Some avocado fruit are very big. Wild Persea species tend to have fruit that are quite big relative to other wild fruit. But relative to cultivated avocado fruit, wild Persea fruit are quite small.
In turn, ‘normal’ commercial avocados are rather small relative to some of the largest cultivated avocado seedling variants!
A 2014 paper by Monforte et al ‘The genetic basis of fruit morphology in horticultural crops: lessons from tomato and melon’ found genes for fruit weight (likely a proxy for size) “co-localized frequently with members of the [Cell Number Regulator/Fruit Weight] CNR/FW2.2 and KLUH/FW3.2 families, as well as co-localizations between [Ovate Family Proteins] OFP family members”. My expansions are [bracketed].
KLUH gene may affect cell division, possibly increasing the number of cell layers in the fruit pericarp, thus affecting ultimate fruit size.
The CLAVATA (CLV) gene in tomato has a signal-receptor complex affecting meristems, wherein a mutation in CLV3 (whose secreted peptide signal binds to the CLV1 receptor) has resulted in larger fruit size in the course of domestication. (Xu et al 2015)
Other genes possibly implicated in regulating cell division are SUN (regulating fruit elongation), and CYP78A (cytochrome P450 of the 78A class).
These gene families were frequently found located on the chromosomes near both OFP family members and areas on the chromosomes associated with fruit shape.
The authors found two genomic regions in melon that had genes affecting fruit weight.
Recent work on the avocado genome has identified some similar regions when compared with tomato (the only genome I bothered to look at – there are others, such as grape, and Arabidopsis in the spreadsheets).
In the following list the first number is the reference number in the first column of the spreadsheet, followed by the function or type of protein, and whether the match between avocado and tomato is partial or complete. There are separate spreadsheets for ‘Mexican‘ and the ‘nodal hybrid’ cultivar Hass.
‘Mexican’ avocado nodal group :
- Cell number regulator
- 492 CNR 2 (complete)
- 14532 CNR 2 (complete)
- 3683 CNR 2 (complete)
- 19056 CNR 6-like (complete)
- 14532 CNR 8 (complete)
- 3787 CNR 8 (partial)
- SUN
- 9572 SUN domain-containing protein 3 – like (complete)
- 14052 SUN domain-containing protein 3 – like (complete)
- 17388 SUN domain-containing protein 3 – like (complete)
- Cytochrome P450 78A class
- 18625 Cyto P450 78A 7 (complete)
- 19185 Cyto P450 78A 7 (partial)
‘Hass’ avocado nodal group hybrid :
- Cell number regulator
- 8056 CNR 2 (complete)
- 1262 CNR 6-like (complete)
- 15483 CNR 6-like (complete)
- 18988 CNR 6-like (complete)
- 4911 CNR 8 (complete)
- 11178 CNR 8 (complete)
- SUN
- 2543 SUN domain-containing protein 3 – like (complete)
- 11026 SUN domain-containing protein 3 – like (complete)
- 17897 SUN domain-containing protein 3 – like (complete)
- 20992 SUN domain-containing protein 3 – like (complete)
- Cytochrome P450 78A class
- 728 Cyto P450 78A 7 (complete)
- 15548 Cyto P450 78A 7 (complete)
- Ovate Family Proteins
- 23546 transcription repressor OFP 6
Fruit heavier than the current dominant ‘Hass’ cultivar are probably undesirable (heavier fruit may be suitable for the restaurant trade, but not so much for today’s small family size).
In summary, very large avocado fruit may result from over-expression of homologs of some of these genes. Other genes may also be involved in the expression of such genes.
Further reading:
Rendón-Anaya,M et al. 2019. The avocado genome informs deep angiosperm phylogeny, highlights introgressive hybridization, and reveals pathogen-influenced gene space adaptation.
PNAS August 20, 2019 116 (34) 17081-17089; first published August 6, 2019 https://doi.org/10.1073/pnas.1822129116
Xu, C., Liberatore, K., MacAlister, C. et al. A cascade of arabinosyltransferases controls shoot meristem size in tomato. Nat Genet47, 784–792 (2015) doi:10.1038/ng.3309
Avocado Gene Expression, Climate, and Society 