PLS 221                                                Instructor:  C.F. Quiros

Solanacea: POTATO: Solanum tuberosum

List of references (includes reading assignments)

Slides


Contents:
 
 

Tuber bearing species, taxonomic classification: ploidy series.

Origin of the cultivated potato: domestication and cultivation of potato in the Andes.

Genomic and cytoplasmic evidence on potato evolution. 

Genome/cytoplasm interaction in interspecific crosses, sterility

Complexity of tetraploid breeding, tetrasomic inheritance, maximum heterozygosity

Triploid block, endosperm balance

2n gametes in diplod potatoes: FDR and SDR

Genetic consequences of 2n gametes and their application in potato breeding. 

Breeding schemes on the basis of 2n gametes: 4x-2x, 2x-4x and 2x-2x crosses, true potato seed cultivars

Techniques to scale down ploidy levels. Dihaploids and monoploids.

Wild species as sources of desirable traits.

Useful aneuploids: primary trisomics

Status of linkage map, development of markers: isozymes. RFLP, gene tagging, markers assisted selection, QTLs and other applications in breeding

Outlook. 

Taxonomy

The cultivated potato belongs to the genus Solanum, species S. tuberosum. The cultivated form in North America and Europe is tetraploid, 2n=2x=48.

Solanum is a very large genus including approximately 2000 species, of which 160 are tuber bearing species. Some of these are rich in toxic alkaloids. Of the 160 tuber bearing species, only 8 are cultivated.

The genetic pool of potato is very extensive, which increases the chances of finding and transferring useful traits for potato improvement. 

 The primary evolution of the potato species has been at the diploid level, where the basic genomic number is x=12.

Polyploidy is a secondary event. However, it is of great significance in the evolution of the domesticated species.

The potato species form an euploid series of :

2n=24 (2x), 36(3x), 48(4x), 60 (5x) and 72(6x) chromosomes.

Distribution

 The potato Solanum species cover a very wide range from southern USA to Southern Chile.  The cultivated potatoes have derived from these South American diploids.

 Most likely that the domestication of the potato took place in South America 10,000 years ago in the high plateau of Boliva –Peru close to the Titicaca Lake. Probably the first step toward domestication was the selection of alkaloid free diploids which could be eaten safely in large quantities.

The archeological record is poor due to the lack of remains because of high humidity which is not conductive to tuber or other plant organ conservation. The best evidence comes from pottery (ceramic 1, ceramic 2) and tools.

The cultivated potato species are listed below by ploidy level:

Diploids:

1) S. goniocalyx:

2) S. phureja:

3) S. stenotomum:

4) S. ajanhuiri:

Triploids:

1) S. x chaucha.

2) S. x juzepczukii.

Tetraploids:

S. tuberosum ssp. andigena,

S. tuberosum ssp. tuberosum,

Pentaploids:

S. x curtilobum

 Spooner et al (2007) Spooner et al (2007), based mostly on SSR markers have now reclassified cultivated potatoes in four major species:

1) S. tuberosum including 2 cultivar groups a) Andigenum group which includes  diploids, triploids and tetraploids, and b) Chilotanum grouo, including lowland Chilean landraces.

2) S. ajanhuiri

3) S. juzepczukii

4) S. curtilobum

 

Potato cultivation in the Andes

The cultivation of the potatoes in the Andes is very peculiar, which is expected due to the immense number of different species and varieties within each species. It has been estimated that at least 2000 to 3000 varieties are found in the Andes. So the implications of such variability in germplasm maintenance and potato evolution are very important. The native agriculture associated with such a diverse germplasm has three consequences:

There is an elaborate folk taxonomy system for classifying cultivated species and their varieties in the Andes associated to the cultivated potato

In spite of the great array of Andean varieties, the growers are pretty good at classifying and maintaining them, according to a survey we did on potatoes grown in 10 fields in the Andes at about 3600 to 3800 m including 100 different native cultivars.

Variety distribution in the field

From surveys of several subsistence potato fields indicate, it is possible to reach the following conclusions:

1) At permissible altitudes, bitter potatoes are grown separately from non-bitter ones.

2) Non-bitter potatoes are often grown mixed, regardless of species, ploidy or morphology, although preferred varieties are grown separately, apart from heterogeneous fields.

3) High yielding modern varieties are grown separately from native ones.

