Single Seed Descent – One Seed per Plant and Generation

Single seed descent is an inbreeding method in which one individual plant from each generation’s progeny is carried forward. Because only one individual passes through per round, the share of heterozygous loci halves each generation without the population having to be selected for the traits that matter later. In cannabis it counts as the most viable method for that at present. Viable here means fast at acceptable cost, not superior: the standard route in dioecious species is mating between relatives, and at equal levels of inbreeding that clears the lethal load more effectively, because homozygosity rises gradually rather than in one jump.

How long it takes and how the endpoint is recognised

The method typically needs six generations to reach functional homozygosity. In the study that first published the full route in this species, a line counted as functionally homozygous once the share of heterozygous alleles across a 1,500-marker panel fell below 3 %. The starting accessions sat between 17 and 35 %.

The decline per generation differed so much between lines that the authors explicitly recommend measuring it rather than inferring it from a generation count. Six rounds are an expectation, not a demonstration.

What the method requires in a dioecious species

In self-pollinating crops it is enough to leave the plant to itself. Cannabis is predominantly dioecious, so every step requires induced sex reversion: female plants form fertile male flowers after treatment with silver thiosulfate, male plants form fertile female flowers after treatment with ethephon. Monoecious accessions manage without such treatment and are therefore the technically easiest to carry.

This does, however, make a trait part of the selection that has nothing to do with the breeding goal. For the route through dioecious female plants it is stated outright: the capacity for sex reversion has to be stabilised where it segregates in the source material, which raises the risk of unwanted spontaneous male flowers in the finished line.

The shortcut available in other crops is not on offer here: doubled haploid approaches have so far failed in cannabis.

The price

From the fourth selfing generation onward, floral abnormalities appeared in every line carried, accompanied by declining seed set. In the line whose seven generations were grown side by side, the proportion of abnormal flowers rose from 0.4 % in the source generation to 13.7 % in the sixth. From that point, seed set could only be rescued by treating different sections of the same plant simultaneously with the ethylene promoter and the ethylene inhibitor.

Loss of vigour, by contrast, was not universal: two of four lines compared fell short of their source accession in dry weight, two did not. The most vigorous seedling was carried forward in every round, however, so the figures reflect selection as well. A line produced this way is unsuited to production in any case; its purpose is the cross with a second one.

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