Plant Breeding – How to make your own new crosses and hybrids

Breeding Basics

Many Gardeners and Plant-lovers find themselves captivated by new and interesting varieties. But how are new plant varieties made – and how can amateurs try this in their own garden?

Well, sometimes you’re lucky and a new variety will just ‘pop up’ as a random mutation or chance cross in a batch of seedlings. However most of the varieties (and especially the best ones) are intentionally bred. This can be accomplished by methodical process of ‘selection’ over the course of many generations, or it can be done by more adventurous (and intentional) crossing of two different varieties in an attempt to get something new and better. It is vital to have a goal when selecting or crossing plants. You need to have an idea of what you want the finished organism to be like, otherwise you can easily end up with something weird, unhealthy or ugly.

Unnatural Selection

This is the oldest process used by people. Most of our major crops have been bred through a process of conscious selection – breeding plants selected only from the very best of the crop. For grain crops the largest yielding plants are selected, for others the criteria may be flavor or oil content. When breeding ornamental plants, the ornamental characteristics take precedence -larger flowers, brighter or newer colors, unusual shapes, and attractive foliage. By growing a large batch of seedlings and selecting the best for breeding, and then repeating the process over and over, any competent gardener can create their own varieties relatively easily.

Crossing Plants

This is really just a controlled version of what the Bees do when they visit and pollinate flowers. You are the Bee.

The plants need to be cross-compatible – in other words they need to be reasonably closely related. Think of it this way – when you cross two plants you are combining their DNA in the same way as if you were trying to cook a meal by combining two different recipes. Some things you can change around, but if too much is different then you will end up with something that just doesn’t make sense and cannot exist as a living organism. Plants have various natural barriers to incompatible crossing – and many plants are even self-incompatible, meaning that they are normally unable to be pollinated by flowers from the same plant or even a plant from the same parents. Such mechanisms are evolutionarily advantageous, as they prevent resources being used to make seeds that cannot grow into viable plants and they prevent inbreeding.

Plants from the same Family may be compatible (but usually not easily), from the same Genus quite likely are, and the same Species are usually totally compatible. Plants that are not in the same family are very unlikely to be compatible.

OK – so down to the business of actual crossing. The key thing here is to ensure control of the pollination – you need to make sure that only the right pollen is landing on the stigma of the flower. The following steps may be used:

  1. Emasculation – removing the anthers from the flower to be pollinated (before they start to release pollen), to ensure self-pollination does not occur.
  2. Bagging – you need to make sure that no other pollen lands on the stigma of the female parent. This can mean using a paper or mesh bag to cover the flower and prevent insects from flying in or pollen blowing in,  in the case of wind-pollinated plants.
  3. Labelling – label your crosses so you know what you did and when. This can help inform future breeding efforts.
  4. Record-keeping – This can be useful in the long term, so try to keep a record of what you’re up to.

Simple Crossing

When you take two easily compatible species or varieties and cross them with each other you may end up with plants that are better and healthier than either parent. This is due to hybrid vigor, or heterosis. This is most common when the two parents come from quite different genetic backgrounds, but are still basically genetically compatible. An example may be two subspecies which have developed in different geographical areas and environments, but are still basically the same species. The offspring in the first generation will have all the evolutionary tricks that each parent will have accumulated, and any potential shortcomings may be covered by a solution found in the other parents different genetic makeup. Some excellent plants can be created in this way, but it is common for future generations to lose this initial vigor as the various traits start to segregate.

Difficult Crossing

This is a broad term here which covers crosses which are not ‘straightforward’, for instance, when the two parents are not closely related (wide crossing), or when they flower at different times of year, making it difficult to cross them in a simple way.

Different techniques can be used to make these crosses work, including pollen storage, forced flowering, mentor pollen, chromosome doubling and other more obscure techniques which are used for breeding Lilies and Amaryllids, such as style grafting, stigma slicing and hormone treatments.

The latter, weird techniques I will not go into. They can be researched, but are usually quite plant-specific so will not work in a lot of cases.

