Ligenioides:Indian Coffee Selection-11

Coffee for a consumer means its fine aroma, flavor and taste that contribute to the cheer of the day. These are the hallmark characters of a beverage brewed from the roasted and ground beans of a single species of the genus Coffea, namely Coffea arabica1,2. This is one unique species of the genus and there are over a hundred other species that are also members of this genus3. The genus Coffea belongs to the flowering plant family Rubiaceae7. Coffea arabica was the only species of the genus to be cultivated until the early 20th Century on account of the quality attributes mentioned above. In the last decade of the 19th Century the cultivated Arabica coffee came under severe attack by the disease caused by the fungus Hemileia vastatrix. This led to the devastation of flourishing coffee plantations of Sri Lanka and Indonesia4,5. This is when the world started looking for alternatives to Arabica coffee and other species started being described and some of them were brought into cultivation by progressive growers. Among those that were introduced to cultivation the most prominent is Robusta (Coffea canephora) and the second species of import is Liberica (Coffea liberica)6. Until then, there was no organized effort to conduct research on the coffee plant.

It is important to understand the biological nature of any important plant in the context of its genetic improvement. At this juncture, it is not out of place to remember that all the organisms are endowed with characters determined by genes that are borne on their chromosomes. Each of the species is characterized by a unique chromosome number8. This number is usually an even number because most of the higher organisms are diploid in nature. The term diploid is derived from Greek “diplous” which means double (written as 2n). Thus, all diploid organisms carry two complete sets of chromosomes, each being contributed by one of the parents. This diploid number is reduced to half in the formation of germ cells like sperms and ova of animals and pollen and egg cells of plants through the meiotic cell division that is also known as the reduction division25. This reduced half number of chromosomes is termed haploid, again derived from Greek “haploos” that means single (written as 1n or n). This haploid set of chromosomes carries all the genes that determine the characters of an organism and is generally characteristic in most of the species of a genus and sometimes even of the family to which a genus belongs. This set is also often called as the base genome of the genus. Such basic set of chromosomes is sometimes referred to as the base number of chromosomes in the genus and/or family (written as x). As already said coffee plants of all species belong to the genus Coffea of the family Rubiaceae. Base number of chromosomes in this family is 11. This is written as n=x=119.

Upon cellular examination of different species of Coffea, it was found that C. arabica carries 44 chromosomes in its cells and all other species including C. canephora and C. liberica have 22 chromosomes in their cells. Thus, C. arabica is a tetraploid carrying four sets of the basic haploid set of chromosomes. This means that each parent of any Arabica coffee plant contributes 22 chromosomes which is the haploid set for this species and the diploid number is 44. However, this diploid number is multiplied four times of the basic haploid set and hence it is considered a tetraploid (written as 2n=4x=44). Further, it is considered an allo-tetraploid, derived by the hybridization of two progenitor species and doubling of chromosomes in their allo-diploid hybrid (allo is a greek word that means combing from different organisms/species). The two progenitor species were identified as C. eugenioides being the female progenitor and C. canephora as the male progenitor, each contributing a complete set of their chromosomes. By an extension of the same line of argument, the diploid chromosome number of all other species of Coffea is 2n=2x=22.

In an evolutionary context, each of the species of various organisms is characterized by a unique chromosome number as already mentioned. However, different species of the same genus may carry the same basic set of chromosomes. They become unique, showing different characters by way of differentiation of chromosome structure within the basic set. This may involve different mechanisms like duplication of some parts of certain chromosomes, deletion of small segments of some chromosomes, inversions of some stretches of chromosomes and sometime translocation of segments of chromosomes from one to another chromosome. These changes can ordain the expression of genes to be different between species leading to the observed differences between them. This chromosomal differentiation can be of an order that can prevent intercrossing of related species, i.e. crossing of such differentiated species would not result in viable hybrids. In such cases the species are said to be reproductively isolated10. Thus, chromosomal structural re-organization could be a powerful isolating mechanism in the context of evolutionary species formation. In some cases, differentiation of chromosomes may be of a lower order. In these cases, apparently different species can cross to produce moderately fertile and viable hybrids. These hybrids are an important source of new genes and new combinations of genes for genetic improvement of a related species that may be important for human consumption.

