Breeding of cross pollination crops

Cross pollinated plants species have different gene pool structure and extent of genetic recombination. genes are shuffled every generation, in self pollinated crops variety developed as improved population. (inbred lines),in cross pollination plants are developed as hybrid varieties (HYB).

There are two methods for breeding of cross pollinated plants.

  • Selection methods – develop open pollination population (OPP)
  • Hybridization methods – develop hybrid varieties (HYB)

Selection method – develop open pollination population (OPP)

Selection is the separation the best individuals from the heterogeneous population. selected plants have variability, and this variability must be heritable. when selection is done population mean for the desired trait has be improved.

Selection methods – Mass selection.

Steps

  1. Variable population
  2. Parent plants are selected phenotypically
  3. selected plants are growth under open pollinated conditions, pollination should be controlled from unwanted pollinators.
  4. Progeny testing
  5. Parent are selected from the progeny test results, and seed of the selected parents are bulked.
  6. Repeat the procedure in 2nd generation.

  • In mass selection variety is developed as heterozygous population. Disadvantage of this methods is reduce the genetic variability selected plants become genetically similar it cause to inbreeding depression, pollination should be controlled but sometimes superior parent can be pollinated with inferior parent

Mass Selection with progeny testing

Progeny testing is the evaluated the performance of the offsprings(progeny) identify the parents with desired characters. however cross pollinated plants does not provide information much informations compared to self pollination plants, it is better to evaluated the performance of the cross pollinated plants.

Progeny plots establishment of the Mass selected cross pollinated crops

  • By self-seed from the selected plants.
  • By crossing the selected plants with the known tester, it can evaluate the combinability of the mother plants (selected plants) with common tester.

Hybridization methods – develop hybrid varieties (HYB)

Hybridization is the generation of F1 by crossing two inbred lines (IBL), this F1 generation is highly heterozygous but uniform.

Hybridized plants display hybrid vigor (heterosis – better performance compare to parent plants)

  • If means of the desired characters in the hybrid is higher than the means of the parent population that generation will be positive heterosis
  • If means of the desired characters in the hybrid is less than the means of the parent population that generation will be negatively heterosis

History of hybrid varieties

Single cross hybrids started with maize in North America in 1909. Single cross hybrids are not commercially viable.

Double cross hybrids started in North America in 1930. Produce by crossing F1 hybrid plants.

Different types of hybrid cultivars
  1. Single cross hybrids (A X B)
  2. There way hybrids (A X B) X C
  3. Four way hybrids (Double cross) (A X B) X (C X D)

Hybrids led to commercialized agriculture because it give opportunity to breeder to maintain the inbred lines and maintains the hybrid seeds, hybrids are adapted to good managements practices and environmental conditions (good uptaken of fertilizers and agrochemicals)

Hybrid seeds are seeds collected from the F1 hybrids, These hybrid seeds are used to cultivate the varieties.

Hybrid variety development – steps

  1. Development of IBLs these plant are used as parent plants. These IBLs are heterozygous plants produce by inbreeding
  2. Selection of the good IBLs and determine their Specific Combining Ability ( SCA)
  3. Improving inbred parent lines by backcross breeding and pedigree selection.
  4. Crossing chosen inbred lines – Pollination control in must when cross two inbred lines, Selfing of the female parent has to be prevented, pollination of unwanted male pollens has to be prevented. there are methods to prevent the self fertilization; hand pollination, pollination by insects, wind pollination
  • The formula for generation for total number of single crosses = n(n-1)/2

n = number of inbred lines

Pollination control

Emasculation – the process of prevention of self-fertilization

Emasculation techniques

  1. Hot water treatment – Useful in emasculating very small flowers (E.g. grasses, rice), that cannot be hand-emasculated. Florets are immersed in hot water (42 – 48 0C) for 10 min.
  2. Using suction to remove anthers – Use a vacuum machine to suck off anthers.
  3. Gametocides – kill pollen with chemicals Ethereal oil prevent the development of the anthers. Alcohol de-activate pollen
  4. Use of male sterility
  5. Use the self-incompatibility

Use the self-incompatibility

Self-incompatibility is physiological barrier between pollination and fertilization.

