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Backcrossing & Selfing

Backcrossing & Selfing

Backcrossing is the pollination of a female flower from a male flower on the same plant. Backcross breeding is repeated crossing of progeny with one of the original parental genotypes. It is cross-pollinating 1 generation back to a previous generation; most often the progeny is crossed with the mother plant. The parent is called the “recurrent parent.” The nonrecurrent parent is the “donor parent.”

Apollo 13 BX’ (Back Cross) from TGA Genetics is a good example of a backcrossed variety.

Backcross breeding is the most widely used form of breeding cannabis to date.
Backcross breeding is simple and can be done with small populations of plants. Most often the goal of backcrossing
is to create a population from the genetics of a single parent (the recurrent parent).

Backcrossing also uses the terms squaring (to denote the second backcross to the same parent) and cubing (to designate the third backcross).

Backcrossing: Incorporating a Dominant Trait
Step One: Cross recurrent parent × donor parent

Step Two: Select desirable plants showing dominant trait, and hybridize selected plants to recurrent parent. The generation produced is denoted BC1 (some cannabis breeders call this generation B×1. [BC1= B×1]).

Step Three: Select plants from BC1 and hybridize with the recurrent parent; the resulting generation is denoted BC2.

Step Four: Select plants from BC2 and hybridize with the recurrent parent; the resulting generation is denoted BC3.

Backcrossing: Incorporating a Recessive Trait
Recessive traits are more difficult to select for in backcross breeding, since their 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
as in the “Backcrossing: Incorporating a Dominant Trait” above.

Step One: Cross recurrent parent × donor F1 hybrid generation

Step Two: Select desirable plants, and create an F2 population via full cross-pollination.

Step Three: Select plants showing the desired recessive trait in the F2 generation. Hybridize selected F2-recessive plants to the recurrent parent. The generation produced is denoted BC1.

Step Four: Select plants from BC1, and create a generation of F2 plants via sib-mating; the resulting generation can be denoted BC1F2.

Step Five: Select desirable BC1F2 plants showing the recessive condition, and hybridize with the recurrent parent; the resulting generation is denoted BC2.

Step Six: Select plants from BC2, and create an F2 population via sib-mating; denote the resulting generation BC2F2.

Step Seven: Select plants showing the recessive condition from the BC2F2 generation, and hybridize to the recurrent parent; the resulting generation is denoted BC3.

Step Eight: Grow out BC3, select and sib-mate the most ideal candidates to create an F2 population, where plants showing the recessive condition are then selected and used as a basis for a new inbred, or open-pollinated, seed line. This new generation created from the F2 is a population that consists of, on
average, about 93.7 percent of genes from the recurrent parent, and only about 6.3 percent of genes leftover from the donor parent.

Note: Only homozygous-recessives were chosen for mating in the BC3F2 generation; the entire resulting BC3F3 generation is homozygous for the recessive trait, and breeds true for this recessive trait. Our new population meets our breeding objective. It is a population derived mainly from the genetics of the recurrent parent, yet breeds true for our introgressed recessive trait. Backcross-derived lines are most often well-adapted to the environment in which they will be grown. Indoor garden rooms are easily replicated, and plants grow in a similar environment to that in which they were bred. Progeny therefore need less extensive environmental field-testing.

If 2 or more characters are to be introgressed into a new seed line, these would usually be tracked in separate backcross programs, and the individual products would be combined in a final set of crosses after the new populations have been created by backcrossing.

Backcrossing has drawbacks. If the recurrent parent is not very true-breeding, the resulting backcross generations segregate, and many of the traits deemed desirable to the line fail to be reproduced reliably. Another limitation of the backcross is that the “improved” variety differs only slightly from the recurrent parent (e.g., one trait). If multiple traits are to be introgressed into the new population, other techniques, such as inbreeding or recurrent selection, may be more rewarding.

Small male flower appears on this Chemdog inbred female flower. Inbreeding often causes male intersex flowers to form on female plants.

Self-Pollinating (Selfing) - “Clone Only” Varieties
Often, 2 hybrid plants are crossed and the “variety” is given a name, but soon the male plant is lost, and the plant is available only as a clone. In this case the plant must be “selfed” to produce male flowers on a female plant. Self-pollinating (aka selfing) is the process of creating seed by pollinating a plant with its own pollen. Self-pollinating is a plant having sex with itself. Self-crossing can derive populations of plants from a single individual.

The first generation population derived from selfing an individual is called the S1 population. An individual from S1 that is selfed again is called S2. Subsequent generations derived in the same manner are denoted S3, S4, etc.

Traits for which the plant is homozygous remain homozygous upon selfing, whereas heterozygous loci segregate and may demonstrate novel expressions of these characters.

We know homozygous loci remain homozygous in future generations upon selfing. Heterozygous loci are increased by 50 percent. Every subsequent generation will be 50 percent more homozygous than the parent from which it was derived.

Repeated selfing, or single-seed descent, is the fastest way to achieve homozygosity within a group or family. The more plants grown from a selfed population, the better probability a breeder has of finding selfed progeny that show all the desired traits.

Self-Pollinating - Breeding
Step One: Identify superior genotypes for the trait under selection.

Step Two: Cross superior genotypes and select improved progeny.

Step Three: Repeat steps One and Two over a series of generations.

This is an excerpt from chapter 25 “Breeding” from the Cannabis Encyclopedia (596 pages, 2,000+ color images, large A4 format) by Jorge Cervantes that is available everywhere in English. Spanish edition will be available autumn 2017.
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