Contrary to popular belief, most plants (at least in an undomesticated state) are not self-fertile, i.e. they cannot fertilise themselves, but normally need a cross-pollinator, another variety of the same species. For cultivated plants or for plants that are to become cultivated plants, self-fertility is certainly a desirable characteristic. In this article, Markus Kobelt shows how Lubera® breeding attempts to obtain self-fertile varieties in Ribes aureum fruit varieties and in Lonicera caerulea, in the Firstberries – using completely conventional breeding methods, by the way. The only miracle cure that makes the process possible is...time. A lot of time. And this also explains the many bags on plants that can be seen in Lubera's breeding fields in the spring...
Picture: younger Lonicera selections in the field. They have now been wrapped for the first time to check for self-fertile selections. The process will be repeated for three years to obtain reliable results.
Picture: Ribes aureum selections in the field. They are only wrapped when the first flower buds are visible. It must be ensured that flowers develop on the wrapped shoots.
Why are plants normally not self-fertile?
At first glance, it seems counterintuitive that plants are not normally self-fertile. Self-fertility would be much easier...But this idea is based on a human perspective, not a plant perspective. A plant that wants to survive – and if possible over 100,000s of years, during which the general conditions can change considerably – needs as much diversity as possible, as many different characteristics as possible, which will facilitate survival in the long term. The chance that there will be a suitable future fit is simply greater with many variants. Purchasing more tickets increases your chances of winning…
Why do we humans want edible berry plants to be self-fertile?
Well, we intuitively want more yield. We will tend to prefer more and more fertile plants, and this will usually also lead to a selection pressure that favours self-fertile plant individuals. In many cases, self-fertility increases fertility. Tomatoes, for example, were originally also self-fertile and only conscious and unconscious human and animal selection has made them almost 100% self-fertile. And once we have arrived in modern agriculture, the best possible self-fertility enables the planting of single-variety blocks (without pollinators) and therefore easier harvesting. This means that it is suddenly possible to plant a single plant in your home garden, which means that you no longer have to worry about fertilisation, as is the case with blackberries, raspberries, currants and most blueberries.
The example of Firstberries® (Lonicera caerulea) and Fourberries® (Ribes aureum)
Although the domestication of Firstberries (=mayberries=honeyberries=Lonicera caerulea) and Fourberries (=golden currants=Ribes aureum) into cultivated plants began several decades ago, these orphan crops still suffer from the disadvantage of lacking self-fertilisation. Of course, other obstacles to domestication need to be overcome (more uniform ripening, fruit size, more sugar, problematic propagation of Ribes aureum), but the lack of self-fertility severely restricts their use in the home garden and also in commercial cultivation: the annoying problem of cross-pollination (with pollinator varieties) must always be solved first. It is now the task of breeders to shorten the evolutionary process, which has taken 10,000s of years in tomatoes, for example, through breeding and, if possible, to breed self-fertile varieties. All in the knowledge, by the way, that 'nature' would probably have decided differently of its own free will and would prefer cross-pollination.
Lubera breeding: This is how we try to obtain self-fertile Firstberries and self-fertile Fourberries
This is done in six steps, some of which overlap. For the sake of this overview and to simplify matters, we will present them one after the other.
- Tactile tests – are there any self-fertile individuals at all?
First of all, we need to know from our plants, sometimes with the help of the literature, whether we have any chance at all of finding deviating self-fertile individuals. If the literature research is inconclusive, we test 100 plus different individual plants; if we find two to three self-fertile plants, we can be (almost) certain that there will be enough self-fertile or partially self-fertile individuals in new crosses.
- Selection of good breeding varieties with horticulturally and agronomically interesting characteristics
But first we want to raise the overall breeding level; it hardly makes sense to determine self-fertility in inferior plants and then continue breeding at such a low level. The best variety candidates are therefore selected from relatively large populations and crosses. In addition to quality and robustness, particular attention is paid to yield, as it is hoped that the selection criterion of yield alone will enable more self-fertile candidates to be fished out of the pond of seedlings. Remember this: self-fertile varieties tend to be more fertile than cross-pollinated varieties.
