(“free crossing”, “polyploid selection”, “plant breeding methods”, “polyploid plants”)
In modern plant breeding, multiple strategies exist for improving plant populations, combining desirable traits and optimizing genetic makeup. Two important methods are free crossing (open crossing) and polyploid selection. Below we also discuss recurrent selection and grafting as complementary techniques.
Recurrent Selection
(“recurrent selection in plants”, “recurrent selection method”)
Recurrent selection is the process in which a group of parent plants is chosen, and then sampling of progeny with desirable traits is performed. These selected progeny are then used as parents in the next cycle of breeding. This cycle—select → cross/produce progeny → select again—is repeated over several generations to increase the frequency of desirable alleles in the population.
Why use recurrent selection?
It accelerates improvement of quantitative traits (e.g., yield, disease resistance) by repeatedly selecting superior individuals.
It enhances the overall genetic potential of a breeding population.
It helps maintain genetic diversity while focusing on targeted traits.
How it works (step-by-step):
Start with a base population of plants with genetic variability.
Identify and select plants that exhibit the desirable traits (e.g., stronger disease resistance, larger fruit, better flower form).
Cross these selected plants or let them inter-mate, producing the next generation (progeny).
Screen the progeny for the desired traits, select the best, and repeat the process for multiple cycles.
With each cycle, the frequency of the favourable alleles rises, and the population gradually shifts to having improved performance.
This method is effective in both self-pollinated and cross-pollinated species. Proper management and tracking of generations is essential for success. It is a key method in population improvement schemes and synthetic variety development.
Grafting
(“grafting plants”, “grafting for disease resistance”, “grafting horticulture”)
Although grafting is not strictly a selection method, it is a powerful technique in horticulture and plant breeding. Grafting involves joining the root-system (rootstock) of one plant with the shoot or scion of another plant that carries favourable fruit or flower traits.
Why graft?
To combine the strong root traits (e.g., disease or pest resistance, tolerance to stress) of one plant with the desirable shoot traits (e.g., flower colour, fruit size, flavour) of another.
Particularly used in woody species (trees, shrubs) but also applicable in many horticultural crops for improved resistance, vigour, or adaptation.
Provides faster improvement for traits that are difficult to breed via conventional crossing (e.g., root disease resistance, soil-borne pathogen resistance).
Helps overcome rootstock-related issues such as weak root systems or susceptibility to soil pests.
Notes for application:
Choose rootstocks known for strong health, disease/pest tolerance, vigour, deep root systems.
Choose scions for the traits you want to propagate (flower colour, fruit quality, yield).
Ensure compatibility between rootstock and scion (graft take, matching vascular systems, similar growth habit).
Monitor for any graft-union issues (delayed incompatibility, vigour differences).
Remember: grafting is a propagation/combination technique rather than a genomic selection method. It complements breeding but does not replace allele-based selection or genetic improvement.
Polyploid Selection
(“polyploidy in plants”, “polyploid breeding”, “polyploid selection method”, “polyploid plants breeding”)
What is polyploidy?
Polyploidy is the condition in which a plant has more than two complete sets of chromosomes (i.e., more than the usual diploid 2n). This may happen due to errors in cell division, chemical or physical mutagens (e.g., colchicine), or irradiation. Polyploid plants may emerge naturally or through induced treatments in breeding programs.
Polyploid plants often exhibit important traits beneficial for breeding: larger organ size (flowers, fruits), increased heterozygosity/variation, novel adaptation potential. �
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Why use polyploid selection?
It enables solving issues of viability or fertility (in hybrids) by doubling chromosomes to restore fertility (e.g., triploid to tetraploid). �
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It is used to create seedless (sterile) varieties (for fruit crops) by manipulating ploidy levels.
It can improve resistance to pests and pathogens, vigour, and stress tolerance (the so-called “gigas effect” where organ size increases due to larger cells) �
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It broadens the genetic platform for breeders: with more chromosome sets, there are more possible allele combinations and more complex inheritance, which may be manipulated for improvement. �
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How does a plant become polyploid?
Chromosome changes that can lead to polyploidy include structural and numerical shifts:
Deletion – loss of a chromosome segment (from Latin: “destruction”).
