Selfing: crossing a plant with itself

Selfing is the fertilisation of a plant with its own pollen. Both halves of the offspring’s genetic information then come from a single genome, that of the mother plant. In cannabis, male and female flowers usually sit on separate plants, so doing this on purpose takes an intervention.

What happens genetically

In a normal cross the two gene copies come from two different parents. In selfing they come from the same plant. That raises the uniformity of the genome: with each generation of selfing, the share of mixed-inherited spots halves on average. The genome becomes step by step more uniform, which is called rising homozygosity.

At a mixed-inherited spot the offspring split in the same ratio as an F2, namely 1:2:1. What the resulting generation is called depends on what the self-pollinated plant was: if it was the F1 from a cross between two parents, its progeny is the F2 of that cross; if it was an individual plant of any other kind, the rounds are counted instead: S1 after the first, S2 after the second, and so on.

Why hidden traits surface

Many traits are recessive, showing only when both gene copies carry the same variant. As long as a plant is mixed at that spot, such traits stay hidden. Selfing brings identical copies together and so makes recessive traits visible, wanted and unwanted alike. For breeding this is a tool to uncover hidden traits.

The other route: sib-mating

Anyone after a true-breeding line has two routes, and selfing is the faster of them. The other is sib-mating, the pairing of two plants from the same progeny; it needs male plants and therefore regular seed, and in dioecious species it is the route the breeding literature works with.

The slower pace has a flip side the time calculation leaves out. At equal levels of inbreeding, sib-mating clears out the variants that are lethal in double copy more effectively, 7.11 remaining variants in the model against 9.3 under selfing, because homozygosity rises step by step rather than in one jump. The model describes natural populations; what transfers is the mechanism, not the number.

What selfing fixes along the way

For a female plant to make its own pollen, sex reversion has to be induced, usually by treatment with silver thiosulfate. How well that works depends on the line, which makes a trait part of the selection that has nothing to do with the breeding goal. Where the capacity for reversion segregates in the source material, it gets stabilised along with everything else, and the finished line carries a raised risk of unwanted spontaneous male flowers.

That did not stay theoretical. In the line whose seven generations were grown side by side, the share of abnormal flowers rose from 0.4 % in the source generation to 13.7 % in the sixth, more than thirtyfold.

The downside: inbreeding depression

The same uncovering has a further price. Among the recessive traits are harmful ones. If continued selfing brings them together in double copy more and more often, vigour and yield decline. This weakening over several generations of selfing is inbreeding depression.

Link to feminized seeds

Because a female plant carries two X chromosomes, selfing such a plant yields almost only female offspring. Feminized seeds rest on this principle. The quality of that seed therefore also hangs on the stability of the parent line: seed from a line in which the capacity for reversion was selected along the way passes the tendency to turn hermaphrodite on.