Xingqi OU,Xinhua LI,Hong QIAO

Department of Agronomy,Henan Institute of Science and Technology,Xinxiang 453003,China

Comparative Analysis between Triple Cross and Double Cross among Three Parents of Crop

Xingqi OU*,Xinhua LI,Hong QIAO

Department of Agronomy,Henan Institute of Science and Technology,Xinxiang 453003,China

In this paper,the triple cross and double cross were compared by analysing the characteristics of genetic variation in progeny of triple cross hybrid and double cross hybrid among three parents,based on the successful breeding practices of Beijing No.10.The results showed that the selection duration of double cross was one year shorter than that of triple cross.The F1generations of single cross hybrids of double cross showed higher breeding value,and the multiple crossing timing of triple cross was more flexible.More wheat varieties have been bred by triple cross,instead of double cross.Moreover,the double cross has a disadvantage of heavy workload.

Triple cross;Double cross;Selection process

C ross breeding is an effective breeding way widely applied at home and abroad.The vast majority of good crop varieties used currently for production in many countries are bred by cross breeding[1-2]. With the development of agricultural production,the standards of varieties have been improved step by step,and the breeding objective traits have been diversified gradually.However,due to the single cross being sometimes difficult to meet the requirements,multiple cross is required[3-4].Among multiple cross patterns,triple cross and double cross are relatively simple and commonly used.The genetic composition is completely the same between triple cross and double cross among three parents,but there is difference in selection process between the two cross patterns[5],which has always been one of the doubts and difficulties in teaching.At last,the differences between triple cross and double cross among three parents were analyzed in this paper so as to help students to understand this difficulty and provide some reference for breeders.

Similarities between Triple Cross and Double Cross among Three Parents

In general,triple cross refers to the cross among any three different parents.In triple cross,the single cross F1is crossed with another parent.Double cross refers to the cross between F1generations of two single cross hybrids.The double cross requires any three or four different parents.

Parents

The three parents for triple cross are exactly the same with those for double cross.The three parents are represented as P1,P2and P3.

Genetic share of parents

Without taking the differences between direct cross and reciprocal cross into account,there are all three combination patterns for each of triple cross and double cross among three different parents.In different combination patterns,the genetic shares of the three parents are also different.If the genetic shares of P1and P2are all assigned as 1/4 and the genetic share of

Breeding target

The parent P3,which has the highest genetic share,should have good comprehensive trait,strong adaptability and high yield.The unfavorable traits of P3are needed to be improved.The parents P1and P2, which have relatively low genetic shares,should have more outstanding objective traits that are absent in P3. Only when the excellent traits of the three parents are integrated together can the breeding goals be achieved.

Genetic Variation in Progeny of Triple Cross Hybrid and Double Cross Hybrid among Three Parents

The genotypes of the three parents are all assumed to be homozygous.P1is dwarf and susceptible to disease infection,and its comprehensive traits are moderate.P2is tall and resistant to diseases,and its comprehensive traits are also moderate.P3is tall and susceptible to diseases,and its comprehensive traits are good.The breeding objective is to breed crops with lower plant height,disease resistance and good comprehensive traits.

Genetic variation of comprehensive trait

The comprehensive traits of P1,P2and P3are represented by AA,BB and CC.The comprehensive traits refer to the traits that are controlled by multiple genes.The A,B and C do not represent single gene or oligogene,but represent three gene groups composed of many genes.They can be also expressed as a1a2a3...an,b1b2b3...bnand c1c2c3...cn.The genes of different gene groups can be recombined freely among different parents.Theoretically, after two parents containing different gene groups are crossed,the genes from the same group may be mostly or entirely gathered together in a certain generations through freedom gene recombination.In the hybrid offspring, the genotype of comprehensive trait in an ideal plant is CC,so in order to briefly describe the issue,the comprehensive trait of each parent is assumed to be composed of one gene group.Therefore,in terms of genetic variation of comprehensive trait,the ideal genotype CC may be shown in the F1of double cross.The genotype CC will be shown in the F2,instead of F1of triple cross.

Genetic variation of objective trait

Dwarfness and tallness and resistance and susceptibility are two pairs of relative traits,which are all assumed to be inherited independently. According to the relationship of dominance and recessiveness between relative traits,four following cases may occur for the two pairs of relative traits.First caseThe dwarfness gene(D) is dominant,whereas the tallness gene(d)is recessive;the resistance gene(R)is dominant,whereas the susceptibility gene(r)is recessive.The genotypes of P1,P2and P3are DDrr, ddRR and ddrr,respectively.In the hybrid offspring,the genotype of objective trait in ideal plants is D_R_.

Second caseThe dwarfness gene (d)is recessive,whereas the tallness gene(D)is dominant;the resistance gene(R)is dominant,whereas the susceptibility gene(r)is recessive.The genotypes of P1,P2and P3are ddrr, DDRR and DDrr,respectively.In the hybrid offspring,the genotype of objective trait in ideal plants is ddR_.

Third caseThe dwarfness gene(D) is dominant,whereas the tallness gene(d)is recessive;the resistance gene(r)is recessive,whereas the susceptibility gene(R)is dominant. The genotypes of P1,P2and P3are DDRR,ddrr and ddRR,respectively.In the hybrid offspring,the genotype of objective trait in ideal plants is D_rr.

