Je m'excuse par avance auprès de l'auteur, car je ne retrouve plus la source de cet article...
il est possible que j'ai récupéré ceci sur un forum (la vibes, icmag...)
je vais faire mon possible pour corriger cela
viendra également la traduction,
peace!!
Merci à Animalxxx et à Rahan
Maintaining old lines: the difference between preservation and conservation
Introduction
We all want to maintain our old seed lines in life. Forgotten by the professionnals, bad transmission due to prohibition, the management of these strains is allways difficult. Panama red, Acapulco gold, Colombian gold, Kerala, Black Magic African, Blond Libanese ..., the list of these mythical strains in danger or near to disappear is long.
However, few seeds of these strains have been preserved by some afficionados during all these years. How to maintain them, without losing their specificity? Two approachs are possible, two visions of the relation with the plan, more or less ambitious.
The first approach can be named "conservation". The goal is here to conserve the strain in its whole complexity, i.e. to conserve a maximum (as close to 100% as possible) the intra specific diversity.
The second approach can be named "preservation". The goal is here to preserve the traits that define the strain, admitting that the diversity as a whole can not be conserved, but that the preserved stuff will be sufficient so that the strain is perfectly recognizable.
Both approach are incompatible. Indeed, when doing conservation, you have to maximize the number of genetic combinations to increase the chances to conserve the total diversity of the strain. At the opposite, when preserving, you have first to select the important traits , so that you fix them in the strain and so obtain a strain that is easier to conserve later. We will see together and in details in a first step what are the biological mechanisms that are involved in these two approachs and in a second step, we will precise how we can use them in our growrooms.
1. Biological concepts
1.a Transmission of a gene and allelic frequency
At each reproduction, genes are transmitted by the gamets which have been created after a meiosis. Each gamet owns one allele for one gene (he is haploid), contrario to the global individual that owns two alleles for each genes (he is diploid). Due to this, there is a random factor in the reproduction event: Which allele will be implied in the reproduction? These processes have been described by Mendel when he determined how genes were quantitatively transmitted. Notice that he did not know at this time that chromosoms existed and what a gene was ! Studying the whole population, and based on the allelic frequence in this population and the probability of 0.5 (one chance over two), the reproduction at the population level can be simplified with a simple table with 4 cases.
Given a locus L for a gene, with two alleles: A (dominant) and a (recessive). AA and Aa will produce phenotype A and aa phenotype a. F (A) = 0.75 is the allelic A frequency in the population. It means that 75% of the individuals have at least one allele A. F(A) + F(a) is allways equal to 1 so F (a) = 0.25.
With the Mendelian table, we can predict phenotype occurence in the descendance:
So if we want to observe the phenotype a, by crossing the individuals randomly, we have 6.125% chances to meet it in the progeny. A contrario, the phenotype A will be present at 56.25 + 18.75 + 18.75 = 93.75% in the progeny, even if 37% of the individuals owns a recessive allele a which does not express as he is dominated by A.
If we selected for phenotype A, we would reproduce only these phenotypes. The new allelic frequencies would be:
F (A) = (0.5625 x 2 + 0.1875 + 0.1875) / total = 1.5 /
0.375 +0.375 + 1.125) = 0.8
whereas a would have a frequency of :
F (a) = 0.1875 + 0.1875 / total = 0.375 /
(0.375+0.375+1.125) = 0.2
The allele a frequency diminished in the population. In the next generation, we obtain:
Whereas we had 6.125% of chances to observe the phenotype a in the F2, we have now only 4% to find it in the next generation! There has been a genetic drift in the allelic frequencies, we will detain this phenomena later.
1.b Transmitting many independant genes
What happens now if we want to study the transmission of two independant genes (that means they are transmitted independantly of each other) ? This study can be interesting, particularly when one of the two genes enhances the other, i.e. for example the protein produced by the first gene is necessary to allow the second to express, or when this protein interact with the protein produced by the second gene.
Given a phenotype A dominant (AA, Aa) and a phenotype a recessive(aa) for locus 1 and B the dominant phenotype(BB, Bb) with b the recessive phenotype (bb) for locus 2. A has a frequency of 0.75, a of 0.25 and F (
= 0.75 ; F ( = 0.25.
The calculation of the next generation is done with :
The work on the strain is now harder: We have quickly rare phenotypes (6.125% in the first progeny for phenotype a .