Male sterility

Male sterility is inability of the plants to produce or releases the functional pollens due to total absence of the male parts, irregularities of the microspores, functional pollen not released due to structural problems of the anthers. Male sterility is commonly called by the genetic factors, however it may influence by the environmental factors such as chemical and temperature.

Genetic control of the male male sterility.

  1. Nuclear male sterility
  2. Cytoplasmic male sterility (cms)

Important terminologies

  • Seed parent (IBL1) – Male sterile (msms)
  • Pollinator line (IBL2) – Male fertile (MsMs)
  • Maintainer line – Isogenic to seed parent except the male fertile (Msms)
  • Propagation Block – Male sterile stock is maintained by crossing male sterile plants with heterozygous fertile plants. (Seed parent (IBL1) X Maintainer line)
  • Crossing Block – Seed parent (IBL1) X Pollinator line (IBL2). The fertile plants in the male sterile rows are removed as soon as they can be recognized. If not it will result in loss of the uniformity of F1 generation.

Nuclear male sterility

  • Determined by nuclear genes. (Ms/ms).
  • Expression of the dominant allele (Ms) produces normal male fertility.
  • Expression of the recessive allele (ms) results in male sterility.

Cytoplasmic male sterility(cms)

This is controlled by cytoplasmic factors and it is maternally inherited.

  • Male sterile system for hybrid seed production – Grow alternative rows of male and female parent lines,harvest F1 hybrid seed produced on the male sterile female parent (seed parent).

Practical problems in using male sterility

  • Male sterility must be stable otherwise parically selfing can occur.
  • Plant morphological and developmental abnormalities occurred when introducing the male sterility. E.g. Southern Corn Leaf Blight, and destroyed some 15% of the entire US corn crop in 1970. (​​Bruns, 2017)​ 

Advantages of the hybrid seeds

  • Obtain high yield than traditional breeding methods.
  • Uniform population even though it gives heterozygous plants.
  • Hybrids fetch a higher price.
  • Good for breeders but not for farmers because can not make seeds for F2 generation.

Disadvantages of the hybrid plants

  1. Difficulty in identifying good inbred parents
  2. Seed production is more complicated and more costly – Inbred line maintains, pollination control.

Multiple hybrids / Synthetics

Multiple hybrids are produced where pollination control is difficult. Multiple hybrids are produced by the combination of the large number of inbred lines, and testing these lines for General combining ability (GCA).

Multiple hybrids steps

  1. Isolation of inbred lines
  2. Testing these lines for high GCA
  3. Intercrossing superior lines to produce the F1 generation.
  4. Maintenance of the F1 generation in isolation under open pollination.

Advantages of the Multiple hybrids

  • Useful where pollination control is difficult
  • These are more variable compare to hybrid plants. therefore more flexible against changing environment conditions, disease and pest infestations.

References

  • Yu, K., Wang, H., Liu, X., Xu, C., Li, Z., Xu, X., Liu, J., Wang, Z., & Xu, Y. (2020). Large-Scale Analysis of Combining Ability and Heterosis for Development of Hybrid Maize Breeding Strategies Using Diverse Germplasm Resources. Frontiers in Plant Science, 11. https://doi.org/10.3389/fpls.2020.00660  

  • ​​Bruns, H. A. (2017). Southern Corn Leaf Blight: A Story Worth Retelling. Agronomy Journal, 109(4), 1218–1224. https://doi.org/10.2134/agronj2017.01.0006 
  • Hand-pollination – Wikipedia. (2022, May 1). Hand-pollination – Wikipedia. https://en.wikipedia.org/wiki/Hand-pollination

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