Planting technique: For home consumption the native, non-bitter varieties are mixed by dropping a few tubers in a single hole made by a foot-plow. The number of tetraploids clones, as a rule, are predominant in the mixture, although it varies from village to village.

Although the commercial cultivars are higher yielding, the natives will keep growing their own varieties because they like better their taste and culinary quality, and in some cases they store better.

Genome evolution:

The value of comparative genome analysis is that it helps to establish evolutionary relationships, and allows the transfer of genes among species. In the Solanum species little is know about genomic relationships among species.

Studies based on chromosome pairing and degree of fertility in interspecific hybrids has led to the postulation of five basic genomes in the potato and related Solanum species, namely: A, B, C, D, and E. All the cultivated potatoes, which range from diploid to pentaploid, are believed to share the same genome which is known as the A genome .

 Comparative mapping using common RFLP (restriction fragment length polymorphism) probes in potato and tomato species (Lycopersicon ssp) discloses that the two genomes of these two crops, in spite of being classified in different genera are essentially homosequential.

Based on chloroplast DNA restriction sites phylograms, Spooner et al. separated the non-tuber bearing Solanum E genome species into the Etuberosum section, and the tuber-bearing species in the Petota section. Furthermore, the study suggests that the tuber bearing species are phylogenetically closer to the species of the genus Lycopersicon than to non-tuber bearing Solanum species. Comparative mapping of the A and E genomes disclosed high conservation for most linkage groups, with the exception of a few possible inversions and translocations. Structural differences between the E-genome and the A-genome accounts for the difficulty introgression of desirable traits from the Etuberosum species into potato.

A map of another Solanum species, S. melongena is now available for comparison. Extensive chromosomal rearrangements distinguishes this species from tomato and potato. Solanacea Genomics Network

Evolution of the modern cultivated potato

It is far from certain the origin of the modern cultivated potato. The cultivated potato has been classified as S. tuberosum ssp. tuberosum, which likely derived from the ssp. andigena.

There are important characteristics differentiating both S. tuberosum subspecies in addition to photoperiod response.
 
 

tuberosum

andigena

Long days

short days

Less dissected leaves, wider leaflets

narrower, more numerous leaflets

Arched lvs, set at wider angle to the stem

Lvs set at acute angle to stem, more dissected 

Shorter stolons

Longer stolons

Less pigmentation

Often wide array of pigmentation 

The native Chilean cultivars and the European cultivars are very similar, not only morphologically but also in their photoperiodic response.

The potato was introduced to Spain in the 1560's to 1600's apparently as ssp. andigena from the Andes, according to the herbariums of the time. Spooner (2008) examined DNA from herbarium specimens of early European, finding that indeed the first introductions were probably from Andigena and Chilean tuberosum introductions took place in early 1800’s.

In 1840 late blight was introduced to Europe and wiped out all the cultivated potatoes, which but that time included both tetraploid types, but Chilean tuberosum predominantly (Ames and Spooner 2008). Selection/hybridization  had to be made most likely from Andigena stock.  Most of  our modern North American and European cultivars derive  from this introduction. Therefore, the genetic base of our potato crop is extremely narrow, which explains in part the poor progress done in potato breeding. The presence of a single chloroplast haplotype for single sequence repeats in NA potatoes supports this observation (Bryan et al. 1999). Practically, the only wild genes introduced into cultivated NA potato varieties come from the species S. demissum, which has been used to develop late blight resistant varieties such as cv `Greta'.

Most of the evidence supporting these hypotheses is cytoplasmic, generated by Paul Grun at Penn State.

Grun and Staub, 1979 found that the cytoplasmic constitution of andigena and tuberosum was different, express in the form of sterility. 

Cytoplasmic factors in potato are non-chromosomal components, most likely genes from mitochondria which cause sterility in the presence of specific chromosomal genes.

In the section Petota 9 of these factors has been identified.

According to their interaction with dominant or recessive nuclear alleles, there are too types of cytoplasmic factors:

-- 2 result from the interaction of recessive alleles in the nucleus.

-- 7 result from interaction with dominant nuclear alleles.

Thus the cytoplasm of the different species is sensitive to dominant or recessive genes introduced from other species.

