Talk:Inbreeding depression

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Page scope

Inbreeding depression covers the loss of fitness that follows inbreeding: how it is measured, the partial dominance and overdominance explanations, purging, the effects of the mating system, stress and small population size, and the characters measured in selfed cannabis. Heterozygosity as a measure belongs to Heterozygosity. The breeding system of cannabis, feminised seed and the account of each selfing study belong to Outcrossing. How lines are made belongs to Inbred line, the recessive characters seen in inbred hemp to Dominance and recessivity, hybrid vigour to Heterosis and the plant numbers for regrowing accessions to Effective population size.

Data verification

Record source conflicts, figures that differ between studies and sources not used here. Keep narrative content on the article.

Discussion

Sources

Charlesworth and Charlesworth (1999), Genetical Research 74: 329–340: fitness components, the inbreeding load, dominance estimates in Drosophila, purging and how to detect it, associative overdominance.
Charlesworth and Willis (2009), Nature Reviews Genetics 10: 783–796: recessive deleterious mutations as the main cause of inbreeding depression and heterosis.
Felsenstein (2019), Theoretical Evolutionary Genetics: mean fitness under inbreeding at one locus (pp. 242–244); inbreeding effects on means and the East and Jones hypotheses (pp. 411–414).
Husband and Schemske (1996), Evolution 50: 54–70, and Winn and colleagues (2011), Evolution 65: 3339–3359: δ across plant species by mating system and life stage.
Crnokrak and Barrett (2002), Evolution 56: 2347–2358: experimental tests of purging.
Yang and colleagues (2017), PLOS Genetics 13: e1007019: incomplete dominance of deleterious alleles in maize hybrids.
Armbruster and Reed (2005), Heredity 95: 235–242, and Bijlsma and Loeschcke (2012), Evolutionary Applications 5: 117–129: stress and inbreeding depression.
Angeloni and colleagues (2011), Biological Conservation 144: 35–43; Caballero and colleagues (2017), Heredity 118: 177–185; Frankham (2015), Molecular Ecology 24: 2610–2618; Davinack (2024), Molecular Ecology & Evolution: population size, purging in small populations, the extinction vortex and genetic rescue.
Kurtz and colleagues (2020), HortScience 55: 1206–1209; Carlson and colleagues (2021), Journal of Experimental Botany 72: 7694–7709; McDonald and Lubell-Brand (2024), HortScience 59: 1795–1799; Srikul and colleagues (2026), Agriculture and Natural Resources 60: 600512; García-de Heer and colleagues (2026), Horticulture Research 13: uhag038: selfed cannabis in greenhouse, field and plant factory.
Serkov and Klimova (2016), Niva Povolzh'ya 2 (39): 52–60 (in Russian), Mishchenko (2011), Plant Breeding and Seed Production 100: 120–130 (in Ukrainian) and Mishchenko and colleagues (2025), Faktori eksperimental'noi evolucii organizmiv 37: 29–34 (in Ukrainian): monoecious hemp in the field over many selfed generations.

Where the sources disagree

Zolotoniski 15 in the twelfth generation. The text of Mishchenko and colleagues (2025) gives stem mass 31.0 per cent, fibre mass 36.1 per cent and seed mass 50.3 per cent below the starting plants, and a rise in thousand-seed weight of 7.1 per cent. Their Figure 2 shows 39.2, 42.5 and 54.5 per cent and a fall of 0.7 per cent.
Technical stem length of Hlukhivski 58. The same text puts the rise by the twelfth generation at about a quarter. Figure 1 of the paper shows 8.5 per cent.
The deepest depression. The same paper says that each cultivar lost most in the character it expressed most strongly, stem and fibre mass in Zolotoniski 15. Its own figures show a larger percentage loss in the seed mass of that cultivar.
Soviet first-generation figures. Serkov and Klimova (2016) report from several authors that depression appeared in 25 to 30 per cent of first-generation lines, and from Kaplunova that productivity fell by 25 to 30 per cent in the first generation. The two statements measure different things.
Hang Kra Rok height. Srikul and colleagues (2026) write that no inbreeding depression was detected for plant height or floral yield. Their table has the selfed plants shorter than the F₁ plants, a difference they attribute to heterosis in the crosses between the two accessions.
Percentage in Husband and Schemske. Their abstract calls the mean δ of selfing species (0.23) 43 per cent less than that of outcrossing species (0.53). The ratio of the two is 0.43, which is a shortfall of 57 per cent.
Heterosis and depression. Felsenstein (2019) calls hybrid superiority inbreeding depression in reverse. Armbruster and Reed (2005) write that inbreeding depression is not simply the reverse of heterosis.

How far the evidence goes

Every cannabis measurement cited comes from a cultivar, breeding line or landrace accession under cultivation. None comes from a wild or feral population, and none gives a cumulative δ over the life cycle or a value of B.
No cited study of cannabis separates partial dominance from overdominance. García-de Heer and colleagues and Kurtz and colleagues discuss the mechanisms in general terms.
The Hang Kra Rok comparison is with F₁ plants between two accessions, so it combines the effect of selfing with any heterosis between the accessions. The Penza generations were each grown in a different year, so generation and season cannot be separated in those means.

Terms

δ: the shortfall of inbred progeny against outbred progeny, as a proportion of the outbred value.
Inbreeding load (B): the slope of the logarithm of a trait's mean on the inbreeding coefficient, counted in lethal equivalents.
S₁, S₂ … and I₁, I₂ …: the first, second and later selfed generations. The English-language papers write S and the Russian and Ukrainian papers I.
Technical stem length: the length of the stem below the inflorescence, the part used for fibre.
Inbred minimum (Ukrainian інбредний мінімум): in Mishchenko and colleagues, the level at which characters settle under repeated selfing.

Further figures from the sources

Hlukhivski 58 in the first selfed generation (Mishchenko and colleagues 2025, Figure 1): total height −4.4, technical stem length −4.2, stem diameter −6.8, stem mass −10.7, fibre mass −12.1, fibre content −1.0, seed mass per plant −11.3, thousand-seed weight −8.1 per cent. In the fourth to sixth generations fibre mass was 29.0 to 29.8 per cent lower.
Mishchenko (2011): male sterility in 1.16 per cent of first-generation Zolotoniski 15 plants against 0.22 per cent in the cultivar. Dwarf plants made up 2.02 per cent of first-generation Hlukhivski 58 plants and 1.68 per cent of the cultivar itself, which accounts for much of that cultivar's total of abnormal plants.
McDonald and Lubell-Brand (2024): seedlings of Wilhelmina with yellow first leaves were 22 per cent of 50 in the first selfed generation, 21 per cent of 48 in the second, 19 per cent of 37 in the third and none of 47 in the fourth.
Serkov and Klimova (2016): plant survival by generation ranged from 73 to 100, 89 to 100, 31 to 100, 0 to 73 and 0 to 100 per cent. The lines lost were five, six, two, one and five.

Images reserved in the article

One image has a place in the article but is not yet uploaded:

File:Cannabis-inbreeding-depression-by-life-stage-in-plants.jpg (Purging and the mating system), a chart to be drawn: four pairs of bars for seed production (selfing 0.05, outcrossing 0.32), germination (0.02, 0.12), survival to reproduction (0.04, 0.15) and growth and reproduction (0.21, 0.27). No crop or species names.