PLS221 Instructor:  C.F. Quiros

Potato: part 2.


2n gametes and their application to potato breeding

The discovery of 2n gamete production by many diploid potato clones in potatoes, and for that matter in diploid forms of other autotetraploid crops, such as alfalfa, opens alternative ways to breed these crops more efficiently and attaining a high level of heterozygosity.

Triploid block: Low yield of triploids from diploid x tetraploid crosses. Zygote-endosperm unbalance? (this topic is covered in detail below). Its strength varies form cross to cross. The following table gives some actual data obtained from 4 progenies:

Advantages of 2n gametes:

1) Expand and capture novel genetic diversity.

2) Transmission of diversity.

What are the mechanisms by which potatoes produce 2n gametes?

There are two main mechanism that we will discuss in detail:

1) First division restitution (FDR).

2) Second division restitution (SDR).

FDR: It results by an abnormal orientation of the spindles right before anaphase II. Non-sister chromatids end up in the same nucleous.

SDR: This is due to premature cytokinesis before the second meiotic division takes place. Sister chromatids end up in the same nucleus.


 
 

 2n pollen producers can be distinguished by:

1) Examining pollen size, 2n pollen (big pollen) has a diameter of 22 to 28 um, while n pollen from 18 to 24 um.

2) Presence of dyads in telophase II.

FDR by presence of parallel or fused spindles.

SDR by presence of cell plate before anaphase II.

Why is it important to know the mechanism of 2n gamete production?

Because each one has important genetic implications:

FDR transmits the parental genotype almost intact especially for genes close to the centromeres. Most of the heterozygosity of the parent is transmitted to the progeny. This mechanism is very important for effectively maximizing the heterozygosity in a tetraploid.

For example, lets take two loci, one close (A) and one far (B) from the centromere.

The following 2n gametic ratio will be obtained:

1 Aa BB

2 Aa Bb

1 Aa bb

Therefore, for a locus close to the centromere, such as A, the heterozygosity from the parent is transmitted 100%

For both loci, A and B, the heterozygosity from the parent is transmitted to 50% of the progeny.

In conclusion, FDR is quite efficient transmitting the heterozygosity of the original parental genotype.

Lets see now the heterozygosity transmission by SDR:


 
 

Two 2n gametic ratio is in this case:

1 AABb

1 aaBb

Notice that there are only two different gametic genotypes, and both are different to the original parental genotype.  In this case, 100% of the progeny will be heterozygous for the locus far from the centromere and 0% for the locus close to the centromere.

Centromere mapping by half tetrad analysis:

This mechanism of 2n gamete production can be used to map genes with respect to the centromere by half tetrad analysis. The name is due to the fact that 2 chromatids out of the 4 forming a bivalent are recovered together in the tetraploid progeny.

The general scheme for this type of mapping is to cross a tetraploid nulliplex by a heterozygote diploid.

For example

aaaa x Aa

If the strain produces 2n pollen by FDR, the progeny should be Aaaa (heterozygous simplex) when the A locus is tightly linked to the centromere.

Any aaaa genotype in the progeny will arise from crossing over, so the frequency of recombinants will give the distance between the locus in question and the centromere.

The general formula is quite simple:

Distance locus to centromere = 2(frequency of nulliplex progeny or aa gametes)x100.

(See Mendiburu and Peloquin, 1979. TAG54:177 for more details.)

A couple of virus resistance genes, Rx, and Ry for PVX and PVY, respectively, and nematode resistance gene. H1 (Globoreda rostochiensis) have been mapped respect ot their centromeres (Wagenport and Zimnoch (1992). This technique is useful to position the centromeres in linkage maps as they become more saturated with markers.

Transmission of heterozygosity by FDR and SDR:

It has been estimated for potato that by FDR, parental heterozygosity is retained in 80% of the gametes, while by SDR only 40%.

This calculations has been done assuming a regular distribution of heterozygous loci along the chromosomes and one cross-over per chromosome . (see Hermsen Iowa State J. of Research 58:411-420.).

Field performance and 2n gametes:

Do higher levels of heterozygosity translate into higher yields or performance superiority in the field? Experimental data on this aspect have been generated by Peloquin and his group comparing yields of tetraploid progenies divided by 4x-4x (conventional) and 4x-2x crosses.

          Tuber yields FDR vs SDR: (lb/plant)
 

Location

4x-2x(FRD)

4x-2x(SDR)

4x-4x

A

7.7

5.1

4.6

B

4.3

3.2

3.2

Mean of 2x parents= 2.5 lb/plant
Mean of 4x parents= 5.2 lb/plant

So these and other data indicate that the use of 2n gametes in potato breeding is an useful alternative for the potato breeder.