If we work on three genes, with the same allelic frequencies (0.75 for the dominant and 0.25 for the recessive), the difficulties rise again:
We have here extremely rare phenotypes: 1.56% of chances to obtain phenotype a b c. We need already 100 individuals to have a good chance to obtain it in our grow !
1.c Transmission of two genes linked together
It is possible that two genes that we want to study are located in the same chromosom (see meiosis functionning for more details). We say that they are linked. If an individual owns alleles A and B in the first version of the chromosom and alleles a and b in the second version, then he can only produce gamets A,B or a,b. However, exceptionnally, crossing over may happen. in this case, the two chromosoms cross each other physically and exchange genetic material. If this cross happens between the two loci, then the individual may produce gamets A,b and a,B.
1.d Other difficulties to take into account
Some genes can be located in the sexual chromosoms. In this case, their expression is linked with sex, particularly when the gene is located on the chromosom X and when the sex is determined with a XY system. Indeed, when we have a male XY, all genes located in the chromosom X express as they have no equivalent in the Y chromosom. So it doesn't matter here wether they are dominant or recessive.
Generally speaking, don't forget also that we simplified a lot the phenotypic expression of the genes by saying that one genes gives one phenotype. In fact, a traits is very often the result of multiple gene interactions.
1.e The selection concept
Selection is a process that may happen after a natural event or after an human event. The selection of the phenotypes was firstly demonstrated by Darwin in his book "On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life", in which he demonstrates that natural selection is the motor of evolution.
Natural selection is simply described with three principles :
- individuals differ each other
- the most adapted survive and they reproduce together
- the adaptative advantages that allowed the survival are able to be transmitted to the progeny
The result is that natural selection explain the adaptation of the species to their environment, with as consequence, the concept of character displacement inside the species.
As we started to see above, this displacement of character in the population is in fact a change of allelic frequencies under the pressure of selection whether natural or from human origin. For example, if the phenotype a was in fact a non potent phenotype, and if human was selecting for potency in the strain, he would give a selective advantage to the phenotype A by eliminating phenotypes a from the reproduction. In this way, the allele A frequency would increase in the population at each generation. If we discovered that in fact, the combination of alleles a,b,c, gives the most potent individuals, due to gene interaction, then we would select this rare phenotype (1.56% of the population) and very quickly, as he is homozygous (a,b and c being recessive, to obtain this phenotype, we need to have individuals aa, bb and cc ) , we would make the population drift to this pheno, to the point where alleles A, B and C would simply and quickly disappear from the population !
For a natural selection example: if phenotype a was an phenotype that flowers in 25 weeks, when grown under temperate climate, he would never have the time to reproduce and so he would disappear from the population as we grow several generations. It is also called genetic drift.
1.e The concept of selection
Selection is a process that may happen after a natural event or after an human event. The selection of the phenotypes was firstly demonstrated by Darwin in his book "On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life", in which he demonstrates that natural selection is the motor of evolution.
Natural selection is simply described with three principles :
- individuals differ each other
- the most adapted survive and they reproduce together
- the adaptative advantages that allowed the survival are able to be transmitted to the progeny
The result is that natural selection explain the adaptation of the species to their environment, with as consequence, the concept of character displacement inside the species.
As we started to see above, this displacement of character in the population is in fact a change of allelic frequencies under the pressure of selection whether natural or from human origin. For example, if the phenotype a was in fact a non potent phenotype, and if human was selecting for potency in the strain, he would give a selective advantage to the phenotype A by eliminating phenotypes a from the reproduction. In this way, the allele A frequency would increase in the population at each generation. If we discovered that in fact, the combination of alleles a,b,c, gives the most potent individuals, due to gene interaction, then we would select this rare phenotype (1.56% of the population) and very quickly, as he is homozygous (a,b and c being recessive, to obtain this phenotype, we need to have individuals aa, bb and cc ) , we would make the population drift to this pheno, to the point where alleles A, B and C would simply and quickly disappear from the population !
For a natural selection example: if phenotype a was an phenotype that flowers in 25 weeks, when grown under temperate climate, he would never have the time to reproduce and so he would disappear from the population as we grow several generations. It is also called genetic drift.
The genetic drift can be quantified mathematically by modifying the calculation of the Mendelian tables. We define w as the fitness of the genotype.
