Types of Seed Populations

Types of Seed Populations

Inbred Line/ Pure Line

Some refer to these as IBLs. An inbred line is a seedlot that has been bred for generations selecting repeatedly for specific traits, to the point where the population reliably reproduces the traits under selection in each successive generation of breeding. These plants are said to breed true for these characteristics. There is little or no variation for these traits, which are thus considered pure. Pure lines make the best breeding stock as the progeny of crosses using known pure parental lines have a predictable outcome in subsequent generations. Inbred lines are uniform in growth patterns and traits and are stable genetically- each generation of inbred seed results in plants similar in most ways to the previous generation. Pure lines are homozygous at most alleles.

Hybrids

Hybrids are a product of a cross between genetically unlike parents. Hybrids retain their distinctive characters if reproduced asexually but fail to reproduce these characters completely or reliably when reproduced sexually. Hybrid cultivars are developed by using available inbred lines or creating new ones from segregating populations, and then coupling selection with inbreeding for homozygosity, evaluation of inbreds for combining ability in hybrid combinations, and subsequent multiplication of selected inbred lines for hybrid seed production.

There are various types of hybrid varieties:

• F1 hybrids- (‘Northern Lights’ x ‘Blueberry’, ‘Northern Lights’ x ‘Haze’, ‘Frostbite’)
• 3-way crosses- (‘Skunk #1’- a cross of (Mexican x Columbian) x Afghani)
• Double cross hybrids- (a cross of two unrelated F1 hybrids ‘Haze’ (Afghani x Thai) x (Mexican x Columbian).

F1 Hybrid Varieties

An F1 hybrid population is obtained by crossing two unrelated, true breeding varieties. F1 hybrids are unique in that they are uniform when grown from seed, but, like all hybrids, are genetically unstable. If reproduced sexually by inbreeding within the F1 population, the subsequent generation will be neither uniform nor similar to the F1 generation.

One of the major benefits of F1 seed to the grower is a condition known as hybrid vigor, or heterosis. Hybrid vigor occurs when the progeny resultant from crossing the two parental inbred lines exceed the performance of the parental lines in some character, or most often in sets of characters (F1 < or > P1 or P2).

F1 hybrids are often bigger and more robust and grow faster than either of the parent populations used in the creation of the F1 population. For example, a (‘Skunk #1’ x ‘Blueberry’) F1 hybrid may grow faster and yield more than either the pure ‘Skunk #1’ or ‘Blueberry” parent populations. Often, heterosis is apparent as a tolerance to adverse environmental conditions.

F1 seed production has benefits to the breeder or seed-maker, as well as the grower. True-breeding seed can be easily reproduced by open-pollination. Most seed companies have no interest in selling easily reproduced seed. This is as true of corn as it is of cannabis. Very few companies that do take time and effort to breed stable parent stock release it in a pure form. Most make and release hybrids, as certain competitor companies’ sole mission is to create knockoff versions of lines released by those who have actually taken time to develop new true-breeding lines. By releasing only hybrids of their pure lines, seed banks ensure the customer comes back to buy more of the F1 seed each time they wish to do a new seed planting of the variety. They also protect the investment of their long-term breeding effort by removing the possibility that a competitor will reproduce their work and sell it as their own.

Unfortunately, breeders of cannabis varieties have no recourse to the law when others reproduce and market their years of work. Due to the illegal nature of the plant, cannabis varieties are not protected by the various plant breeder’s rights legislation around the world.

Variety

A subdivision of a kind, group, or family that is distinct in some characters. Within the variety, all plants exhibit a set of defined morphological, physiological, or other chemical characteristics that differentiate the variety from all other varieties. The variety must be uniform. Variations in essential and distinctive characters are described and characterized by the breeder. The variety will remain unchanged to a reasonable degree of reliability in its essential and distinctive characteristics and its uniformity when reproduced.*

Cultivar

A term derived from “cultivated variety”, a population of cultivated plants clearly distinguished by any number of morphological, physiological, cytological, or chemical characteristics. When reproduced sexually or asexually the population retains its distinguishing characteristics.*

New selections derived from a cultivar or variety such that the selections shows sufficient variation from the parent cultivar to render it worthy of a name, are to be regarded as a distinct cultivar.

