The idea here is to select only
individuals that carry the desireable genes, and discard
the rest.
A) Principles of selection
B)
Progeny
tests