Let's take a population of 100 individuals. In this population, we have 25 individuals that are AA; 50 that are Aa and 25 that are aa. Allelic frequencies of A and a are respectively p= 0.5 and q = 0.5.
Let's consider that the 25 AA individuals, 90% of the Aa individuals and 80% of the aa individuals will be able to reproduce. By consequence, w(AA) = 1 ; w(Aa) = 0,9 and w(aa) = 0,8
When reproducing, each individual produces two gametes. With the reproduction rates we just defined, we obtain 1 x 2 x 25 + 0,9 x 2 x 50 + 0,8 x 2 x 25 = 180 gametes.
What are the new allelic frequencies?
The 25 AA individuals produce 50 gametes A.
The 45 Aa individuals produce 45 gametes A and 45 gametes a.
The 20 aa individuals produce 40 gametes a.
Finally, we obtain 95 gametes A and 85 gametes a so by consequence, the new allelic frequencies are p = 0,53 and q = 0,47.
The allele A has an increase in its allelic frequency between generation n and generation n+1, because of its better ability to being reproduced when he is represented by AA individuals.
Let's imagine the simplified case where the phenotype a can not reproduce at all because of a too high flowering time. We have there:w(aa) = 0. If phenotype A reproduces perfectly, we have: w(AA) = 1 and w(Aa) = 1.
In this example, phenotype a is considered as lethal, which means that this phenotype can not reproduce.
In this context, we can calculate the effect of selection on the allelic a frequency with the relation:
qg = q0 / (1 + g x q0)
where qg is the allelic frequency of allele a at generation g, q0 is the allelic a frequency at initial generation and g is the number of generations since initial generation.
If we use q0 = 0,25, then, after 10 generations,
Q10 = 0,08
Allele a became rare !
Obviously, the studied allele is not necessarily recessive, it can also be dominant or co-dominant and the maths demonstrate that this status and the allelic frequency at the beginning, the genotype can have a more or less stability during the following generations.
2. A strategy to maintain old strains
2.1 What do we want and what are we working?
This two questions may seem evident but they are really important to conduct the work.
What are we working? On a variety of Cannabis sativa, obviously. But it is not enough !
How can we caracterize it? Which criterias do you use to recognize it? Are these criterias constant between individuals? Or do they appear on only a part of the population? Does the strain come from a wild area where human is not here or a contrario, did she appear from selection of humans since decennias?
Two approaches are possible considering the material you have to preserve or conserve :
- Some varieties have never been frequent, because the traits that have been selected in these varieties correspond to particular environment. In this case, the populations of this variety were never big and so that strain has already known consaguinity episods which conducted to drastic selection events of particular phenotypes. This is the case for example, for varieties that were selected by humans or that were adpated to very narrow ecological niches.
- Some other varieties, a contrario, developped in environments wild or semi-wild, giving strains that are adpated to very variable environments. In this case, their genetic stuff is very diverse, allowing to adapt to changing conditions, owning many alleles in its genetic stuff. In other words, these varieties have not "put their eggs in the same basket". This is the case for the strains growing freely near the road or in wild places where man doesn't put his hands too strong, and where environmental conditions may vary.
Initially, Cannabis sativa is a very generalist species, able to adapt to very variable conditions, from an extreme to another, hence its success. This kind of species shows a great plasticity, allowing to adapt easily against local conditions; with as consequencies many phenos that can differ greatly. The difference between indica varieties and sativa varieties is a good example of the plasticity of Cannabis sativa for the leaves shape. The Ducksfoot is an interesting variety in this purpose. The difference of potency between wild hemp and drug type hemp is also a good illustration of this plasticity of Cannabis sativa.
Often, as we said, wild or semi-wild variety will show a great heterogeneity in their phenotypic expression of the genotype diversity ; this genetic diversity being favored to have a solution in case of brutal changes in the grow conditions. We will have multiple phenos, with some rarer hard to observe.
What do we want to do? The amateurs of Cannabis sativa are often enthusiastic and this particularly since few years, and they answer often "conserve the genepool". The conservation of a variety is a very complex project and at this point, we will often have two opinions fighting each others :
- The realistics / defaitist which argue that we need at least 1,000 individuals to preserve in a correct manner and they propose whether to abandon the project, whether to stock a huge number of seeds (possibly from a first reproduction if the original number was not sufficient), waiting for a better future.
- The optimistics / naives that argue that we do not know really what is our future and that it is better to do something when we have the things in hand rather than stock them and wait.