*Note that the Guidelines for Classifying Cultivated Plant Populations (1978) stipulate that variety and cultivar are considered equivalents.

Strain

Strain is not a scientifically accepted botanical term, although in the cannabis industry many use the term when discussing seedlots for purchase, for lack of a more accurate term. Strain is a term incorrectly applied to selections of cultivars or varieties. In the cannabis seed industry, very few seedlots could be considered true varieties or cultivars, because they are not uniform or do not breed true. All plants within the population do not reproduce the defining characteristics, and, usually the variations in the defining characteristics are not described. Very often, commercially sold seeds are nothing more than hybrids of hybrids with names, and there are no defining characteristics of the “strain”. Perhaps “family” or “group” are more appropriate terms.

Open pollinated Varieties

Non-hybrid populations reproduced by random pollination within the variety. All pistillate individuals have the potential to mate with all staminate individuals as the pollen spreads randomly, ensuring preservation of the genetic diversity within the breeding population. In cannabis, open pollination is carried out by planting the breeding population together in a given plot isolated from other pollen sources and left to the will of the wind. To maintain varietal purity, hemp breeders ensure there is no non-varietal pollen source within four miles upwind, and one mile downwind- which should demonstrate just how far cannabis pollen can travel on the wind.

Heirloom Varieties/ Heirloom Seeds

The product of many years of selective planting and seed saving. The original seeds bore a plant or flower that had particular traits the grower liked- typically flavor, color, or psychoactive effect. The grower then saved the seeds from the desirable plant and repeated the process the next season selecting for similar type plants. The term “heirloom seeds” came about because the selection process for some cultivars has been going on for generations, often passed along within a family and/or shared with friends.

Heirloom varieties are non-hybrid (open-pollinated). This simply means that they breed relatively true. Thus, growers can save seeds from their crops, plant them the following year, and expect to see offspring that are very much like parent generation. Any off-types in each generation should be rogued out of the breeding population to keep it pure, as they are likely the result of pollen contamination from an external source.

Multi-line

Two or more pure-breeding lines, which are very similar, but differ is a small part of the overall phenotype (i.e., maturation, disease resistance). The varieties are grown and bred separately but are subsequently mixed together and sold in the same seed package. These packs are a benefit to growers if the grower’s given environment is inconsistent from year to year, or for growers who are experimenting with growing in a new location. For example, a multi-line may include a slightly earlier-maturing variety with a slightly more mold-resistant variety; most other traits are equivalent in each population. The variations in performance of each variety with regard to mold or earliness of maturity ensure that there will be some harvest even in a year where only the early varieties finish (as a result of early rains), or even if mold is more prevalent during the particular grow season. If a grower is new to an area, multi-lines may be useful for the first few years of planting. It is always a shame to plant a single variety, only to find it is not suitable for the particular climate, thus wasting the year production. The grower may not reap the highest yield as may be possible from a single hybrid variety particularly suited to the climate, but the degree of variation present in multi-lines helps to ensure at least some plants are harvested.

Synthetic Variety

An interbreeding population derived from intermating a group of specific genotypes, each of which were selected for good combining ability in all possible hybrid combinations, Subsequent maintenance of the variety is achieved by open pollination and usually rounds of recurrent selection over a series of generations.

Intersexuality

Intersexuality is a trait that can be expressed due to a multitude of causes, both genetic and environmental. There are intersex plants which are strictly genetic; these plants have inherited a gene that triggers the intersex condition, even given a perfect growing environment. They produce both pistillate and staminate flowers on the same individual under typical environmental conditions. Strict negative selection against these plants is required by breeders and growers in order to eliminate the intersex trait from the breeding population. Cultivators and breeders alike have wisely selected against plants that show the slightest degree of intersexuality. They know even a single male flower on an otherwise female plant can result in the majority of the crop being pollinated, and thus seeded.