Duplication – a segment of a chromosome is duplicated (copied).
Inversion – a chromosome segment is reversed end-to-end (180° rotation).
Translocation – segments between different chromosomes are exchanged, resulting in new combinations of genetic material.
These structural changes can accompany or trigger polyploidization and reshape the genome of the plant. Because of these changes, selection of polyploids must be handled carefully: while there are many advantages, there are also risks related to plant health, ecological stability, and biodiversity.
Considerations, advantages & risks
Advantages:
Larger organs (flowers, fruits, foliage) due to larger cells (“gigas effect”). �
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New trait combinations, increased heterozygosity, improved hybrid vigour.
Potential to overcome sterility barriers in hybrids by chromosome doubling.
Expanded genomic toolkit for breeders: multiple sets of homologous chromosomes offer flexibility for selection and manipulation.
Risks and challenges:
Potential fertility issues: odd‐ploid levels (e.g., triploids) may be sterile or low‐seed producers.
Complexity of inheritance: more chromosome sets mean more allele copies and complex behaviour in meiosis. �
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Possible ecological or biodiversity impact: polyploids may out‐compete diploid relatives, or their novel genetics may affect gene flow or ecosystem stability. �
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Commercial and breeding challenges: selection in polyploids may require special tools, larger populations, more complex genetics and statistical modelling. �
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Practical breeding applications
Artificial induction of polyploids using chemicals (e.g., colchicine) or irradiation.
Using inter‐ploidy crosses (diploid × tetraploid) and then doubling chromosomes to produce fertile tetraploids. �
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Selecting among polyploid progeny for desirable traits (size, yield, disease resistance).
Combining polyploid breeding with other methods (free crossing, recurrent selection, grafting) to create robust improved varieties.
Free Crossing (Open Crossing)
(“free crossing in plant breeding”, “open pollination breeding”, “plant breeding free crossing”)
Free crossing, sometimes called open crossing, refers to allowing plants to cross freely (within a selected population) without strict controlled single crosses, or using many parents and combining their progeny into a population. This method is useful for establishing base populations with high genetic variability, useful for subsequent selection.
Benefits of free crossing:
Maintains a wide genetic base and high diversity, providing more opportunity for favourable allele combinations to emerge.
Less labour-intensive than strict one-to-one hybrid crosses, especially in early stages of breeding.
Good for building synthetic cultivars or composite cross populations, especially in cross-pollinated species. �
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How to implement:
Identify a set of parent plants (with desirable combined traits) and plant them together, allowing them to cross naturally (by insect or wind pollination).
Harvest the progeny seeds, mix them (bulking) and grow them as a population.
Then apply selection (e.g., recurrent selection) on this bulked population to move toward improved traits.
Repeat this scheme until the population stabilises with the desired traits.
Relation to other methods: Free crossing often serves as the starting point. Once the population is established and desirable traits begin to cluster, you can shift into recurrent selection or controlled crosses for fine-tuning. Controlled crosses may then enforce specific parents to bring together particular alleles more rapidly. �
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Integrating Methods for an Effective Breeding Program
In a well-designed plant breeding program, these methods can be integrated:
Start with free crossing: assemble a broad base of genetic diversity by allowing many‐parent crosses or open pollination among selected parents.
Use grafting (where applicable) to bring together rootstock/scion advantages in perennial or woody crops, or propagate elite genotypes.
Apply recurrent selection: cycle through selection of progeny with favourable traits, inter-mate them, and repeat to raise allele frequency of desired genes.
Introduce polyploid selection (if applicable): induce or select for polyploid plants when larger organ size, seedlessness, or overcoming fertility barriers is needed.
Monitor risks: especially when using polyploids, take precautions for fertility, inheritance complexity, ecological impact and biodiversity.
Screen and evaluate: in each generation, evaluate plants for traits like disease resistance, yield, quality, stress tolerance. Use well‐designed trials to ensure selected lines truly perform under target conditions.
Refine via controlled crossing: once you have good parental lines, make specific crosses (rather than purely open) to combine specific traits and fix them in new varieties.
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