Fourth caseThe dwarfness gene (d)is recessive,whereas the tallness gene(D)is dominant;the resistance gene(r)is recessive,whereas the susceptibility gene(R)is dominant. The genotypes of P1,P2and P3are ddRR,DDrr and DDRR,respectively. In the hybrid offspring,the genotype of objective trait in ideal plants is ddrr.

In terms of genetic variation of objective trait,the ideal genotype can be shown in both triple cross F1and double cross F1in the first case;in the second,third and fourth cases,the ideal genotype can be shown in neither triple cross F1nor double cross F1,but it may appear in triple cross F2or double cross F2.

Differences in Triple Cross and Double Cross among Three Parents

According to the genetic variation characteristics in offspring of triple cross hybrid and double cross hybrid among three different parents,one typical example is taken to analyze thedifferences between the two cross patterns.Beijing 10 was bred by Academician Zhuang through crossing double cross(Huabei 672/Xinshimai) F1with double cross(Zaoshu No.1/ Huabei 672)F1in 1965.Huabei 672 was a high-yielding variety in Northern China around 1955,and it had good comprehensive traits.Compared with Nongda 183,Huabei 672 had higher high-yielding potential.But its maturity was later,and the occurrence of rust was more serious.So Huabei 672 had not been promoted in production.So when the cross was started in 1957 and 1958,the rust-resistance and early-mature parents were not adopted.Therefore,using the early-mature Xinshimai and rust-resistant Zaoshu No.1,the double cross combination (high yield×early maturity)F1×(high yield×disease resistance)F1[6-7].

Selection duration

The selection duration of double cross is one year shorter than that of triple cross.The parents of the double cross are all singlecross F1,and the plants integrating the excellent traits (high yield,early maturity and disease resistance)of all the three parents may appear in the double cross F1.In the parents of triple cross,only one parent is singlecross F1,and the other one is a variety.So the plants integrating the excellent traits of all the three parents may appear in the triple cross F2. Therefore,there may be a difference of one year in the selection duration between double cross and triple cross.

It has to be pointed out that the early maturity and rust resistance are all controlled by dominant genes,and their inheritance is relatively simple. But the inheritance of high yield and adaptation is complex.

Breeding value of F1generation of single cross hybrids in double cross

The singlecross F1of double cross has higher breeding value.The F1generations of two single crosses of double cross,such as(Huabei 672× Xinshimai)F1and(Zaoshu No.1× Huabei 672)F1,all contained the highyielding genes derived from Huabei 672.During the double cross,if the two singlecross F1generations are selected simultaneously,the early-mature and high-yielding varieties or the rust-resistant and high-yielding varieties may be bred out.Therefore,the singlecross F1of double cross has higher breeding value.If the singlecross F1of triple cross,such as(Xinshimai×Zaoshu No.1)F1,is selected,although the early-mature and rust-resistant varieties may be bred, the high-yielding varieties cannot be bred directly.So the singlecross F1of triple cross has relatively low breeding value[8-10].

Multiple crossing timing of double cross

The multiple crossing time of double cross is more flexible.If the double cross is not varied out at the F1generation,it can still be carried out at F2or F3generation,till which the ideal plants (early-mature and high-yielding varieties or rust-resistant and high-yielding varieties)may have been bred out.So double cross has more flexible multiple crossing timing,and it may produce better multiple crossing effect.For triple cross,the multiple cross is mainly carried out at the F1generation.If the multiple cross is carried out at the F2or F3generation,the breeding duration will be undoubtedly prolonged.It is because that the isolation,integration and stabilization of high-yielding trait require a process of selection.

Conclusions

Compared with triple cross among three parents,double cross among three parents has a few advantages, so double cross is considered to be applied more commonly.However,among the bred wheat and crop varieties,the varieties bred by triple cross are more than those bred by double cross.In practical breeding works,the triple cross is adopted more commonly.This may be due to the following reasons:①For first hybridization, triple cross requires only one singlecross,while double cross requires two singlecross simultaneously.②For second hybridization,triple cross requires less hybrid seeds compared to double cross,which is because that double cross F1shows greater separation compared to triple cross F1.③In actual breeding practices,the successful breeding of an excellent variety generally requires breeding a large amount of combinations.The breeding of both double cross hybrid and triple cross hybrid all has a very low success rate.So in actual breeding practices, breeders are often willing to adopt triple cross with less workload,instead of double cross with greater workload.

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Responsible editor:Tingting XU

Responsible proofreader:Xiaoyan WU

作物三亲本三交与三亲本双交对比分析

欧行奇*，李新华，乔红
（河南科技学院农学系，河南新乡453003）

该文通过比较三亲本三交与三亲本双交杂种后代的遗传变异特点，并结合北京10号小麦良种选育成功的经典实例分析后认为，双交比三交的选择进程可能早一年，双交的单交F1育种价值较高，双交的复交时机比较灵活机动。又从采用三交比采用双交育成更多的小麦等作物品种的事实出发，同时也指出了双交也具有杂交工作量大的缺点。

三交；双交；选择进程P3 is assigned as 1/2,the cross combination should be P1/P2//P3for triple cross among three parents,and should be P1/P3//P2/P3for double cross among three parents.

欧行奇（1964-），男，河南周口人，教授，主要从事作物育种学教学及小麦新品种选育和种子生产工作，E-mail：ouyangxq@163. com。*通讯作者。

2015-02-13

修回日期 2015-05-30

*Corresponding author.E-mail:ouyangxq@163.com

Received:February 13,2015 Accepted:May 30,2015