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 Deadkndys420
Registered: 08/28/12
Posts: 8,703
Loc: █████
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 Cannabis Genetics Part 1.5 
#663660 - 03/26/13 02:37 PM (11 years, 7 months ago) |
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This is a continuation from "Cannabis Genetics Part 1"
Part 1.5
Large amounts of hybrid seed are most easily produced by planting two strains side by side, removing the staininate plants of the seed strain, and allowing nature to take its course. Pollen- or seed-sterile strains could be developed for the production of large amounts of hybrid seed without the labor of thinning; however, genes for sterility are rare. It is important to remember that parental weak nesses are transmitted to offspring as well as strengths. Because of this, the most vigorous, healthy plants are al ways used for hybrid crosses.
Also, sports (plants or parts of plants carrying and expressing spontaneous mutations) most easily transmit mutant genes to the offspring if they are used as pollen parents. If the parents represent diverse gene pools, hybrid vigor results, because dominant genes tend to carry valuable traits and the differing dominant genes inherited from each parent mask recessive traits inherited from the other. This gives rise to particularly large, healthy individuals. To increase hybrid vigor in offspring, parents of different geo graphic origins are selected since they will probably represent more diverse gene pools.
Occasionally hybrid offspring will prove inferior to both parents, but the first generation may still contain recessive genes for a favorable characteristic seen in a parent if the parent was homozygous for that trait. First generation (F1) hybrids are therefore inbred to allow recessive genes to recombine and express the desired parental trait. Many breeders stop with the first cross and never realize the genetic potential of their strain. They fail to produce an F2 generation by crossing or self-pollinating F1 offspring. Since most domestic Cannabis strains are F1 hybrids for many characteristics, great diversity and recessive recombination can result from inbreeding domestic hybrid strains. In this way the breeding of the F1 hybrids has already been accomplished, and a year is saved by going directly to F2 hybrids. These F2 hybrids are more likely to express recessive parental traits. From the F2 hybrid generation selections can be made for parents which are used to start new true-breeding strains. Indeed, F2 hybrids might appear with more extreme characteristics than either of the P~ parents. (For example, P1 high-THC X P1 low-THC yields F1 hybrids of intermediate THC content. Selfing the F1 yields F2 hybrids, of both P1 [high and low THC] phenotypes, inter mediate F1 phenotypes, and extra-high THC as well as extra-low THC phenotypes.)
Also, as a result of gene recombination, F1 hybrids are not true-breeding and must be reproduced from the original parental strains. When breeders create hybrids they try to produce enough seeds to last for several successive years of cultivation, After initial field tests, undesirable hybrid seeds are destroyed and desirable hybrid seeds stored for later use. If hybrids are to be reproduced, a clone is saved from each parental plant to preserve original parental genes.
Back-crossing is another technique used to produce offspring with reinforced parental characteristics. In this case, a cross is made between one of the F~ or subsequent offspring and either of the parents expressing the desired trait. Once again this provides a chance for recombination and possible expression of the selected parental trait. Back-crossing is a valuable way of producing new strains, but it is often difficult because Cannabis is an annual, so special care is taken to save parental stock for back-crossing the following year. Indoor lighting or greenhouses can be used to protect breeding stock from winter weather. In tropical areas plants may live outside all year. In addition to saving particular parents, a successful breeder always saves many seeds from the original P1 group that produced the valuable characteristic so that other P1 plants also exhibiting the characteristic can be grown and selected for back-crossing at a later time.
Several types of breeding are summarized as follows:
1 - Crossing two varieties having outstanding qualities (hybridization).
2 - Crossing individuals from the F1 generation or selfing F1 individuals to realize the possibilities of the original cross (differentiation).
3 - Back crossing to establish original parental types.
4 - Crossing two similar true-breeding (homozygous) varieties to preserve a mutual trait and restore vigor.
It should be noted that a hybrid plant is not usually hybrid for all characteristics nor does a true-breeding strain breed true for all characteristics. When discussing crosses, we are talking about the inheritance of one or a few traits only. The strain may be true-breeding for only a few traits, hybrid for the rest. Monohybrid crosses involve one trait, dihybrid crosses involve two traits, and so forth. Plants have certain limits of growth, and breeding can only pro duce a plant that is an expression of some gene already present in the total gene pool. Nothing is actually created by breeding; it is merely the recombination of existing genes into new genotypes. But the possibilities of recombination are nearly limitless.