Indoors, where growers attempt to mimic Mother Nature, plants often undergo stresses which are not present under natural conditions. When plants are stressed by being grown in an inhospitable environment, the typical expression of characteristics can be altered. Intersexuality, for example, can also be induced in cannabis by a grower’s influence as a result of an inconsistent growth environment.

Environmentally stressed female plants have been known to show the occasional male flower. Interrupted dark cycles and other types of stressors can result in the development of staminate flowers on otherwise pistillate individuals. Environmental conditions which may provoke sexual reverse include an inconsistent photoperiod, nutrient toxicities and deficiencies, pH issues, or drastically fluctuating temperatures during the flowering cycle. Females severely stressed, for any reason, are more prone to develop a few male flowers. These stresses cause changes in the levels of a plant hormone called ethylene.

Ethylene is one of only a few known plant hormones, and plays many roles in plant development across a range of species. In cannabis, one of ethylene’s major roles is its involvement in the determination of sex. It regulates which flowers should be produced- stamin or pistil. We know this because applying high enough concentrations of ethylene to staminate individuals in the flowering cycle results in the formation of pistils. Conversely, applying ethylene-inhibiting agents to pistillate individuals as they enter flowering results in the formation of stamens in place of pistils. This practice can be of use to breeders in the creation of “feminized” seeds, or all-female (gynoecious) seedlots.

All female seeds are produced by obtaining pollen from one female individual, and subsequently fertilizing another female plant.

When we previously discussed chromosomes, we said there were 20 chromosomes in each cell of the plant. The 10th pair of chromosomes, the smallest pair, are the sex chromosomes. Female cannabis plants have two copies of the X chromosome, therefore their genotype is XX. Males plants have only one copy of the X chromosome, and a Y chromosome instead of a second X chromosome. The genotype of male plants in terms of the sex chromosomes is XY.

When pollen is created within the plant, one of each of the chromosomes pairs is packaged into the cells that develop into pollen. Each pollen grain or ovule contains 10 chromosomes, 1 copy of each pair. When the pollen deposits the genetic material into the ovule unite to make a total of 20 chromosomes, a full genetic compliment.

A Punnet square from a typical male:female cross

Male genotype = XY

Female genotype = XX

A Punnet square from a female:female cross-

Some growers intentionally use the pollen from intersex plants to fertilize females. They have found that the seeds and subsequent offspring produced from this union will be predominately female. The major problem with this technique is that these plants will have intersex tendencies. By selecting parent plants that have intersex tendencies, we ensure that some of the progeny will also have intersex tendencies. Using pollen from an intentional selection for intersexuality-like begets like.

Some seed companies market “feminized seeds”, which are produced by collecting pollen from carefully selected, latent, stress-induced hermaphrodites, and use it to pollenate female plants. This process is time consuming and arduous, but yields mostly female plants when frown without stress. However, under the stresses that resulted in the intersexuality for the pollen parent, the progeny will often show some degree of intersexuality as well. Again, like begets like.

There are hormone treatments, which, when applied to cannabis, result in the formation of staminate flowers on otherwise pistillate plants. To select against the intersex condition, we take our chosen female breeding candidates and grow them under stressful conditions that may lead to the formation of male flowers- irregular light cycle, high heat, etc. Only plants that resist intersexuality under these conditions should be considered as potential breeding parents for the creation of all-female seed lines. We call these intersex-resistant plants “true females”. Intentional selection against intersex plants is the only way to ensure intersex-free offspring.

Clone copies of these pistillate intersex-resistant plants are then sprayed with our hormone treatment and placed into the flowering cycle and allowed to develop stamens. It typically takes three to five weeks for the plants to enter dehiscence and shed pollen. True female candidates that also resist intersexuality under typical stresses, are pollinated by pollen obtained by our hormone-treated, gender-reversed, stamen-bearing female plants. The result is a true gynoecious population, consisting entirely of female plants.

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