C. arabica history : movements and genetic diversity
Thanks to significant genetic studies since 2020 (Krishan et al, 2021; Montagnon et al, 2021; Montagnon et al, 2022)., the genetic landscape of Arabica coffee is much better known than before. To facilitate understanding of this genetic landscape, we can consider the following three categories:
- Coffea arabica genetic groups that have formed naturally over time.
- Introgressed arabica coffee plants.
- The new generation of hybrids between both previous groups.
The basic genetic groups of C. arabica
There are several Coffea arabica genetic groups that have formed naturally over time.
Three were formed in the species’ Native Habitat zones, i.e. where the species was (and still is) found in its natural state: in south and south-western Ethiopia and southern Sudan.
Two groups (Core Ethiopia 1 and 2) are found in south-western and southern Ethiopia. It is highly probable that these two genetic groups developed over the ages on either side of the Rift Valley. However, human trade and transport of coffee beans between these two regions has erased this probable geographical distribution. Today, both groups can be found on either side of the Rift Valley. For simplicity’s sake, we can speak of a single “Core Ethiopia” group. All Ethiopian landraces originate from this group. Some of these landraces have travelled around the world: either as the result of surveys such as the FAO and Orstom missions in 65 and 66, or in more or less traceable forms. Most of these movements of Ethiopian Landraces have taken place since the 20th century.
The third group, corresponding to the Native Habitat of C. arabica, is the “Southern Sudan” group, found only in Southern Sudan, which is quite different from Core Ethiopia and has never participated in the domestication of the species, with the sole exception of Rume Sudan.
In fact, the source of domestication of C. arabica in Yemen is located east of the Rift Valley, in the Gedeo and Sidama regions and gave rise to the “Ethiopian Legacy” group. The “Ethiopian Legacy” group is the only one with representatives to be found in both south Ethiopia and south-west Yemen. It is therefore the species’ exit point from Ethiopia to Yemen: the center of its domestication.
This “Ethiopian Legacy” group gave rise to three other genetic groups: the “Typica/Bourbon” and “New-Yemen” groups, to the south and north of the Yemeni coffee zone respectively, as well as the “Harrar” group, which today is found only in the Harrargue region of eastern Ethiopia, but which all the evidence suggests was reintroduced from Yemen.
Among the first coffee trees to conquer the world from Yemen in the early 18th century, only the “Ethiopian Legacy” and the “Typica/Bourbon” groups were represented. In fact, the vast majority of Arabica coffee varieties grown worldwide belong to the “Typica/Bourbon” group: Typica and Bourbon, of course, but also SL-28, Jackson, Kent, Coorg…. Of course, the famous varieties derived by mutation and/or crossing between Typica and Bourbon also belong to this large group: Caturra, Catuai, Mundo Novo, Maragogype…Of the “Ethiopian Legacy” group, only a few varieties are significant today, notably SL-34 and K7.
To date, no representatives of the “New-Yemen” and “Harrar” groups have been observed outside the area where they differentiated: Northern Yemen and the Ethiopian Harrargue region respectively. As for the “New-Yemen” group, its distribution to the north of Yemen’s coffee-growing zone, far from the ports of Aden and Mocha, explains its absence from the Out of Yemen of the early 18th century.
Between the beginning of the 18th century and the 70s and 80s, Arabica coffee trees grown outside South Sudan, Ethiopia and Yemen, and representing over 90% of world production, were predominantly of the “Typica/Bourbon” group and a minority of the “Ethiopian Legacy” group.
In other words, three genetic groups (South Sudan, New-Yemen and Harrar) did not participate in Coffea arabica‘s conquest of the world. If the genetic diversity of the C. arabica species as a whole is itself reduced, we are using only a small part of this genetic diversity.
Introgressed Arabica coffee plants
In the second half of the 20th century, coffee leaf rust reached the American continent, posing a serious threat to world coffee production. It soon became clear that the Arabica species itself could not provide the necessary resistance genes. However, in the early 20th century, natural hybrids between the Arabica and Canephora (Robusta) species had been discovered in East Timor. These interspecific hybrids, dubbed Timor Hybrids, were resistant to most races of Coffee Leaf Rust, thanks to genes from Robusta. They were then used as parents for crosses with traditional Arabica varieties to form “introgressed” varieties. Varieties are said to be introgressed when a certain number of genes from a distant donor genetic background are included in a recipient genetic background. The main groups of introgressed Arabica coffee plants are Catimors, Sarchimors and Cavimors (see ….).
It should be noted that these introgressed varieties only have three different cultivars from the “Typica / Bourbon” genetic group as parents on the Arabica side: Caturra, Villa Sarchi and Catuai. These three varieties are actually very close to the Bourbon cultivar alone. Caturra and Villa Sarchi are mutations of the Bourbon variety, while Catuai is a cross between Mundo Novo (from Bourbon x Typica) and Caturra. On the Timor Hybrid side, there are only four parents involved in the creation of introgressed varieties: codes 832/1, 832/2, 1343 and 2570.
The new generation of hybrids between both previous groups.
In plant breeding, there’s a well-known phenomenon known as “Heterosis” or “Hybrid Vigor”. This phenomenon is expressed when two genetically distant parents are crossed, and the offspring between these two parents is superior to the best of the two parents.
Knowledge of the genetic diversity structure of a species enables us to select genetically distant parents, often belonging to different genetic groups, for crosses which are generally called F1 hybrids. For Arabica coffee, it was only in the early 2000s that F1 hybrids were developed and offered to coffee growers.
Why not before? In fact, for reasons linked to the reproductive biology of the C. arabica species, which can self-pollinate (autogamy), mass production of F1 hybrid cultivar plants requires clonal multiplication. Clonal propagation by conventional cuttings is complicated and costly. What changed in the 2000s was the mastery of the somatic embryogenesis in vitro culture technique, enabling large-scale production of F1 hybrid plants. This technical advance has made F1 genetic progress available to coffee growers.
For some plant species, breeders use a trick to produce large quantities of F1 hybrid seeds (rather than clones) at low cost. Without going into too much detail, this trick is called “male sterility”, which refers to the inability of certain plants to produce pollen. Such “male sterile” trees have been discovered in C. arabica. This led to the recent release of the first F1 hybrid coffee variety distributed by seed: Starmaya (selected by Ecom and Cirad). Other similar varieties have since appeared, such as Star1, Star2 and Star3 (selected by Nestlé).
There is no doubt that the recent advances in our knowledge of the diversity of C. arabica, mentioned above, will have a major impact on the future of Arabica coffee breeding, namely regarding the development of seed produced F1 hybrids.