The most common use of hybridization is to cross two outstanding varieties. Hybrids can be produced by crossing selected individuals from different high-potency strains of different origins, such as Thailand and Mexico. These two parents may share only the characteristic of high psycho activity and differ in nearly every other respect. From this great exchange of genes many phenotypes may appear in the F2 generation. From these offspring the breeder selects individuals that express the best characteristics of the parents. As an example, consider some of the offspring from the P1 (parental) cross: Mexico X Thailand. In this case, genes for high drug content are selected from both parents while other desirable characteristics can be selected from either one. Genes for large stature and early maturation are selected from the Mexican seed-parent, and genes for large calyx size and sweet floral aroma are selected from the Thai pollen parent. Many of the F1 offspring exhibit several of the desired characteristics. To further promote gene segregation, the plants most nearly approaching the ideal are crossed among themselves. The F2 generation is a great source of variation and recessive expression. In the F2 generation there are several individuals out of many that exhibit all five of the selected characteristics. Now the process of inbreeding begins, using the desirable F2 parents.
If possible, two or more separate lines are started, never allowing them to interbreed. In this case one accept able staminate plant is selected along with two pistillate plants (or vice versa). Crosses between the pollen parent and the two seed parents result in two lines of inheritance with slightly differing genetics, but each expressing the desired characteristics. Each generation will produce new, more acceptable combinations.
If two inbred strains are crossed, F1 hybrids will be less variable than if two hybrid strains are crossed. This comes from limiting the diversity of the gene pools in the two strains to be hybridized through previous inbreeding. Further independent selection and inbreeding of the best plants for several generations will establish two strains which are true-breeding for all the originally selected traits. This means that all the offspring from any parents in the strain will give rise to seedlings which all exhibit the selected traits. Successive inbreeding may by this time have resulted in steady decline in the vigor of the strain.
When lack of vigor interferes with selecting phenotypes for size and hardiness, the two separately selected strains can then be interbred to recombine nonselected genes and restore vigor. This will probably not interfere with breeding for the selected traits unless two different gene systems control the same trait in the two separate lines, and this is highly unlikely. Now the breeder has produced a hybrid strain that breeds true for large size, early maturation, large sweet-smelling calyxes, and high THC level. The goal has been reached!
Wind pollination and dioecious sexuality favor a heterozygous gene pool in Cannabis. Through Anbreeding, hybrids are adapted from a heterozygous gene pool to a homozygous gene pool, providing the genetic stability needed to create true-breeding strains. Establishing pure strains enables the breeder to make hybrid crosses with a better chance of predicting the outcome. Hybrids can be created that are not reproducible in the F2 generation. Commercial strains of seeds could be developed that would have to be purchased each year, because the F1 hybrids of two pure-bred lines do not breed true. Thus, a seed breeder can protect the investment in the results of breeding, since it would be nearly impossible to reproduce the parents from F2 seeds.
At this time it seems unlikely that a plant patent would be awarded for a pure-breeding strain of drug Cannabis. In the future, however, with the legalization of cultivation, it is a certainty that corporations with the time, space, and money to produce pure and hybrid strains of Cannabis will apply for patents. It may be legal to grow only certain patented strains produced by large seed companies. Will this be how government and industry combine to control the quality and quantity of "drug" Cannabis?
Acclimatization
Much of the breeding effort of North American cultivators is concerned with acclimatizing high-THC strains of equatorial origin to the climate of their growing area while preserving potency. Late-maturing, slow, and irregularly flowering strains like those of Thailand have difficulty maturing in many parts of North America. Even in a green house, it may not be possible to mature plants to their full native potential.
To develop an early-maturing and rapidly flowering 8train, a breeder may hybridize as in the previous example. However, if it is important to preserve unique imported genetics, hybridizing may be inadvisable. Alternatively, a pure cross is made between two or more Thai plants that most closely approach the ideal in blooming early. At this point the breeder may ignore many other traits and aim at breeding an earlier-maturing variety of a pure Thai strain. This strain may still mature considerably later than is ideal for the particular location unless selective pressure is exerted. If further crosses are made with several individuals that satisfy other criteria such as high THC content, these may be used to develop another pure Thai strain of high THC content. After these true-breeding lines have been established, a dihybrid pure cross can be made in an attempt to produce an F1 generation containing early-maturing, high-THC strains of pure Thai genetics, in other words, an acclimatized drug strain.