Breeding schemes proposed with 2n gametes:

1) 4x-2x crosses, where the 2x parent is a highly heterozygous line, with good combining ability developed by breeding at the diploid level.

2) 2x-4x crosses. Male fertile progeny will be obtained in most cases, since the 2x female parent normally has insensitive cytoplasm.

3) 2x-2x crosses. This is called bilateral polyploidization.

The advantage of potato is that once you hit a good combination, it may become a variety, since this crop is vegetatively propagated.

There has been efforts to synthesize tetraploids by fusing protoplasts from diploids plants. This would be equivalent to 2x-2x crosses, using heterozygous and horticulturaly desirable protoplast diploid donors.

True potato seed varieties

There has been efforts to develop sexually propagated varieties. These are called TPS varieties. The basis of this scheme rests again on the generation of 2n gametes by FDR. Since by this mechanism the genotype of the parental plant is transmitted largely intact, it might result in uniform high yielding progenies.

Triploid block

Triploid block refers to the low frequency of triploids obtained in 4x-2x crosses. This is due to post-fertilization breakdown of the endosperm leading to embryo abortion. It varies in strength.

The triploid block is an important mechanism for polyploid evolution, allowing effective introgression between diploid and tetraploids in the absence of sterile triploids. Thus, polyploidy is not always a strong mechanism of species isolation.

4x-2x

6x endosperm 

4x embryo

5x endosperm 

3x embryo

2x-4x

6x endosperm 

4x embryo

4x endosperm 

3x embryo

2x-2x

3x endosperm 

2x embryo 

6x endosperm 

4x embryo 
 

This rule, however fails to explain the few triploids arising from 4x-2x crosses, or the natural triplod species.

Endosperm Balance Number (EBN):

Nishiyama and Inomata (1966) suggested that successful endosperm development depended on a 2:1 maternal:paternal genomic ratio in the endosperm regardless of the ploidy level of the embryo.

This concept was later refined into the polar-nuclei activation hypothesis in which a value (activation index) is assigned to the male gamete and a value (response index) to the female gamete based on hybrid seed development and viability.

Johnston has devised a system to predict success of interploidy crosses based in the concept of Endosperm Balance Number, which is the ratio female to male `factors' in the endosperm.

For normal development the endosperm has to be have a 2:1 (female:male endosperm factors).   He has classified the species that cross as having the same EBN number. The system is useful to predict success of the crosses in Solanum but the genetic determination of the system is unknown. In other words, we do not know why this ratio is necessary, even less the molecular basis for this phenomenon.

Male parent>>> 

Female parent 

Cultivated diploids 

S. acaule (4x) 

EBN 2

Cultivated tetraploids 

Hexaploid species 

EBN 4

Cultivated diploids 

S. acaule (4x) 

EBN 2

Viable hybrids 

Endosperm ratio 2:1 

Embryos: 2x, 3x, 4x

Unviable hybrids 

Endosperm ratio 2:2

Cultivated tetraploids 

Hexaploid species 

EBN 4

Unviable hybrids 

Endosperm ratio 4: 1

Viable hybrids 

Endosperm ratio 4:2 (2:1) 

Embryos 4x,5x,6x

Dihaploids:

We have seen so far the use of diploids generating 2n gametes to produce tetraploids. Interestingly enough, 4x-2x crosses in potato can work in the opposite way, to generate dihaploids, or tetraploid derived diploids, maintaining the required EBN 2:1 ratio .

In the 2n pollen producing clones, haploid pollen is also produced at various frequencies. It is doubled to 2n after meiosis, by fusion of the generative nuclei during pollen mitosis. These 2n pollen produced during mitosis will fertilize the endosperm only, failing to fertilize the egg, which develops parthenogenetically. Thus the diploids from the 4x-2x crosses are parthenogenetic dihaploids. The frequency of dihaploids can be monitored by the presence of embryo markers in the male parent, such as embryo spot (Bd) which is dominant.

Androgenesis

Dihaploids and monoploids have also been reported by anther culture (androgenesis).  These can be obtained at higher rates than using the pollinator method. Normally dihaploid ratios by androgenesis range between 1 to 10%, depending on the anther donor plant..[ See Wenzel, TAG 59:333-340(1981), Cappadocia TAG69:139 (1984)].

Mapping and gene assignment into chromosomes

Potato cytogenetics, as far as mapping activities and development of cytogenetic stocks is concerned, was quite undeveloped before the advent of RFLPs and other molecular techniques compared to tomato and maize. Now extensive maps of molecular markers including disease and pest resistance genes are available for this crop, generated mostly for diploid strains. The challenge still remains to apply this information effectively to tetraploid potato.

Cytogenetic stocks:

A series of primary trisomics, developed by Ramanna and Wagenvoort are available in potato.

The pachytene chromosomes are somewhat similar in morphology to those of the tomato. They are less differentiated, however, which makes difficult their identification.

