Sex Reversion – Induced Flowers of the Opposite Sex

Sex reversion is the formation of flowers of the opposite sex on a plant with a fixed chromosome set. It can occur at any stage of the life cycle, triggered by environmental conditions or deliberately by chemical treatment, and it changes nothing about the chromosomes: a masculinised plant stays genetically XX, a feminised one stays XY.

The two triggers

The lever in both directions is ethylene. Genetically female plants form male flowers after treatment with silver thiosulfate, which inhibits ethylene action; genetically male plants form female flowers after treatment with the ethylene promoter ethephon. What matters for breeding is that both induced flower types are fertile – pollen from a masculinised plant fertilises, and the ovules of a feminised plant set seed.

Reversion from female to male is genotype dependent and does not work equally well in every line. In later inbreeding generations, induced male flowers often produced very little pollen, which accounted for a substantial share of the failed inbreeding attempts.

What the method is for

The best-known purpose is feminised seed: pollen from a masculinised XX plant onto an XX mother yields exclusively female offspring, since no Y chromosome is involved. The second purpose concerns breeding itself. In a dioecious species, self-pollination is impossible without sex reversion, and without self-pollination there is neither an F2 from a single selected plant nor single seed descent.

Selfing male plants also answers a question that stood open for a long time. Across three lines and more than 500 offspring the sex ratio came out at one to one. Viable YY plants, which would be a shortcut to masculinised seed, do not exist by this account; one explanation is that Y-bearing ovules are not viable.

What the method does to a line

Inbreeding by selfing turns the capacity for reversion into a trait under selection. Where it segregates in the source material it has to be stabilised along the way, and the finished line then carries a raised risk of unwanted spontaneous male flowers. That is the substantive core of the reservation against selfed lines, and it holds however carefully the work is done.

There is a measured side effect on top. From the fourth selfing generation, floral abnormalities appeared, and seed set could only be rescued by treating different sections of the same plant simultaneously with the ethylene promoter and the ethylene inhibitor. Since ethylene governs sex expression in this species, that points to inbreeding interfering with the signalling pathway itself.

How far the plasticity of this species reaches shows in the extremes from the same study: XX plants without a single female organ and XY plants without male organs, both without chemical intervention.

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