Crosses made without a clear goal in mind lead to strains that acclimatize while losing many favorable characteristics. A successful breeder is careful not to overlook a characteristic that may prove useful. It is imperative that original imported Cannabis genetics be preserved intact to protect the species from loss of genetic variety through excessive hybridization. A currently unrecognized gene may be responsible for controlling resistance to a pest or disease, and it may only be possible to breed for this gene by back-crossing existing strains to original parental gene pools.
Once pure breeding lines have been established, plant breeders classify and statistically analyze the offspring to determine the patterns of inheritance for that trait. This is the system used by Gregor Mendel to formulate the basic laws of inheritance and aid the modern breeder in predicting the outcome of crosses,
1 - Two pure lines of Cannabis that differ in a particular trait are located.
2 - These two pure-breeding lines are crossed to pro duce an F1 generation.
3 - The F1 generation is inbred.
4 - The offspring of the F1 and F2 generations are classified with regard to the trait being studied.
5 - The results are analyzed statistically.
6 - The results are compared to known patterns of inheritance so the nature of the genes being selected for can be characterized.
Fixing Traits
Fixing traits (producing homozygous offspring) in Cannabis strains is more difficult than it is in many other flowering plants. With monoecious strains or hermaphrodites it is possible to fix traits by self-pollinating an individual exhibiting favorable traits. In this case one plant acts as both mother and father. However, most strains of Cannabis are dioecious, and unless hermaphroditic reactions can be induced, another parent exhibiting the trait is required to fix the trait. If this is not possible, the unique individual may be crossed with a plant not exhibiting the trait, inbred in the F1 generation, and selections of parents exhibiting the favorable trait made from the F2 generation, but this is very difficult.
If a trait is needed for development of a dioecious strain it might first be discovered in a monoecious strain and then fixed through selfing and selecting homozygous offspring. Dioecious individuals can then be selected from the monoecious population and these individuals crossed to breed out monoecism in subsequent generations.
Galoch (1978) indicated that gibberellic acid (GA3) promoted stamen production while indoleacetic acid (IAA), ethrel, and kinetin promoted pistil production in prefloral dioecious Cannabis. Sex alteration has several useful applications. Most importantly, if only one parent expressing a desirable trait can be found, it is difficult to perform a cross unless it happens to be a hermaphrodite plant. Hormones might be used to change the sex of a cutting from the desirable plant, and this cutting used to mate with it. This is most easily accomplished by changing a pistillate cutting to a staminate (pollen) parent, using a spray of 100 ppm gibberellic acid in water each day for five consecutive days. Within two weeks staminate flowers may appear. Pollen can then be collected for selfing with the original pistillate parent. Offspring from the cross should also be mostly pistillate since the breeder is selfing for pistillate sexuality. Staminate parents reversed to pistillate floral production make inferior seed-parents since few pistillate flowers and seeds are formed.
If entire crops could be manipulated early in life to produce all pistillate or staminate plants, seed production and seedless drug Cannabis production would be greatly facilitated.
Sex reversal for breeding can also be accomplished by mutilation and by photoperiod alteration. A well-rooted, flourishing cutting from the parent plant is pruned back to 25% of its original size and stripped of all its remaining flowers. New growth will appear within a few days, and several flowers of reversed sexual type often appear. Flowers of the unwanted sex are removed until the cutting is needed for fertilization. Extremely short light cycles (6-8 hour photoperiod) can also cause sex reversal. How ever, this process takes longer and is much more difficult to perform in the field.
Genotype and Phenotype Ratios
It must be remembered, in attempting to fix favorable characteristics, that a monohybrid cross gives rise to four possible recombinant genotypes, a dihybrid cross gives rise to 16 possible recombinant genotypes, and so forth.
Phenotype and genotype ratios are probabilistic. If recessive genes are desired for three traits it is not effective to raise only 64 offspring and count on getting one homozygous recessive individual. To increase the probability of success it is better to raise hundreds of offspring, choosing only the best homozygous recessive individuals as future parents. All laws of inheritance are based on chance and offspring may not approach predicted ratios until many more have been phenotypically characterized and grouped than the theoretical minimums.
The genotype of each individual is expressed by a mosaic of thousands of subtle overlapping traits. It is the sum total of these traits that determines the general phenotype of an individual.
