This article throws light upon the top four breeding methods applicable to cucumber. The methods are: 1. Routine Backcross Programme Method 2. Pedigree Method 3. Use of Sex Inheritance and Chemicals in Breeding Method 4. Population Improvement and Extraction of Inbred Lines Method.
1. Routine Backcross Programme Method:
This has been quite useful for transferring characters governed by single genes, e.g. disease resistance or quality traits from donor lines to more stable recurrent parents. The typical examples falling in this case could be transfer of qualitatively inherited traits like determinate growth habit (de), downy mildew resistance (dm), nematode resistance (mj) into an otherwise superior inbred cultivar (recurrent parent).
Often, six generations of selection and backcrossing to the recurrent parent are required to recover the desired genotype (recurrent parent + additional trait). Two versions of backcross method depending upon whether the character to be transferred is governed by a dominant or a recessive gene, are employed.
For transfer of a character governed by a recessive gene, the F1 is backcrossed to the recurrent parent. In one scheme, F1 is self-pollinated to produce F2. The F2 plants that possess the trait under transfer are backcrossed to the recurrent parent to produce BC1.
The BC1 is self-pollinated to produce BC1F1 which segregates into 1: 1 ratio (Rr: rr). BC1F1 is evaluated and the plants having rr genotypes are selected and backcrossed to the recurrent parent (RR). This process is repeated till BC6 where best individuals are self-pollinated and selected for trait under transfer. Selfing precedes each backcrossing to identify rr plants.
For transfer of trait governed by a dominant gene (anthracnose-Ar, bacterial wilt-Bw, target leaf spot-Cca), the recurrent parent is crossed with donor parent and F1 is backcrossed to recurrent parent. BC1 is backcrossed to recurrent parent and process is continued till BC6 from where the best individuals are self-pollinated and selected for homozygous expression of the trait using progeny testing.
2. Pedigree Method:
This includes selection of single plant-segregates in segregating generations F2, F3 etc. derived from crosses between desirable parents. Pedigree breeding is the most common method of breeding in cucumber. Two or more adapted parents are chosen which complement each other in desirable traits.
After crossing two parents, F1, F2, F3, F4, F5 generations are produced with selection of desirable plants in each segregating generation. Commercially available F1 hybrids could also be starting materials for this method of breeding. Beginning at F4 or S4 generation, selection puts emphasis on family row performance for quantitative traits and superior plants within superior family rows are selected.
For next generation, F6/S6 are uniform and can be handled as inbred cultivars. Staub (2008) have suggested the population size/cross as 54 F2 plants, 36 F3 families/rows, 24 F4 families/rows and 18 F5 lines during selfing and selection.
Single Seed Descent (SSD) Method:
This is a modified pedigree method of breeding where each F2 plant is advanced to F3, F3 to F4 and F4 to F5 through single seed from each plant.
This is a rapid method to develop inbred lines by self-pollination in greenhouses and winter nurseries in western world and Europe without selection till S3 to S5 or S6 from where selected individual plants are grown as progeny rows for effective evaluation. This can be used to improve yield and earliness rather than resistance to diseases.
3. Use of Sex Inheritance and Chemicals in Breeding Method:
In this system gynoecious lines are treated with Ag NO3 to stimulate production of staminate flowers and such lines are allowed inter-pollination to build up a source population with wide genetic variation.
The gynoecious lines treated with Ag NO3 pollinate a mixture of same lines which have been sown about 2-3 weeks later and rogued from any staminate flower segregates. A gynoecious population emerges from the harvested seed and serves as a gene pool for carrying out further selection of desired genotypes and lines.
4. Population Improvement and Extraction of Inbred Lines Method:
This method is based on recurrent selection and aims at long term gains for the characters having low to moderate heritability. The source population should have exotic and even un-adapted germplasm into it. Fig. 26.2 as adapted from Lower and Edwards (1986) illustrates this procedure.
A few points worth noting in this approach are:
(i) Adequate variance be maintained in the population through few cycles of recombination without selection to minimize linkage disequilibrium.
(ii) Yielding ability should get priority in initial stages to ensure retention of high yielding genotypes to realize maximum long term gains.
(iii) After about three cycles of recurrent selection for yield, selection pressure may be applied for fruit quality, disease resistance and other traits.
(iv) Before extracting inbreds, it is important to ensure that the frequency of genes for desirable traits is adequate enough to recover them at an early inbreeding stage.
Hybrid varieties of cucumber are becoming predominant day by day. Their number is continuously increasing in USA, Germany, South-East Asia etc. and thus, extraction of superior inbreds and determination of best hybrid combinations appears to be of great importance. This scheme is diagrammatically illustrated in Fig. 26.3 as adapted from Tatlioglu (1993).
According to Staub (2008), hybrids are usually made as:
gynoecious x gynoecious lines or
monoecious x monoecious lines.
Even though publically bred open-pollinated populations are often genetically broad-based and provide a source for further plant improvement, hybrids provide an avenue for proprietary protection of commercial inbred lines.