Cytoplasmic sensitivities of cultivated potato and its relatives:

Cytoplasmic Sensitivity Factors
 
 

Species

dfs

los

Sps

SMs

Ins

TAs

ASFs

VSAs

Fms

ssp tub(NA)

-

+

+

+

+

+

+

+

+

Chile

-

+

+

+

+

+

+

+

+

andigena

-

+

-

-

-

-

-

-

-

sten/phu

+

-

-

-

-

-

-

-

-

- absence of cytoplasm factor.  + presence

Molecular studies of chloroplast DNA (cpDNA) by restriction analysis in the different potato species (Hosaka et al 1984,1986, 2002) shed more light into the problem of the potato origin and evolution.

On the basis of 5 restriction endonucleases, 5 main chloroplast genomes were identified.

Types of chloroplast DNA and species where they predominate
 
 

W

C

S

A

T

S. chacoense

S. sparsipilum

S. demissum

cv. 'Greta'

S. acaule

S. canasense

S. juzepczukii

S. goniocalyx

S. phureja

S. stenotomum

S. chaucha

S. curtilobum

S. tuberosum

ssp. andigena

S. maglia

S. tuberosum

ssp. tuberosum

At least three more types of cpDNA found in various wild species.

On the basis of this fact, Hosaka has postulated the following evolutionary steps responsible for the origin of the cultivated species, based on their cpDNA constitution. T cp has a 241 deletion diagnostic of Chilean tuberosum types, also it is also found in a few Andigena accessions.


                                                                     polyploidization    selection

(adapted from Hosaka K and Hanemann R, 1988)

Sterility in ssp. tuberosum:

Sterility is quite widespread in most of the North American and European potato cultivars. Many of the best potato clones of ssp tuberosum are either male or female sterile, being dead ends for genetic improvement activities.

The difficulties and frustrations of the potato breeder can be summarized by the statement written by S. R. Livermore, an earlier potato breeder at Cornell University. He said:

" Nine times out of 10, one of two clones will not flower, if both flower, 9 out of 10 both will be male sterile. If one is male sterile, nine out of ten the other will be female sterile. If 1 berry is set, 9 out of 10 very few seed will be obtained and they will not germinate. If they germinate, 9 out of 10 plants are very weak but if they are good, strong and with nice tubers, 9 out of 10 that they will be sterile".

Due to these sterility problems, in most cases varieties developed 50 to 100 years old are still in use. Very few newly released varieties have been widely adapted for use. Still one of the most popular varieties is Russet Burbank, develop early this century.

In summary, the slow progress in potato breeding is due to:

1) A narrow genetic base, originating from a few or maybe only one Chilean accession, Rough Garnet Chile, which resulted in the creation of the old varieties Early Rose and Kathanin, early this century. In the US. 80% of the varieties can be traced to this early material.

2) Sterility problems already discussed.

3) Tetraploidy.

Although the tetraploid cultivated potato might have originated from the hybridization of two species, S. stenotomum and S. sparsipilum, it behaves like and autotetraploid because the genomes of both parental species are virtually identical.

The genetics of autotetraploids is much more complex than that of a diploid. Having 4 chromosomes instead of 2 means larger progenies must be grown to recover homozygous recessive genotypes.

For example:

Selfing a heterozygous diploid at one locus will produce 25% homozygous recessive genotypes. Diploid: Aa F2 = 3A_ : 1 aa

Selfing an heterozygous autotetraploid of constitution AAaa at a single locus, will produce less than 3% homozygous recessive individuals. AAaa F2: 35A____ : 1aaaa

This is only a case with 2 alleles, which is the simplest example. Multiple alleles for many traits are the rule.

Basic studies on tetraploids indicate that maximizing the number of tetra-allelic loci or maximizing the heterozygosity of loci involved in the determination of important quantitative traits, such as yield, earliness, tuber size, etc. results in the maximum expression of heterosis. Therefore, the breeder of an autotetraploid crop must aim to create cultivars with maximum heterozygosity.

Nakamura and Hosaka (2010) report possible role of methylation in the regulation of heterosis in diploid potatoes.

 From the segregation ratios described above, it is difficult to maintain the level of heterozygosity high. It has been calculated that from a random mating population, the probability of drawing a tetra-allelic plant is <10%.

A common method used by the breeders is the double cross and select progenies of desirable individuals. For this to happen large progenies need to be grown and still it is going to be difficult to find the right recombinant genotypes. This is however, the most efficient way to maximize heterozygosity, when working with tetraploids. An alternative breeding scheme proposed for potatoes is to work at the diploid level and then scale the ploidy up back to tetraploidy, as explained in the following section.


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Last modified: March 25, 2010
© Carlos F Quiros, 1998