We are often asked where the dwarf growth properties of Lowberries®, but also of Maloni® and Pironi®, our compact growing, bushy pear and apple trees, come from. How do breeders get such properties, how do they recognise and develop them?
The breeder "coincidence" also plays and played a crucial role here. In one of our first apple crosses (Resi x Delbard Jubilé) I noticed already in the seedling stage that there was an exceptionally high number of "genetic dwarfs", only very slowly growing seedlings, partly with lethal characteristics, partly (but also vital) with only very short internodes. The demand from other breeders 25 years ago resulted in the following feedback: simply remove these plants, we cannot use them in our breeding, as most of them are not viable...Out of ignorance and curiosity, I kept 50 "genetic dwarfs", grew them and later planted them on their own roots. After about 10 years we selected our first two Maloni® varieties Sally® and Lilly® from this population. Later research and analysis showed that obviously the crossing (Resi x Delbard Jubilé) of these "genetic dwarfs" had not worked at all, but that the super compact plants must be the self-fertilised 'Resi' (Resi x Resi)...
Picture: dwarf apple tree Maloni® Sally® in the selection field
Dwarf genes from the depths of evolution
How did we come up with that? First of all, both daughter varieties point to the mother and do not carry any conspicuous characteristics of 'Delbard Jubilé'. But above all the subsequent crosses with Sally® and Lilly® (Sally x normal growing variety) showed that the compact gene in these varieties is recessive. If Sally® or Lilly® is crossed with a normal growing variety no compact growing seedlings will result. Only the sibling cross Sally x Lilly (or backcrossing with the mother variety 'Resi') produces compact trees again...
Picture: dwars apple tree Maloni® Lilly® with fruits
Here is the genetic explanation for the phenomenon: obviously 'Resi' had been partially self-fertilised at the crossing with 'Delbard Jubilé', i.e. in some cases Resi's own pollen succeeded in pollinating 'Resi' flowers (this is also the case with apples, but only in a small single-digit percentage). Due to this selfing, the invisible recessive compact gene slumbering in 'Resi' had come to the surface and become visible.
But how could these genes survive in nature and why are they still there? To answer this question, we have to do a short thought experiment: what happens to super-compact apple trees in nature, in the process of evolution? Well, as a rule, they will not survive. Plants that are too small are overgrown, receive too little light, and lose out in the competition for nutrients and other resources; they get trampled down, eaten and destroyed. This also means, however, that the dominant compact genes in evolution, i.e. those that always show themselves and that visibly reproduce and develop from generation to generation, die out. They simply have no chance to survive. Potentially negative genes or mutations for survival (besides dwarfism this can also be reduced fertility, double flowers, etc.) can only survive if they are recessive. One can imagine this as follows: at some point a (always undirected, random) spontaneous potentially lethal mutation takes place (which leads to this dwarfism, for example); if it is dominant, it will soon die out again; if it is recessive, it has a good chance of surviving, even if only hidden and by definition mostly invisible and ineffective.
Recessive compact genes in Lubera breeding
Pictures: compact growing Lowberry® raspberry and blackberry in pots on patio
In the breeding programme at Lubera® we have discovered such recessive compact genes in blackcurrants (Lowberry® Little Black Sugar®, various test numbers), raspberries (Lowberry® Little Sweet Sister®), apples (the Maloni® variety group) and blackberries (Lowberry® Little Black Prince®) and probably also gooseberries (various breeding numbers). Through careful crossing programmes, sibling crossings and backcrossings, we have succeeded in continuously expanding this "accidentally" found growth trait and making it fertile. The corresponding breeding programme for apples has now been taking place for 25 years – with an output of four compact to super-compact growing varieties (20-50% the size of normal growing varieties).