This is the kind of debate that we see in a recurrent manner, and we will see now the scientific elements that can be used in this discussion. Obviously I have my opinion on this topic, but I prefer to let you make yourself your opinion, studying facts and not words. Let's see what we have to know and what can cause problems for amateurs like us ; when we have seeds of a rare strain with few seeds and a great willing to see it grow and reproduce, in the best conditions. When concluding this text, I'll give you my opinion
2.2 The concept of efficient population
The main issue that we will encounter is the population size. Most of the following will only explain the problems encountered when reducting population size.
The population size is a problem at two levels: First, the number of individuals that we can grow and second, the number of individuals that we can bring to reproduce. The last quantity is named "efficient population", which is, the number of individuals in the population that will have the same chance to reproduce and transmit their genetic stuff to the next generation.
The efficient population is calculated in two way. First when reproducing, this will be the number of individuals that are active in the reproduction event, with a consideration of the sex ratio.
It is then calculated following the relation:
Ne = [4 x Nm x Nf] / [Nm + Nf]
Where Nm is the number of males, Nf lthe number of females and Ne, lthe size of the efficient population.
For example, if we take a population of 200 individuals, it is not the same in reproduction success if we obtain 100 males and 100 females or if we obtain 120 males et 80 females. Ne diminishes from 200 to 72, translating in this way the diminution of the number of possibilities of genetic exchange during the reproduction of the population.
But the efficient population is also a dynamic concept, because we know all that if a generation has been reproduced with 3 individuals, it is not worth the pain to reproduce it with 2000 individuals at the following generation. We say that the strain undergoes a bottle neck.
We can calculate the consequences of bottlenew with the following relation :
Ne = 1 / [1/t ( 1/N1 + 1/N2 + … + 1/Nt)]
Where t is the number of generations.
Imagine that we have reproduced N1 = 50 individuals then N2 = 8 individuals then N3= 50 individuals.
Ne = 1 / [1/3 x (1/50 + 1/8 + 1/50)] = 18,2
Whereas the mean number of individuals was 36, the bottle neck in the second generation conducted to a diminution of the mean efficient population to 18 individuals !
2.3 The concept of founder effect
When we own only a small number of seeds of a particular variety, the diversity of this sample has big probabilities to be inferior to the real diversity of the whole variety. This population, that comes from these seeds, undergoes the founder effect, which is the fact that its diversity will allways be restricted to the initial representativeness compared to the real whole population.This is a problem when we own only a small sample that has a bad representativeness of the diversity of the strain and when it will never be enriched by new individuals grown outside this sample. This is the case when we own only 25 seeds of a rare variety that disappeared. The positive side of the thing is that maintaining such a population is probably easier than maintaining the diversity of a strain for which we have 10,000 seeds, once the first reproduction ensured, obviously...
2.4 The concept of genetic drift
As we have seen before, this risk happens when population are of small size. Indeed, the allelic frequencies are valids when we work on big numbers. In case of small numbers, it is possible that randomly, we find a rare phenotype that is over represented in the next generation, if, for example, it is the only female that survives.
In fact, the genetic drift is mostly not avoidable with our grow conditions. We have seen that the most important factor modifying the allelic frequencoes was the ability to reproduce. And this will be precisely the big difficulty (aside with the capacity to grow large populations) that we will have to fight when growing landraces.
Outdoor, the main problem will be the lack of light and the flowering time. When growing under temperate climate, it is hard to finish pure tropical sativas due to freeze temperatures in december/january. And the sun has not the same intensity than under tropical conditions. Nothing is balck or white, and we may observe most of the potential of the strain and even test its full potential. But for the phenotypes that are rare and extrem (which are the ones that need huge light and long flowering time ; and also a great number of individuals to have a chance to see them ,for example), it will be hard to observe and reproduce them like they would do in thier native environment. The genetic drift is most probable at more or less long term, as a function of grow conditions and grower abilities.
Indoor, we can escape from the problem of the long flowering time but not from the problem of the light (which is different here, it is more likely to be a problem of spectra) nor from the population size. Often, we will have also the problem of the size of the individuals which is reduced indoor, very far from natural size of the outdoor individuals. Indoor, the grower has a very limited vision of the potential of the strain, which is here the potential of the strain adapted indoor. As a paradox, indoor is better adapted for the reproduction, allowing to pollinate easily and to ensure that pollination will not be polluted by other strains coming from the neighbourhood (the pollen of C. sativa can travel for dozen of kilometers or more...).
