| Potato originated from the Andes before8000years has become one of the most important food crops and plays a vital role in global food security, feeding more than one billion people with an annual production of over300million tonnes in more than one hundred countries. Due to the vegetative propagation, the degeneration caused by viral diseases of the seed potato is the key factor to cause low yield and benefit of potatoes and it is also the most important constraint of the potato production in China. The production of virus-free seed potatoes based on in vitro tuberization has greatly alleviated this problem. So to understand the genetic basis of in vitro tuberization is not only an important aspect of theoretical research, but also critical to improve potato yield and quality. But as an autotetraploid species, the complexity of tetrasomic inheritance and the lack of pure lines increase the difficulty of genetic analysis of the inherited characteristics of cultivated potatoes (Solanum tuberosum L.) and the genetic analysis of in vitro tuberization is hard and lacking.To understand the complex genetic basis of in vitro tuberization of the cultivated potato, we constructed a tetraploid population (F1) of237genotypes segregating on in vitro tuberization, developed linkage maps of this population (F1) and mapped QTLs for the percent of in vitro tuberized plantlets (%IVT). The main results include:1. Based on the performance of in vitro tuberization in two different photoperiods, we evaluated the phenotype of119genotypes from4crosses, and we found that the8h/d photoperiod promoted the in vitro tuberization compared with16h/d; different crosses showed different sensitivity to the different photoperiods. The results of the analysis of the segregation ratio between tuberized and nontuberized genotypes suggest that the ability to tuberize in these cresses is controlled by a pair of major genes, and they have different interaction modes in different photoperiods, additive in8h/d photoperiod and complementary in16h/d photoperiod. 2. From these119genotypes, we selected E108(tuberized well and quickly both in8h/d and16h/d photoperiod) and E20(nontuberize in either photoperiod) to generate the segregation F1population MT I which consisted of237individuals.3. Used the same method, we evaluated the phenotype of this tetraploid potato population MT I. The distribution of the phenotypic data in the population MT I was skewed both in8h/d photoperiod and16h/d photoperiod. In8h/d photoperiod, the segregation ratio of the tuberized genotypes to the nontuberized genotypes in the population was200:37which was consistent with a5:1ratio (p=0.6629). The ratio of the well tuberized (%IVT>20%) genotypes to the poorly tuberized (%IVT<20%) genotypes was117:120which was consistent with a1:1ratio (p=0.8415). In16h/d photoperiod, a large part of the population had not tuberized and the segregation ratio of the tuberized genotypes to the nontuberized genotypes was74:163. All of the74genotypes which tuberized in16h/d photoperiod also tuberized in8h/d photoperiod.4. Based on the711useful markers resulted from primers of128AFLP、65SSR and3candidate genes, we constructed the genetic linkage maps of two parents. The paternal map for E108(well tuberized) consisted of315markers, covered a total length of948cM and included12linkage groups, all of which contained all four homologous chromosomes. The maternal map for E20(nontuberized) consisted of341markers, covered a total length of1286cM and included14linkage groups,12of which contained all four homologous chromosomes. All12chromosomes of potato were tagged using the104SSR markers.5. The results of the marker-trait association analysis showed that8and3markers were significantly (P<0.01) associated with the phenotypic data in8h/d and16h/d photoperiod and one of them was the identical marker. There were both positive and negative alleles affecting in vitro tuberization under each photoperiod and the same positive allele was detected both in8h/d and16h/d photoperiod. The total10markers associated with the phenotypic data in8h/d or16h/d photoperiod were located on chromosome V of E108and chromosome V, IX and XII of E20. 6. We utilized the IM routine of TetraploidMap for almost all linkage groups identified. Four QTLs were identified using the phenotypic data in8h/d photoperiod, but none was detected in16h/d photoperiod. A major QTL (MT05) with additive effect was detected on chromosome V of E108which explained16.23%of the variation for%IVT, and three minor QTLs (mt01-1, mt05and mt09) displaying duplex dominant and simplex dominant effects were located on chromosome I, V and IX of E20which explained6.60%,5.33%and4.81%of the variation for%IVT, respectively. Based on the additive model of MT05, the segregation ratio of the gametic genotypes (Q-:qq=5:1) matched the ratio of the tuberized genotypes to the nontuberized genotypes in the population suggesting that the segregation of in vitro tuberization in this population is controlled by a major-effect gene or genes.7. Based on the interaction analysis of the alleles between the major QTL (MT05) and minor QTLs (mt05and mt09), we found that there was interaction (may be epistasis) between mt09and allele3in MT05, but the effect of mt05was not significant and could be ignored. Although, the present study could not elucidate the minor QTL effect as to whether it is additive, dominant, or interactive, the major QTL effect with minor modifiers of in vitro tuberization were clearly confirmed in this population. Furthermore, we could conclude that the major QTL may control individuals of the progeny to tuberize or not, and the minor modifiers could influence individuals to tuberize well or poorly.8. We also tested the segregation of three candidate genes (StSP6A, StCO and StCDFl) in our population. There was no polymorphism in the candidate gene loci StSP6A and StCO. Only candidate gene StCDFl was mapped on the north arm of chromosome V of E108and the location of StCDFl in our population was near the position reported in the potato genome sequence superscaffold and pseudomolecule information, but out of the one-LOD support interval of MT05. The mapping results of these important candidate genes indicated that the QTL causal genes detected in our study are new. 9. The QTL support interval of MT05was matched the segment on potato (DM) genome through e-PCR, which harbored216genes. Compared these216genes with genes that were differentially regulated by day length or sucrose content identified in E26, we found that54genes of them were identical to the genes which differentially regulated by sucrose content. The result suggests that MT05causal genes may be involed in the sucrose controlled in vitro tuberization.In this study, we developed the almost complete linkage maps of a tetraploid population, identified a major QTL on chromosome V affecting in vitro tuberization, suggested a major-effect gene with minor modifiers model controlling this trait and found that the QTLs identified here correspond to new tuberization genes. Our work provides additional confirmation of previous researches which suggested that few-gene models are responsible for the tuberization process of cultivated potatoes, and we also provide the possibility that there are new genes involved in the tuberization process in tetraploid populations. |
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