Picture: raspberry Lowberry® Little Sweet Sister® at the Hampton Court Flower Show
Spontaneously occurring dwarf genes
Obviously, there are also other compact genes that are dominant, i.e. they grow normally with an approximate portion of 50% when crossed compact growing x normal growing. Examples are the columnar apples, which inherit the dominant columnar growth; the compact gene for Pironi® and Pirini® (mini pears and columnar pears) is also dominant and goes back to a bud mutation and variety discovered in the 19th century called 'Nain vert'. An article about the history of Pironi® and Pirini® can be found here. The dwarf mini summer raspberry Ruby Beauty(S) also has a compact gene that is obviously dominant (compact x normal in every cross).
Picture: compact growing pear tree Pironi® Joy of Kent®
How can that be possible? Does this not contradict our above thesis that negative, potentially lethal plant traits can only survive recessively? The answer to this question is the gardener. As already mentioned, mutations are always undirected and also holistically diverse: they can lead to positive or negative or even to completely incomprehensible traits (always from the point of view of man and the gardener), the traits can be recessive or dominant, quantitative or qualitative, also genetically much more complex based on several genes or gene groups. If a compact mutation only occurs in nature and remains unobserved, it survives only recessively. However, if it occurs during cultivation, it is observed and propagated by humans, and at best is used for breeding, then it can of course also be dominant. The gardener helps it to survive. The chance that a compact mutation will survive during cultivation is even greater in the case of a dominant genetic trait than in the case of a recessive one since it is consciously or unconsciously much easier to use and make productive in breeding and agronomy.
Dwarf varieties at Lubera
Picture: established dwarf peach tree in the house garden in full bloom
For me, compact breeding is one of the most exciting sub-disciplines of fruit and berry breeding for the home garden. You explicitly profit from the ubiquitous breeder "coincidence", you only have to take a look at your work...and then you can systematically reproduce it, expand it and make it fertile. It is our goal to discover such compact genes in more and more fruit species and then use them for breeding. As already mentioned, we currently have such genes in apples (recessive and dominant), pears (dominant), peaches (dominant), raspberries (recessive and dominant), blackberries (recessive), gooseberries (rather recessive) and blackcurrants (recessive). I am still somewhat uncertain about the blueberries: the compact growth comes from Vaccinium angustifolium, but in contrast to other fruit species it seems to be much more continuous (all growth types possible from super compact to semi-normal...). The "compact" trait seems to be based on several genes and has a largely dominant effect.
Picture: blueberry Lowberry® Little Blue Wonder® on patio
The limits of dwarf breeding or how many dwarf varieties should there be in the world?
Despite the breeder's enthusiasm and all the hype about urban gardening and container cultivation on terraces and balconies, the demand and market share of compact fruit and berry varieties should not be overestimated. The sales figures at Lubera’s online shop show that this market share for raspberries is currently around 10-20%. The reason is this: if in almost all cases the fruit and berry dwarfs have succeeded in providing good fertility and fruit quality, they naturally produce less plant volume, less shoot length and assimilation area. This cannot be fully compensated by the shorter length of the internodes and so compact dwarf varieties systematically give less yield. It is one of the main goals of compact breeding to compensate this effect as much as possible, but this will never succeed completely. There are natural boundaries or even counter-trends that oppose this: if the internodes become shorter and almost all buds fertile, this leads to an increase in alternation (intermediate years with less yield, especially with tree fruit). From a breeding point of view, for example, we have succeeded in largely masking this effect with the Malini® Subito® columnar apple variety. In general, however, an even more compact and even more fertile plant (such as dwarf raspberries) that grows, blooms and produces fruit in the same year will reach its physiological limits at some point in the future, and will take a very long time to expand...
Picture: columnar apple tree Malini® Subito® in the selection field during ripening time
To make a long story short: if you are a hobby gardener who wants to harvest kilograms of fruit to fill your freezer, you will continue to benefit from normal growing varieties of plants😉. And accordingly, we will continue to concentrate a large part of our breeding work on normal growing or moderately compact growth types, despite all the joy we have in breeding compact plants.