As part of the Solanaceous crop project, the potato genome is being sequenced. See progress made to date at the Potato Genome Sequencing Consortium

Molecular marker maps

RFLP maps: These have been developed by Bonierbale et al. (1988) using tomato probes, by Gebhart et al. (1989, 2004) at Max-Plank Institute in Germany and and Rivard et al. (1989), Canada, with potato probes.

Putting the maps together, 1400 markers mapped in potato, spaced at 0.7cM on the average. Tanksley's group recently developed a new map based on a progeny of 2x S. tuberosum x S. berthaultii. (Yancho et al. 1990)

Recently a smaller map integrating molecular markers, transposons , isozymes and morphological markers was constructed by Jacobs et al (1995). Additionally, extensive AFLP maps have become available recently. Microsatellite markers (Provan, 1996, Kawchuk, 1996, Milbourne et al. 1998, Feingold et al 2005) and inter-simple sequence repeat  (ISSR) markers (Prevost and Wilkinson, 1998) are also available in potato and its relatives.

For updated map versions, go to the Solanacea Genomics Network

Gene tagging and marker assisted selection

A number of genes of economic importance have been tagged with markers.

Tuber and flower traits: Pigment producing genes: Tuber/sprout/flower color determined by three independent loci

P purple pigment , petunidin. This locus is epistatic to R that produces red pigments pelargonidin and cyanidin in the flower.

p=red, P masks R alleles R and Rw . Rpw in homozygous condition results in lack red anthocyanins

Locus I self-colored; i white tubers

P_R_I_ Blue tuber (blue flw)

P_R_ii white tuber (blue flw)

P_RpwRpwI_ blue tuber (pale blue flw)

pp RpwRpwIi pink tuber (white flower)

ppRpwRpwii white tuber (white flower)

ppR_I_ red tuber (red flw)

ppRRii white tuber (red flw)

Recently it has been proposed to change the name of locus R locus to D, which maps on chromosome 2. In this case, pink color tuber is determined by genotype ppddI_, due to the expression of I in the absence of D.

Locus P in on chromosome 11.

Two additional loci, B and F determine distribution of anthocyanin.

B has multiple alleles: Bd = spots in embryo, nodes and floral abscission zone and tuber eyebrow, Bc= Bd but there is no pigmentation in the nodes. Ba = floral abscission pigmentation only.

Locus F, ff=flecked flowers

B-I-F are linked. This groups has been mapped recently with RFLP markers on chromosome 10. Round tuber shape, determined by allele Ro is also present in this group.

Another locus, Pf, linked also to I determines tuber flesh anthocyanin.

For latest review on flower and tuber color, see van Eck (1994).

Tuber flesh color Y= white, y= yellow . Other loci may be involved for yellow intensity.

Disease resistance genes

Late blight, caused by Phytophthora infestans. For race specific resistance eleven R genes have been reported (R1 to R11), according to race specificity. Four of them have been mapped, R1, R2, R3, R6 and R7. It is becoming evident that some of the R genes are also involved in the non-specific resistance, perhaps due to different alleles. Some of the QTLs map on top of the resistance loci. Much recent research activity on this disease due to the presence of new virulent races.

PVX virus resistance. There are two types of monogenic resistance for this disease, hypersensitive and extreme resistance. Genes Nb and Nx control hypersensitive resistance. Loci Rx1 and Rx2 controlling extreme resistance have been mapped on chromosomes 12 and 5, respectively. Nb maps on chromosome 5, close to Rx2, to Gpa (resistance to Globodera pallida) and to a R1, conferring hypersensitive resistance to late blight .

PVY virus resistance. Also for this disease there are hypersensitive and extreme types of resistance. Resistance is controlled by gene family Ry. One of these genes, Ryadg has been mapped on chromosome 11, tightly linked to 5 RFLP markers. (Hamalainen et al. 1997). A second Ry locus, Rysto   from S. stoloniferum also maps on chromosome 11 approximately 5 cM from the other Ry locus.

Cyst-nematode resistance (Globodera rostochiensis pathotype). There are five pathotypes, Ro1 to Ro5. Monogenic resistance is controlled by gene Gro1 (syn. Fb) derived from S. spegazinii. It maps on chromosome 7 close to RFLP markers. A second locus, H1, from ssp andigena confers resistance to pathotypes Ro1, Ro2 and Ro4. It maps on chromosome 5 and also is linked to RFLP markers, but far from Rx2 and R1. On the other hand, the resistance gene to G. pallida is also on chromosome 5 close to Nb and to Rx2. A second resistance locus to this nematode, Gpa2, has been mapped on chromosome 12, linked to Rx1.

