Short answer. Cross photo x auto, grow the F1 (all photoperiod), then either sib-cross the F1s (F2: ~25% autos) or backcross an F1 to the auto (~50% autos). To keep a photoperiod line's character while adding autoflowering, backcross towards the photo parent, using a "reveal" generation or a DNA marker each time to keep the hidden auto allele.
What's happening
In the populations mapped by Toth et al. (2022), autoflowering is controlled by a recessive allele at Autoflower1 on chromosome 1; F2s fit 1:2:1 genotypes with about 25% autos [1]. Heterozygotes flowered about two weeks earlier in the field but did not autoflower [1]. Not all auto sources have been shown to share this locus, so crossing two different auto lines may not always give autos if they carry different mechanisms (Contested) [1].
Paths compared
| Plan | Generations | Auto share | Photo-parent genome (avg) |
|---|---|---|---|
| F1 then F2 | 2 | ~25% of F2 | ~50% |
| F1 x auto parent (backcross to auto) | 2 | ~50% | ~25% |
| Backcross to photo with reveal steps (F1, BX1, BX1-F2 reveal, BX2, BX2-F2 reveal) | 5 | ~25% in each reveal generation | ~87.5% |
| Backcross to photo with a marker (F1, BX1, BX2, BX3 picking carriers, then F2) | 5 | ~25% in final F2 | ~94% |
Averages from standard backcross genetics [3]; actual recovery varies.
Step by step: marker-free route
- P: Photo line female x stable auto male (or reverse). Record IDs.
- F1: All photoperiod. Pick the best F1s.
- F2: Sib-cross F1s. Grow under 18/6 or longer; autos are the ones that flower at 3-5 weeks [1].
- Select autos with the photo parent's best traits.
- Backcross chosen autos to the photo parent (BX1, all photoperiod, all carriers).
- Reveal: sib-cross BX1s; select autos again.
- Repeat 5-6 until the line looks like the photo parent in the traits you care about.
- Stabilise: grow 50+ seeds from your auto x auto line under long days; any plant that doesn't flower at the expected age is an off-type. Aim for 0 photoperiod off-types in a 50-plant test before release.
Step by step: marker-assisted route
If a validated marker for your auto source exists (check the lab's validation in your germplasm), backcross F1 x photo parent each generation and test seedlings, keeping only carriers [1]. Reveal autos only in the final generation. This saves time but costs lab fees, and markers may not predict every auto source [1].
Growing tips for selection
House position: autoflowers. An auto runs on a strict countdown from germination, so any setback costs it time it can't get back. Plant it straight into its final pot: transplanting is almost a no-no, because of how the auto's taproot reacts (the founder's observation, compared with photoperiods). Train with LST, don't top: topping will never produce what proper low-stress training does on an auto. Grower practice, house-endorsed.
- Sow selection plants straight into their final pots and train with LST only, so every plant gets the same clean run and setbacks don't mask differences.
- Score autos on days to flower and total cycle length under the same long-day schedule (18/6 or 20/4).
- Expect more variation in size and yield than in photoperiod hunts: you can't veg longer to equalise.
- Clones of autos flower on the parent's age clock; select from seed populations.
Numbers that matter
| Test | Plants for 95% chance of ≥1 auto |
|---|---|
| F2 (p = 0.25) | 11 |
| BX to auto (p = 0.5) | 5 |
| Want ≥5 autos in an F2 | 34 |
Common mistakes
- Expecting F1 autos.
- Releasing a "stable auto" that throws photoperiod plants (incomplete fixation or a second photoperiod source).
- Crossing two auto lines of unknown origin and getting photoperiod offspring (possible if mechanisms differ) [1].