It is often difficult to determine if the characteristic being selected is one trait or the blending of several traits and whether these traits are controlled by one or several pairs of genes. It often makes little difference that a breeder does not have plants that are proven to breed true. Breeding goals can still be established. The selfing of F1 hybrids will often give rise to the variation needed in the F2 generation for selecting parents for subsequent generations, even if the characteristics of the original parents of the F1 hybrid are not known. It is in the following generations that fixed characteristics appear and the breeding of pure strains can begin. By selecting and crossing individuals that most nearly approach the ideal described by the breeding goals, the variety can be continuously improved even if the exact patterns of inheritance are never deter mined. Complementary traits are eventually combined into one line whose seeds reproduce the favorable parental traits. Inbreeding strains also allows weak recessive traits to express themselves and these abnormalities must be diligently removed from the breeding population. After five or six generations, strains become amazingly uniform. Vigor is occasionally restored by crossing with other lines or by backcrossing.
Parental plants are selected which most nearly approach the ideal. If a desirable trait is not expressed by the parent, it is much less likely to appear in the offspring. It is imperative that desirable characteristics be hereditary and not primarily the result of environment and cultivation. Acquired traits are not hereditary and cannot be made hereditary. Breeding for as few traits as possible at one time greatly increases the chance of success. In addition to the specific traits chosen as the aims of breeding, parents are selected which possess other generally desirable traits such as vigor and size. Determinations of dominance and recessiveness can only be made by observing the outcome of many crosses, although wild traits often tend to be dominant. This is one of the keys to adaptive survival. However, all the possible combinations will appear in the F2 generation if it is large enough, regardless of dominance.
Now, after further simplifying this wonderful system of inheritance, there are additional exceptions to the rules which must be explored. In some cases, a pair of genes may control a trait but a second or third pair of genes is needed to express this trait. This is known as gene inter action. No particular genetic attribute in which we may be interested is totally isolated from other genes and the effects of environment. Genes are occasionally transferred in groups instead of assorting independently. This is known as gene linkage, These genes are spaced along the same chromosome and may or may not control the same trait. The result of linkage might be that one trait cannot be inherited without another. At times, traits are associated with the X and Y sex chromosomes and they may be limited to expression in only one sex (sex linkage). Crossing over also interferes with the analysis of crosses. Crossing over is the exchanging of entire pieces of genetic material between two chromosomes.
This can result in two genes that are normally linked appearing on separate chromosomes where they will be independently inherited. All of these processes can cause crosses to deviate from the expected Mendelian outcome. Chance is a major factor in breeding Cannabis, or any introduced plant, and the more crosses a breeder attempts the higher are the chances of success.
Variate, isolate, intermate, evaluate, multiplicate, and disseminate are the key words in plant improvement. A plant breeder begins by producing or collecting various prospective parents from which the most desirable ones are selected and isolated. Intermating of the select parents results in offspring which must be evaluated for favorable characteristics. If evaluation indicates that the offspring are not improved, then the process is repeated. Improved off spring are multiplied and disseminated for commercial use. Further evaluation in the field is necessary to check for uniformity and to choose parents for further intermating. This cyclic approach provides a balanced system of plant improvement.
The basic nature of Cannabis makes it challenging to
breed. Wind pollination and dioecious sexuality, which
account for the great adaptability in Cannabis, cause many
problems in breeding, but none of these are insurmountable. Developing a knowledge and feel for the plant is more important than memorizing Mendelian ratios. The words of the great Luther Burbank say it well, "Heredity is indelibly fixed by repetition."
The first set of traits concerns Cannabis plants as a whole while the remainder concern the qualities of seedlings, leaves, fibers, and flowers. Finally a list of various Cannabis strains is provided along with specific characteristics. Following this order, basic and then specific selections of favorable characteristics can be made.