The self-incompatibility S locus in potatoes maps on chromosome 1, as it does in tomato. Alleles S11, S13 and S14 have been cloned. An inhibitor gene of self-incompatibility , gene Sli (S-locus inhibitor) of sporophytic action has been mapped on chromosome 12 (Hosaka and Henneman, 1998, Birhman and Hosaka 2000). It is dominant and lethal in homozygous condition.

In general, the tomato and potato maps are very similar to each other, except that the potato map is half the size of the tomato in cM due to reduced recombination.

Detectable chromosome changes between potato and tomato are paracentric inversions and involve whole arms. Four in the short arm of chr 5, 9, 11 and 12; one in the long arm of 10. Breakpoints seem to correspond at the centromere.

Other marker applications

Rivard et al. (1989) used RFLP genotypes to determine the origin of diploids derived by anther culture. They could derive from spontaneously doubled haploids from microspores, from anther tissue or from unreduced microspores by FDR or SDR.

Microsatellite (SSR), ISSR and AFLP markers are used for fingerprinting and variety identification (Provan et al. 1996, Provan and Wilkinson, 1998, McGregor 2000, Coombs et al 2004). Some primers were designed after searching sequence databank for microsat like motifs. A total of 16 primer pairs polymorphic and with multiple alleles were designed and used successfully. Ashkenazi et al. (2000) has developed a larger number of SSR markers. Only a few SSR markers amplify both potato and tomato species.

Glycoalkaloid aglycone QTLs which are toxic to humans have been detected in progenies involving wild species containing these compounds (Yencho et al. 1998).

Potato Genome Sequencing

An international consortium is working on this project to be completed in early 2011. A diploid variety and a doubled monoploid are being used for this task

http://www.potatogenome.net/index.php/Main_Page

Tissue culture

This area has taken quite a bit of emphasis in the potato in the past few years, mostly because it is a vegetatively propagated crop.

Somaclonal variation.
There are earlier reports by Shepard that of 35 traits evaluated, variation was found for 22 of these in protoplast derived plants (protoclones), such as vine morphology, tuber yield and composition, maturity, photoperiod response, disease resistance etc. This claim remain unsubstantiated.

Somatic hybridization.

Protoplast fusion has been applied to diploid cultivated species by Helgeson and his group. They fused two S. tuberosum diploids obtaining vigorous tetraploids. Both diploids had distinct morphological traits such as yellow flesh, red pigmented flesh, light colored and dark colored skins among other traits. Several novel traits not present in either parent surfaced in the hybrid.

Somatic hybridization has also been applied to diploids S. phureja and S. brevidens (E genome) for the transfer of potato leaf roll virus resistance and bacterial rot resistance. These can cross sexually but with difficulty resulting in highly sterile hybrids. The somatic hybrids were resistant to virus and fertile. Also hexaploid hybrids were obtained by fusion of S. tuberosum + S. brevidens protoplasts.

Intergeneric hybrids: Tomato-potato hybrids: Pomato (potato cytoplasm) and Topato (tomato cytoplasm) hybrids, are highly sterile but allow derivation of backcross plants. Tetraploid hybrids (2x tomato + 2x potato) are less fertile than 6x hybrids (4x potato + 2x tomato) . (Jacobsen et al. 1994).

Genetic engineering.

Transformation techniques are well developed for potatoes, based on Agrobacterium, and electroporation. New co-cultivation technique of aerial minitubers with resistance to virus introduced by gene coding coat protein of PVX and PVY. After inoculation of transgenic plants, drastic reduction in virus observed. Interferes with virus infection, multiplication and symptom expression. Bt potato varieties resistant to Colorado potato beetle are available.

Also transformation used to attempt to improve protein quality for improved nutritional value. Yang et al. (1989) introduced a synthetic gene of 292 bp (HEAAE-DNA= high essential aa encoding DNA). Protein increased in 1%. To be useful, it is necessary to have expression in tubers. Transformation with the Bt gene have also been accomplished in potato, resulting in some resistance to tobacco horn worm and tuber moth (Kumar et al 2010).

Transposable elements: Ac from corn was introduced to potato by Knapp et al. (1988), providing evidence that it moves in potatoes. It may be used for gene tagging.

Outlook

Potato genetics is an exciting research area, especially research dealing with disease and pest resistance. The linkage of resistance genes in clusters is and important finding, but its significance remains obscure. Research is certainly going to continue at an increased pace, aimed to understand this phenomenon which will help to develop disease and pest resistant varieties. This will include gene cloning and expression studies. Comparative mapping activities will also continue for a more efficient use of marker assisted selection, which is now possible to apply in potato breeding programs quite effectively for a large number of traits. The challenge will be to apply the information developed in the simpler diploid system to the polyploid crops, which are the one we are most interested from the economical point of view.

 

 


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Last modified: April 26, 2011
Carlos F Quiros, 1998