To conclude with this genetic drift, you must absolutely understand that genetic drift is a random process that affects both lethal and positive alleles. This means that a population can both fix lethal alleles as the generations goes AND /OR fix beneficial alleles which will make it adapt better to its environment.
2.5 the concept of degree of consanguinity
In small populations, the probability of consanguinity is higher. This consanguinity conducts to the increase of homozygous (which, again, is not strictly synonymous to inbreeding depression!)
The degree of consanguinity in a population can be determined with the coefficient of consanguinity F:
F = (2 pq - H) / 2 pq
Where 2 pq is the expected frequency of heterozygous and H the frequency of heterozygous really observed in the population.
In a small population, heterozygous rate will decrease at each generation, as a function of efficient population size. It will decrease as much as the efficient population size is small, following the relation:
Ht / H0 = [1 – 1/ (2 x Ne)]t
Where H0 is the initial frequency of heterozygous, Ht is the frenquency of heterozygous after t generations and Ne is the mean size of the efficient population.
What we call inbreeding depression is the increase of the homozygous rate for lethal alleles. But this increase may also concern favourable alleles.
In some species, this inbreeding depression may in fact save the species if it is immediately followed by a selection event. It is what is called (in french) the purge of the genetic load. The selection eliminates drastically the homozygous for lethal alleles and only individuals homozygous for favourable alleles survive. The strain is globally improved.
Autofecundation is a particular characteristic that speed up the fixing of the recessive traits, whether good or bad. Some species that use this process doesn't have anymore reduction of their fitness as the generations goes on: natural and/or human selection events, with autofecundation, i.e. strong consanguinity, have removed the worst alleles and conserved only the favourable ones.
For other species, animal species particularly, the consanguinity conducts quickly to disappearance, since it conducts quickly to the fixing of lethal alleles at the level of reproduction functions.
2.6 The ex-situ conservation
This is the kind of conservation that interests us here. It is simply the conservation of varieties out of their natural environment. How to minimize the effects of this conservation on genetic diversity?
If we use again the formula dedicated to calculate the degree of consanguinity, we can propose a solution to maximize heterozygous in the population :
Ht / H0 = [1 – 1/ (2 x Nfo)] x [1 – 1/ (2 N (Ne / N)]^t-1
Where N is the real population, Ne the mean efficient population on the t generations and Nfo the founder population.
If we analyse this relation, we can see three principles to follow :
- Use the highest number of individuals when reproducing the founder population
- Maximize the ratio Ne/N so that the maximum number of individuals reproduce at each generation
- Minimize t, which means, minimize the number of generations
Conclusion
We have seen in this text shortly what we have to know in genetics to maintain old school lines. If you read a little bit more in the books (and you'll do it after this marvelous post, don't you? ), you will realize quickly that these elements stay at the theoretical level, because they act at a large scale, and we have difficulties to link them with the thousands of genes that are in living organisms.
If you stay at the mathematical level, you may be discouraged but I advice you to look at it quietly. Please note that without the real allele frequencies, for each variety, we can not determine anything precisely and then we use these concepts to inspire ourselves, think to the problem, define an attitude to have when growing...
Make your opinion! Look closely at your varieties: are they diverse? What are the criteria that define them? How many individuals can you grow at a time? How many grow per year?
We know we should conserve each strain that crosses our roads. But you and I know the difficulties of such projects... Are we really able to save these lines? Would we be able to do it even if the laws change? Look at all the problems, and read with a critical eye what it written in the forums, whether it comes from professionnals, amateur or biologists...A preservation that is well done (i.e. with a careful selection) is far away better than a global conservation project that missed a step. You have here the elements that prove it.
Concentrate yourself on your possibilities and please don't stop on the words. In this forum, you will never see someone criticizing you if you are testing something. Read and jump ! Surf on the wave !
If you feel alone with your project, contact other members and discuss with them. Preserving is a difficult art but it has the advantage to be exciting for your brain and it is nice to discuss it with other people, to share ideas. The old lines allways had difficulties to be maintained, because of their necessity to grow in a particular environment, but also because some men want to destroy them or even worst want to hide them to exploit them better. Will we be able to do something best than in the past?
Original Post by Rahan
-
1
3 Commentaires
Commentaires recommandés