- All selected varieties are tested for self-fertility (at least three years, three results as clear as possible)
Then the compulsory breeding work of self-fertility tests begins. For this purpose, entire plants or branches are bagged and wrapped so that they cannot be reached by pollinating insects (and thus by foreign pollen). The varieties and selections that do set fruit inside the bags must actually be self-fertile. At best, they could also develop parthenocarpically (i.e. without fertilisation). In this way, we have found 10-15 candidates for self-fertility in both the Fourberry and the Firstberry.
Picture: Packed shoots of Lonicera selections in the field. Here we test whether a selection is self-fertile. This is the case if the packed shoots produce fruit.
Picture: A look inside the flower prison - The first self-fertile varieties
Possibly, and with a bit of luck, the first self-fertile varieties can already be selected from these candidates. This will be decided in the next two to three years.
- Conversion of the entire fruit variety to self-fertility
But actually, as breeders, we want to go even further. Wherever possible, we want to completely eliminate self-infertility, so that in a few years we will only have self-fertile varieties in our breeding material. It is important to know that the trait of self-fertility is usually recessive, i.e. hidden; it does not manifest itself as a visible trait if it is only present on one chromosome. It only comes to light when it is present on both chromosomes. This genetic structure is easily explained by the originally evolutionarily advantageous role of self-infertility (more diversity!), so evolution has not allowed a dominant self-fertility trait.
However, if we now cross our new self-fertile selections with each other, we can expect that all offspring (or almost all) will be self-fertile. To prevent a genetic bottleneck and to allow as much diversity as possible in other traits within the self-fertility, we do not make individual crosses between the self-fertile varieties at this stage but mix them as freely as possible with each other by having them fertilised by bumblebees in a closed tent.
Pictures: Already secured self-fertile Firstberries® are placed in groups in the insect-proof tent, where bumblebees take over mutual fertilisation. The aim here is to create populations that are completely self-fertile.
The best varieties (taste, size, harvest time, aroma, sugar, pickability) can then be selected from these populations. Although the potential new varieties will ultimately also be tested for self-fertility, it is almost certain that they will produce fruit reliably even without cross-pollination.
- Self-fertility becomes a matter of course in breeding; all breeding material is self-fertile
Sufficient diversity has now been introduced into the new world of self-fertile varieties (Lonicera caerulea, Ribes aureum) so that there is no longer any threat of a genetic bottleneck or genetic dead end. From now on, it is possible and sensible to make targeted crosses (within the self-fertile extended family) for faster quality development and faster promotion of certain traits. We have finally arrived at the breeding level of more advanced domesticated varieties.
Where are we currently in this process?
We have now reached stage 5 for both species, Ribes aureum and Lonicera caerulea. We are currently trying to make the two fruit species self-fertile on a broad front by freely crossing interesting self-fertile selections from stages 3 and 4 with each other – leaving the hard crossing work to the bumblebees.
Picture: The flowers of Ribes aureum are already visible here. Now they have to be wrapped up or put in the crossing tent.
Picture: The bumblebees are already at work here
How long does this entire process of developing self-fertile varieties from barely domesticated fruit species take?
In any case, if you performed the calculation below at the outset of the process, you would barely begin.😉 So if you have similar plans, you are advised not to read this last section...
- Tactile trials: 2-3 years
- Production of large populations, usually from as many open-pollinated good varieties or populations as possible to obtain as much diversity as possible. From these, selections of the best varieties: 6-7 years
- Testing of the resulting selections for self-fertility: 3 years
- At best, the first varieties with self-fertility: 3 years
- Production of seedling populations that are as genetically diverse as possible, but 100% self-fertile, and again the selection of the best varieties. Establishment of a broad and diverse parent stock with self-fertile varieties: 6 years
You see that the whole process takes at least more than 20 years. Our only consolation is that we are already in year 14...The worst is probably behind us!