List of Favorable Traits of Cannabis
in Which Variation Occurs
1. General Traits
a) Size and Yield
b) Vigor
c) Adaptability
d) Hardiness
e) Disease and Pest Resistance
f) Maturation
g) Root Production
h) Branching
i) Sex
2. Seedling Traits
3. Leaf Traits
4. Fiber Traits
5. Floral Traits
a) Shape
b) Form
c) Calyx Size
d) Color
e) Cannabinoid Level
f) Taste and Aroma
g) Persistence of Aromatic Principles and Cannabinoids
h) Trichome Type
i) Resin Quantity and Quality
j) Resin Tenacity
k) Drying and Curing Rate
I) Ease of Manicuring
m) Seed Characteristics
n) Maturation
o) Flowering
p) Ripening
q) Cannabinoid Profile
6. Gross Phenotypes of Cannabis Strains
1. General Traits
a) Size and Yield - The size of an individual Cannabis plant is determined by environmental factors such as room for root and shoot growth, adequate light and nutrients, and proper irrigation. These environmental factors influence the phenotypic image of genotype, but the genotype of the individual is responsible for overall variations in gross morphology, including size. Grown under the same conditions, particularly large and small individuals are easily spotted and selected. Many dwarf Cannabis plants have been re ported and dwarfism may be subject to genetic control, as it is in many higher plants, such as dwarf corn and citrus. Cannabis parents selected for large size tend to produce offspring of a larger average size each year. Hybrid crosses between tall (Cannabis sativa-Mexico) strains and short
(Cannabis ruderalis-Russia) strains yield F1 offspring of intermediate height (Beutler and der Marderosian 1978). Hybrid vigor, however, will influence the size of offspring more than any other genetic factor. The increased size of hybrid offspring is often amazing and accounts for much of the success of Cannabis cultivators in raising large plants. It is not known whether there is a set of genes for "gigantism" in Cannabis or whether polyploid individuals really yield more than diploid due to increased chromosome count. Tetraploids tend to be taller and their water re quirements are often higher than diploids. Yield is determined by the overall production of fiber, seed, or resin and selective breeding can be used to increase the yield of any one of these products. However, several of these traits may be closely related, and it may be impossible to breed for one without the other (gene linkage). Inbreeding of a pure strain increases yield only if high yield parents are selected. High yield plants, staminate or pistillate, are not finally selected until the plants are dried and manicured. Because of this, many of the most vigorous plants are crossed and seeds selected after harvest when the yield can be measured.
b) Vigor - Large size is often also a sign of healthy vigorous growth. A plant that begins to grow immediately will usually reach a larger size and produce a higher yield in a short growing season than a sluggish, slow-growing plant. Parents are always selected for rich green foliage and rapid, responsive growth. This will ensure that genes for certain weaknesses in overall growth and development are bred out of the population while genes for strength and vigor remain.
c) Adaptability - It is important for a plant with a wide distribution such as Cannabis to be adaptable to many different environmental conditions. Indeed, Cannabis is one of the most genotypically diverse and phenotypically plastic plants on earth; as a result it has adapted to environ mental conditions ranging from equatorial to temperate climates. Domestic agricultural circumstances also dictate that Cannabis must be grown under a great variety of conditions,
Plants to be selected for adaptability are cloned and grown in several locations. The parental stocks with the highest survival percentages can be selected as prospective parents for an adaptable strain. Adaptability is really just another term for hardiness under varying growth conditions.
d) Hardiness - The hardiness of a plant is its overall resistance to heat and frost, drought and overwatering, and so on. Plants with a particular resistance appear when adverse conditions lead to the death of the rest of a large population. The surviving few members of the population might carry inheritable resistance to the environmental factor that destroyed the majority of the population. Breeding these survivors, subjecting the offspring to continuing stress conditions, and selecting carefully for several generations should result in a pure-breeding strain with increased resistance to drought, frost, or excessive heat.
e) Disease and Pest Resistance - In much the same way as for hardiness a strain may be bred for resistance to a certain disease, such as damping-off fungus. If flats of seedlings are infected by damping-off disease and nearly all of them die, the remaining few will have some resistance to damping-off fungus. If this resistance is inheritable, it can be passed on to subsequent generations by crossing these surviving plants. Subsequent crossing, tested by inoculating flats of seedling offspring with damping-off fungus, should yield a more resistant strain.
Resistance to pest attack works in much the same way. It is common to find stands of Cannabis where one or a few plants are infested with insects while adjacent plants are untouched. Cannabinoid and terpenoid resins are most probably responsible for repelling insect attack, and levels of these vary from plant to plant. Cannabis has evolved defenses against insect attack in the form of resin-secreting glandular trichomes, which cover the reproductive and associated vegetative structures of mature plants. Insects, finding the resin disagreeable, rarely attack mature Cannabis flowers. However, they may strip the outer leaves of the same plant because these develop fewer glandular tri chomes and protective resins than the flowers.
