Solanum curtilobum

Description

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Solanum curtilobum distribution map Map note

Solanum curtilobum is perhaps better included among the cultivated species than the wild, but as species that has two-fifths wild genetics, the dividing line isn’t entirely clear.  This species is treated more like a wild species in breeding programs.  Of the three frost-resistant hybrid species, this is the most similar to domesticated tetraploid potatoes and the easiest to breed with.  The specific epithet means “short nightshade,” which might refer to the whole plant, but probably refers to the often short, broad leaves. 

This species is cultivated primarily in the highest elevations of the Andes, from northern Peru to southern Bolivia, where S. tuberosum group andigenum will not survive due to frost.  None of the accounts of this species that I have found indicate that it has ever been found in Colombia, but the USDA lists three accessions from Colombia, which is a puzzle.  Plants look similar to Andean tetraploids and grow quite large under short day conditions, with some plants producing vines as much as seven feet long.  Tubers most commonly have blue skin and white flesh.  They do not have the kind of diversity of form found in either parent species and are typically round to oval with shallow eyes.  The flowers are usually blue.  Some accessions of this species are high in glycoalkaloids and are only consumed after processing into the freeze-dried form known as chuño, but the accessions available in the USA are all non-bitter and edible fresh.

Relationships Between Andean Domesticated Potatoes
Relationships Between Andean Domesticated Potatoes

There are two proposed origins of S. curtilobum.  The first case is hybridization between tetraploid S. tuberosum group andigenum and S. juzepczukii (Hawkes 1962), making its genetic composition 60% S. tuberosum and 40% S. acaule. The tuberosum parent is tetraploid and 4EBN, while S. juzepczukii is triploid and 2EBN.  To cross successfully, S. juzepczukii must have provided an unreduced (2n) gamete, giving the combination of a 2x S. tuberosum gamete and a 3x S. juzepczukii gamete for a 5x (pentaploid) progeny.   The second case is hybridization between triploid S. tuberosum group andigenum and S. acaule (Gavrilenko 2013).  In that case, the triploid andigena would have acted as the female parent with a 2n ovule and S. acaule would have fertilized with normal haploid pollen.  The second case seems less likely, simply because 2n ovules are produced in lower numbers than 2n pollen.

Although it has an odd ploidy, S. curtilobum is self-compatible and is able to cross with other 4EBN potato species.  These crosses typically produce near-tetraploid aneuploids; that is, tetraploids with some extra chromosomes.  If these aneuploids are back-crossed to the tetraploid parent or produce seed through self-pollination, they tend to lose the extra chromosomes over several generations.

S. curtilobum has not been synthesized by crossing the putative parent species.  This might be because it is hard to get the required 2n gamete from S. juzepczukii or it could be an indication that the hypothesized parents are not correct.

Resistances

Vega (1995) found that this species is a little more frost tolerant than S. tuberosum.

All three accessions that we have grown here had strong frost resistance, surviving frosts undamaged that killed 99% of domesticated potatoes.

Condition Type Level of Resistance Source
Drought Abiotic Somewhat resistant Machida-Hirano 2015 
Frost Abiotic Somewhat resistant Machida-Hirano 2015 
Meloidogyne spp. (Root Knot Nematode) Invertebrate Somewhat resistant Machida-Hirano 2015 
Phytophthora infestans (Late Blight) Fungus Somewhat resistant Bachmann-Pfabe 2019
Potato Virus X (PVX) Virus Somewhat resistant Machida-Hirano 2015 

Glykoalkaloid content

This species is generally used to make chuño or similarly processed potatoes, typically an indication of high glycoalkaloids.  Osman (1978) found TGA levels ranging from 3.8 to 29.0 mg / 100 g for this species, levels that are not particularly high, but that do span the safety limit of 20 mg / 100 g.

The accessions available in the USA are mostly non-bitter, or barely bitter, probably safe enough to eat in moderate portions.  In my experience, seed grown progeny of S. curtilobum segregate strongly for bitterness, with many plants that are more bitter than the parent variety.

Images

Mixed Solanum curtilobum potatoes
Mixed seed-grown Solanum curtilobum potatoes
Aerial plant of the potato species Solanum curtilobum
Solanum curtilobum plant
Aerial plant of the potato species Solanum curtilobum
Solanum curtilobum plant
Flower of the hybrid potato species Solanum curtilobum
Solanum curtilobum flower
Tubers of the USDA potato accession PI 604208
USDA PI 604208
Potato tubers of USDA PI 604208
USDA PI 604208
Tubers of the USDA potato accession PI 604207
USDA PI 604207
Potato tubers of USDA PI 604207
USDA PI 604207
Tubers of the USDA potato accession PI 604206
USDA PI 604206
Potato tubers of USDA PI 604206
USDA PI 604206
   

Cultivation

Plants are large and tuberize under short days, so they need wide spacing.  18 inches (45 cm) is probably a good separation for this species.

This species is self compatible and will produce viable seed, but the seed does not produce true S. curtilobum.  Instead, first generation seeds usually produce aneuploids between tetraploid and pentaploid.  Subsequent self-pollinated generations converge toward tetraploidy.  True S. curtilobum can only be propagated clonally.  Perhaps “true” is not the right word.  The seeds do not produce pentaploids, but I suppose that tetraploids of this species are still S. curtilobum.

Breeding

I think it is kind of surprising that there hasn’t been more breeding done with S. curtilobum as a tetraploid.  After a few generations of selfing or crossing between varieties, they stabilize at the tetraploid level.  Pentaploid S. curtilobum has 3 copies of the andigenum genome and 2 copies of the acaule genome.  During the process of tetraploid stabilization, it will lose one copy of the andigenum genome, so the tetraploid is 50% andigenum and 50% acaule.  There ought to be plenty of interesting traits to explore in that mix.

S. curtilobum sets much less seed per berry than domesticated tetraploids and I would guess that this is because the uneven distribution of chromosomes doesn’t work out in a way that produces a viable seed much of the time.  Seed set improves with each generation of outcrossing in my experience.

Crosses with S. tuberosum

Female Male Berry Set
Seed Set Germ Ploidy Source
S. curtilobum S. tuberosum (4x) High Low Yes 4x+  
S. tuberosum (4x) S. curtilobum Low Low Yes 4x+  

Crosses with other species

Female Male Berry Set
Seed Set Germ Ploidy Source
             

References

Solanum curtilobum at Solanaceae Source

Solanum curtilobum at GRIN Taxonomy

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