In the genus Coffea, the various diploid species which manifest large differences in morphology and other characters can cross to produce moderately or fairly fertile hybrids11,12,13. Some diploid species can also cross with C. arabica and give rise to triploid hybrids which are moderately fertile and can be used in back crosses to C. arabica to transfer the genes of interest from diploid species to the tetraploid Arabica14. This is because the differentiation of chromosomes among the species of Coffea is not so complete as to reproductively isolate them15. Thus, different species of Coffea carry weakly differentiated chromosomes of the basic set. Thus, chromosomes of different species of Coffea carry large homologous segments which are also homosequential16. This means that the order of genes on these homologous segments of chromosomes is the same among the apparently different species. This situation auger well for the transfer of genes between different species of Coffea and is very useful in the improvement of the commercially very important C. arabica17.

In the mid and late 20th Century, research efforts were devoted to understand the genetic relationships among the apparently different species of Coffea through interspecific hybridization. These studies led to the classification of the different species of Coffea into three sections in the context of Arabica coffee breeding18. Thus, the various varieties of Arabica that cross well within themselves and the tetraploid Arabicoid interspecific hybrids used in Arabica breeding19 were placed in the primary gene pool. So far, this is the main source of genes for all the coffee breeding programmes. The diploid species of Coffea that cross well with C. arabica to produce moderately fertile triploid hybrids were placed in the secondary gene pool. These triploid hybrids can be back crossed with Arabica to evolve synthetic types carrying the characters of Arabica and the diploid species that can be stabilized as varietal characters through appropriate selection and breeding techniques. Coffea canephora and C. liberica and several other species like C. racemosa, C. eugenioides and C. excelsa are in this section. The third section includes C. stenophylla, C. salvatrix and some others that do not cross with C. arabica. However, these species can cross with some species of the secondary gene pool to produce fertile or moderately fertile hybrids which can be used in Arabica breeding. Application of cytogenetic techniques and skills is needed to handle the transfer of genes from diploid species to tetraploid Arabica.

In India, efforts to understand genetic relationships of Coffea species began in 1950s and 1960s when initial crossing of various diploid species was conducted. One of the crosses is between the diploid species C. liberica and C. eugenioides with C. liberica as the mother parent11,15. One of the plants in this line of hybrids has given rise to a sucker which manifested Arabica-like characters12,20. This was found to be tetraploid with 44 chromosomes in its cells20. This sucker was utilized to develop four plants by vegetative propagation. The four clones were fertile and produced seed upon self pollination. Further propagation of these plants was by seed. This was named as Ligenioides21, combining the specific epithets of the two parent species. This is an event of natural doubling of chromosomes in a hybrid of C. liberica and C. eugenioides, the two diploid species. Resemblance of these plants to C. arabica, their self-compatibility and beverage quality of their seed prompted the breeders to propose that C. arabica might have arisen by a hybridization of the species C. liberica and C. eugenioides in the evolutionary past26,27.

This material was given for cultivation as Selection-11 in the late 1970s in India. This selection possesses strong resistance against leaf rust and white stem borer. However, bean size of this selection is small and this has not become popular with growers on account of this. Even so, beverage quality of Ligenioides is found to be good with FAQ and above score24. To improve bean size, Ligenioides was crossed with Hibrido de Timor which was also manifesting high resistance to coffee leaf rust. In the first generation and backcross progenies, an improvement of bean size was recorded. Rust resistance in these hybrids also remained high. Yield and quality of these hybrids are also impressive over several years of observation. A comprehensive study of these materials indicated that Ligenioides could be a source of new genes for breeding Arabica varieties resistant to rust22. It is also possible to incorporate resistance to white stem borer through Ligenioides23.


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