Non-glandular cannabinoids and other compounds produced within leaf and stem tissues which possibly inhibit insect attack, may account for the varying resistance of seedlings and vegetative juvenile plants to pest infestation. With the popularity of greenhouse Cannabis cultivation, a strain is needed with increased resistance to mold, mite, aphid,- or white fly infestation. These problems are often so severe that greenhouse cultivators destroy any plants which are attacked. Molds usually reproduce by wind-borne spores, so negligence can rapidly lead to epidemic disaster. Selection and breeding of the least infected plants should result in strains with increased resistance.
f) Maturation - Control of the maturation of Cannabis is very important no matter what the reason for growing it. If Cannabis is to be grown for fiber it is important that the maximum fiber content of the crop be reached early and that all of the individuals in the crop mature at the same time to facilitate commercial harvesting. Seed production requires the even maturation of both pollen and seed parents to ensure even setting and maturation of seeds. An uneven maturation of seeds would mean that some seeds would drop and be lost while others are still ripening. An understanding of floral maturation is the key to the production of high quality drug Cannabis. Changes in gross morphology are accompanied by changes in cannabinoid and terpenoid production and serve as visual keys to deter mining the ripeness of Cannabis flowers.
A Cannabis plant may mature either early or late, be fast or slow to flower, and ripen either evenly or sequentially.
Breeding for early or late maturation is certainly a reality; it is also possible to breed for fast or slow flowering and even or sequential ripening. In general, crosses between early-maturing plants give rise to early-maturing offspring, crosses between late-maturing plants give rise to late-maturing offspring, and crosses between late- and early-maturing plants give rise to offspring of intermediate maturation.
This seems to indicate that maturation of Cannabis is not controlled by the simple dominance and recessiveness of one gene but probably results from incomplete dominance and a combination of genes for separate aspects of maturation. For instance, Sorghum maturation is controlled by four separate genes. The sum of these genes produces a certain phenotype for maturation. Al though breeders do not know the action of each specific gene, they still can breed for the total of these traits and achieve results more nearly approaching the goal of timely maturation than the parental strains.
g) Root Production - The size and shape of Cannabis root systems vary greatly. Although every embryo sends out a taproot from which lateral roots grow, the individual growth pattern and final size and shape of the roots vary considerably. Some plants send out a deep taproot, up to 1 meter (39 inches) long, which helps support the plant against winds and rain. Most Cannabis plants, however, produce a poor taproot which rarely extends more than 30 centimeters (1 foot). Lateral growth is responsible for most of the roots in Cannabis plants. These fine lateral roots offer the plant additional support but their primary function is to absorb water and nutrients from the soil.
A large root system will be able to feed and support a large plant. Most lateral roots grow near the surface of the soil where there is more water, more oxygen, and more avail able nutrients. Breeding for root size and shape may prove beneficial for the production of large rain- and wind-resistant strains. Often Cannabis plants, even very large ones, have very small and sensitive root systems. Recently, certain alkaloids have been discovered in the roots of Cannabis that might have some medical value. If this proves the case, Cannabis may be cultivated and bred for high alkaloid levels in the roots to be used in the commercial production of pharmaceuticals.
As with many traits, it is difficult to make selections for root types until the parents are harvested. Because of this many crosses are made early and seeds selected later.
h) Branching - The branching pattern of a Cannabis plant is determined by the frequency of nodes along each branch and the extent of branching at each node. For examples, consider a tall, thin plant with slender limbs made up of long internodes and nodes with little branching (Oaxaca, Mexico strain). Compare this with a stout, densely branched plant with limbs of short internodes and highly branched nodes (Hindu Kush hashish strains). Different branching patterns are preferred for the different agricultural applications of fiber, flower, or resin production. Tall, thin plants with long internodes and no branching are best adapted to fiber production; a short, broad plant with short inter nodes and well developed branching is best adapted to floral production. Branching structure is selected that will tolerate heavy rains and high winds without breaking. This is quite advantageous to outdoor growers in temperate zones with short seasons. Some breeders select tall, limber plants (Mexico) which bend in the wind; others select short, stiff plants (Hindu Kush) which resist the weight of water without bending.
i) Sex - Attempts to breed offspring of only one sexual type have led to more misunderstanding than any other facet of Cannabis genetics. The discoveries of McPhee (1925) and Schaffner (1928) showed that pure sexual type and hermaphrodite conditions are inherited and that the percentage of sexual types could be altered by crossing with certain hermaphrodites.
