Inbreeding

Inbreeding is nothing more than crossing a group, family, or variety of plants within themselves with no additions of genetic material from an outside or unrelated population. The most severe form of inbreeding is the self-cross, in which only one individual's genetic material forms the basis of subsequent generations. Ml

A male plant is fertilizing a female.

Subsequent Inbreeding
Inbreeding consists of crossing a group, family; or variety of plants with themselves.

hybrid populations are only slightly less narrow, derived from the genetic materia! of 2 individuals. Such tight or narrow breeding populations lead to a condition called "inbreeding depression" upon repeated self-breeding or inbreeding.

Inbreeding depression is a reduction in vigor (or any other character) due to prolonged inbreeding. This can manifest as a reduction in potency or a decrease in yield or rate of growth. Progress of depression is dependent, in part, on the breeding system of the crop. Earlier, when we discussed dioecy, we said cannabis is an out-crossing or cross-pollinating species. Cross-pollinated crops usually exhibit a higher degree of inbreeding depression when "selfed," or inbred, than do selfing crops. For example, tomato (an inbreeding or selfing species) can be selled for 20 generations with no apparent loss in vigor or yield, whereas some experiments have shown that the yield of corn per acre is decreased quite dramatically when inbred for 20 generations.

In cross-pollinated crops, deleterious genes remain hidden within populations, and the negative attributes of these recessive traits can be revealed or unmasked via continual inbreeding. Inbreeding depression can be apparent in S! populations after a single generation of self-fertilization. When breeding cannabis using small populations, as is often the case with continual 1:1 mating schemes, inbreeding depression typically becomes apparent within three to six generations. To deal with this problem, breeders often maintain separate parallel breeding lines, each of which are selected for similar or identical sets of traits. After generations of inbreeding, when each of the inbred lines, or selfed populations, begin to show inbreeding depression, they are hybridized or out-crossed to each other to restore vigor and eliminate inbreeding depression while preserving the genetic stability of the traits under selection.

The vast majority ol texts written to date on the subject of breeding cannabis have espoused

1:1 mating strategies, much to the detriment and health of cannabis germplasm. Sadly, this is the preferred breeding scheme used today by the majority of commercial seed banks. These breeders don't realize that cannabis is naturally an out-crossing or cross-pollinating species and existed in wild breeding populations of hundreds if not thousands of individuals. Within these many individuals lies a wide range of versions of different genes. When we select only one or two plants from this vast array as our breeding population, we drastically reduce the genetic variability found in the original population (a genetic bottleneck). This variability is lost from the populations, and unavailable to future generations.

Outbreeding - the process of crossing or hybridizing plants or groups of plants with other plants to which there is no, or only a distant, relation. Any time a breeder is hybridizing using plants that reside outside of the family, group, or variety, hybrid seed is produced. For example, an Fl hybrid seed is the first generation offspring resulting from a cross of two distinct true-

This pyramid shows the evolution of breeding cannabis.

3311

BREEDING

breeding plants or populations. Each of the parent populations were hybridized (out-crossed to each other) to produce the new generation, which is now comprised of genetics from both parental populations. Out-crossing results in the introduction of new and different genetic material to each of the respective pools.

Filial Breeding - A type of breeding system where siblings of the same progeny lot and generation are inter-mated to produce new generations. The first hybrid generation of two distinct true-breeding lines is denoted the F1 generation (F, filial). If two Ft siblings are bred, or the Ft population is allowed to be open pollinated, the resulting generation is labeled F2.

Mating siblings chosen from the F2, results in the F3 population. F4, F5, F6 generations, etc., are obtained in the same manner, by crossing plants of the same generation and progeny lot. Note that as long as any number of siblings of a generation (F[n]) are mated, the resulting generations is denoted (F[n+1]).

Filial inbreeding with selection for specific traits is the most common method for establishing a pure or a true-breeding population, when breeding cross-pollinated species such as cannabis.

Backcross Breeding - A type of breeding that involves repeated crossing of progeny with one of the original parental genotypes; cannabis breeders most often cross progeny to the mother plant, This parent is known as the recurrent parent. The non-recurrent parent is called the donor parent. More widely, any time a generation is crossed to a previous generation, it is a form of backcross breeding. Backcross breeding has become one of the staple methods clandestine cannabis breeders use, mainly because it is a simple, rapid method when using greenhouses or growrooms, and requires only small populations. The principal goal of backcross breeding is to create a population of individuals derived mainly from the genetics of one single parent (the recurrent parent).

The donor parent is chosen based on a trait of

Sprayed with 'Stamen-It!' from PC-Solutions, this plant that started out as a female is now expressing only male flowers.

interest that the recurrent parent lacks; the idea is to introgress this trait into the backcross population, such that the new population is comprised mainly of genetics from the recurrent parent, but also contains the genes responsible for the trait of interest from the donor parent.

The backcross method is a suitable scheme for adding new desirable traits to a mostly ideal, relatively true-breeding genotype. When embarking on a backcross breeding plan, the recurrent parent should be a highly acceptable or nearly ideal genotype (for example, an existing commercial cultivaror inbred line). The ideal traits considered for introgression into the new seed line should be simply inherited and easily scored for phenotype. The best donor parent must possess the desired trait, but should not be seriously deficient in other traits. Backcross line production is repeatable, if the same parents are used.

Backcross breeding is best used when adding simply inherited dominant traits that can easily be identified in the progeny of each generation (Example 1). Recessive traits are more difficult to select for in backcross breeding, since their

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S3ns expression is masked by dominance in each backcross to the recurrent parent. An additional round of open pollination or sib-mating is needed after each backcross generation, to expose homozygous-recessive plants. Individuals showing the recessive condition are selected from F2 segregating generations and backcrossed to the recurrent parent (see Example 2).

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