Pollen Storage – this is very simple but can be variable too. Each plant species has its own rules for this but most can have their pollen dried and then frozen for at least a few months -sometimes years. Some plants will have their pollen degrade over time, others seem to benefit from the storage process. Go figure. In every case however it is pretty easy: Just harvest the pollen in a small sealable (airtight) plastic or glass container. Place the open container with the pollen in a sealed jar with fresh silica gel. This jar is your ‘drying chamber’. It needs to be quite a lot of silica gel, maybe 50-100g – but it can be ‘recharged’ afterwards in an oven and used many times. After 24 hours at room temperature the pollen should be dry, the container can then be removed from your drying chamber, sealed, then frozen. This allows you to use a plant flowering NOW to pollinate a different plant in the future. When using the pollen, leave it sealed, take out of the freezer and place in the fridge for an hour. Then take out of the fridge and leave sealed at room temperature for another hour, or until it has acclimitised. Why? Because if you warm it up too quickly it can damage the pollen, and if you use cold pollen you will get water condensing and this makes the pollen wet and unusable in future.

Forced flowering- this is possible when two different species flower at different times of the year but might otherwise be able to cross. Different techniques work for different species, including altering the day/nigh ration through artificial lighting, use of ethylene or other plant hormones, temperature and moisture variation. By getting the two plants to flower at the same time it is possible to cross them with each other in both directions. This is useful, because sometimes a cross will work in one direction but not in the other direction.

Mentor pollen- This is a useful technique when the crosses are wide or when you are trying to overcome self-incompatibility. Basically, you mix an easily compatible pollen (this is the mentor)   with the pollen you want to use, and use the mixture to pollinate. Some breeders try to make the mentor pollen This has dubious efficacy.

Chromosome Doubling- This involves the use of chemical treatments to cause the cells of the growing point of a plant to not divide at the normal time – so the DNA has fully replicated but the cells just don’t divide. After a while the chemical stops working and the cells start to divide normally again, but now they have twice (or more) the normal amount of DNA. Such cells are called polyploid, and a whole organism made up of such cells is also called polyploid. This is something that actually happens a lot in nature, with the majority of existing species having an origin from a polyploidy event,  but the chemicals used to induce this artificially need to be handled with great care, and requires a great deal of research and responsibility. The point of this process for breeding purposes is for broadly two reasons:

  1. Fertility Restoration: You have made a cross successfully but the parent plants had different ploidy levels (chromosome count), so the offspring cannot form viable haploid sex cells, so they are infertile (unable to produce seeds). Doubling the original count makes it possible to easily divide the DNA into two equal sets again, so fertility is restored. Modern Strawberries are the result of this breeding method.
  2. Bridging a Gap: You want to make a cross but the ploidy levels are sufficiently different to prevent successful crossing. By ensuring that at least one of the parents has a ‘double set’ of DNA, you can ensure that the offspring will have at least the basic instructions for viability – OR you can make the parents a better match (so they have a more similar or identical chromosome count).

Chromosomes are tricky things -they are basically packages of data and the cells will be looking to make neat bundles that match up. In some cases the count might be the same, but the chromosome pairings may not be analogous-which makes every cell division a mathematical nightmare for the cellular equipment that is trying to get the chromosomes to line up neatly in little pairs. The doubling process can overcome many of these issues, but new traits from one parent can become lost for several generations due to a phenomenon sometimes called ‘genetic shock’ – basically that the plants are not able to easily integrate the ‘foreign’ genome so it stays inactive due to DNA methylation employed as an evolutionary safety measure. The new DNA is still there, but it has been temporarily deactivated. In nature this would help to maintain genetic equilibrium in a population when the population is healthy. Successive selected generations can allow the new DNA to become introgressed into the new genome through the normal crossing-over process, but this takes a little time.

Closing remarks

This document is only a very brief overview, and should act as a launchpad for anyone interested in getting into their own breeding efforts. The breeding of newer and fitter forms of life is a noble occupation.

Nature exists in every form where it is possible to thrive and survive, and as a result we have many different plant and animal species – all interconnected by complex relationships within various biomes – but the world we live in is largely shaped by one of nature’s most ingenious of evolutionary influences – us.

Humans have left their mark in many ways; changing the layout and appearance of the land, altering the functions and mechanisms of ecosystems, introducing new varieties and species, producing food and commodity crops through large-scale monocropping, and in every other way changing our environment by our justifiable and selfish daily need to survive and thrive.

There are negatives to this ability of ours – we make mistakes sometimes. But we should not imagine ourselves to be separate from nature – and respect that we are a part of the natural world, and not some kind of foreign invader. All our mistakes will instruct our future successes, and then pave the way for probably more mistakes, followed hopefully by more successes. Always aim for success!


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