Potato: part 2.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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)].
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.
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.
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.
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.
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.
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.
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.
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).
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).
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.
Last modified: April 26, 2011
© Carlos F Quiros, 1998