Since then it has generally been assumed by researchers and breeders that a cross between ANY unselected hermaphrodite plant and a pistillate seed-parent should result in a population of all pistillate offspring. This is not the case. In most cases, the offspring of hermaphrodite parents tend toward hermaphrodism, which is largely unfavorable for the production of Cannabis other than fiber hemp. This is not to say that there is no tendency for hermaphrodite crosses to alter sex ratios in the offspring. The accidental release of some pollen from predominantly pistillate hermaphrodites, along with the complete eradication of nearly every staminate and staminate hermaphrodite plant may have led to a shift in sexual ratio in domestic populations of sinsemilla drug Cannabis. It is commonly observed that these strains tend toward 60% to 80% pistillate plants and a few pistillate hermaphrodites are not uncommon in these populations.
However, a cross can be made which will produce nearly all pistillate or staminate individuals. If the proper pistillate hermaphrodite plant is selected as the pollen-parent and a pure pistillate plant is selected as the seed-parent it is possible to produce an F1, and subsequent generations, of nearly all pistillate offspring. The proper pistillate hermaphrodite pollen-parent is one which has grown as a pure pistillate plant and at the end of the sea son, or under artificial environmental stress, begins to develop a very few staminate flowers. If pollen from these few staminate flowers forming on a pistillate plant is applied to a pure pistillate seed parent, the resulting F1 generation should be almost all pistillate with only a few pistillate hermaphrodites.
This will also be the case if the selected pistillate hermaphrodite pollen source is selfed and bears its own seeds. Remember that a selfed hermaphrodite gives rise to more hermaphrodites, but a selfed pistillate plant that has given rise to a limited number of staminate flowers in response to environmental stresses should give rise to nearly all pistillate offspring. The F1 offspring may have a slight tendency to produce a few staminate flowers under further environmental stress and these are used to produce F2 seed. A monoecious strain produces 95+% plants with many pistillate and staminate flowers, but a dioecious strain produces 95+% pure pistillate or staminate plants. A plant from a dioecious strain with a few inter sexual flowers is a pistillate or staminate hermaphrodite. Therefore, the difference between monoecism and her maphrodism is one of degree, determined by genetics and environment.
Crosses may also be performed to produce nearly all staminate offspring. This is accomplished by crossing a pure staminate plant with a staminate plant that has produced a few pistillate flowers due to environmental stress, or selfing the latter plant. It is readily apparent that in the wild this is not a likely possibility. Very few staminate plants live long enough to produce pistillate flowers, and when this does happen the number of seeds produced is limited to the few pistillate flowers that occur. In the case of a pistillate hermaphrodite, it may produce only a few staminate flowers, but each of these may produce thou sands of pollen grains, any one of which may fertilize one of the plentiful pistillate flowers, producing a seed. This is another reason that natural Cannabis populations tend toward predominantly pistillate and pistillate hermaphrodite plants. Artificial hermaphrodites can be produced by hormone sprays, mutilation, and altered light cycles. These should prove most useful for fixing traits and sexual type.
Drug strains are selected for strong dioecious tendencies. Some breeders select strains with a sex ratio more nearly approaching one than a strain with a high pistillate sex ratio. They believe this reduces the chances of pistillate plants turning hermaphrodite later in the season.
2. Seedling Traits
Seedling traits can be very useful in the efficient and purposeful selection of future parental stock. If accurate selection can be exercised on small seedlings, much larger populations can be grown for initial selection, as less space is required to raise small seedlings than mature plants. Whorled phyllotaxy and resistance to damping-off are two traits that may be selected just after emergence of the embryo from the soil. Early selection for vigor, hardiness, resistance, and general growth form may be made when the seedlings are from 30 to 90 centimeters (1 to 3 feet) tall. Leaf type, height, and branching are other criteria for early selection. These early-selected plants cannot be bred until they mature, but selection is the primary and most important step in plant improvement.
Whorled phyllotaxy is associated with subsequent anomalies in the growth cycle (i.e., multiple leaflets and flattened or clubbed stems). Also, most whorled plants are staminate and whorled phyllotaxy may be sex-linked.
Part 2
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