Coffea arabica L - A Compilospecies: Implications for Breeding

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1 Index Table of contents Coffea arabica L - A Compilospecies: Implications for Breeding A. SANTARAM Central Coffee Research Institute, Coffee Research Station , Chikmagalur District, Karnataka, India SUMMARY There are two schools of thought regarding the origin and evolution of Coffea arabica L. the world s most important non-alcoholic stimulant beverage. The first or conventional school of thought based their inference on the evidence raised from observations on morphological, cytological, biochemical and reproductive biological features. The school proposes that C. arabica evolved by spontaneous hybridization of C. eugenioides with C. canephora, C. congensis or C. liberica and doubling of the chromosomes in the natural hybrid. An interesting offshoot of this thought is the belief that C. arabica is a segmental allotetraploid. The second or technological school of thought hypothesizes that natural hybridization of C. eugenioides or a sub-species by an unreduced gamete of C. canephora and spontaneous stabilization of chromosome number in the progeny of triploid hybrid resulted in the evolution of C. arabica on the basis of evidence from molecular marker studies. Evidence from some of the marker studies also suggests that C. arabica may be sharing considerable genomic homologies with C. racemosa and C. congensis. Molecular cytogenetic evidence also supports that C. eugenioides and C. congensis are the probable evolutionary parents of C. arabica. The distribution of C. arabica outside the area of distribution of all diploid species was attributed to the events of Pleistocene. Both schools agree that C. eugenioides or a subspecies of it is the most probable female progenitor of C. arabica. While they differ in the matter of male progenitor, in that, a species of the canephoroid group or liberio-excelsoides group is considered to be the likely male progenitor of C. arabica. Considering the genetical evidence from plant breeding studies, the disease resistance genes of C. liberica and C. canephora are inherited by C. arabica. All these available evidence points to the possible compilospecies nature of C. arabica. This has strong implications for the breeding practices, as inheritance patterns in compilospecies are considerably different from those observed in diploids and allopolyploids. These aspects are discussed and a possible breeding model with integration of vegetative selection to maintain traits of interest in the commercially exploited arabicoid derived materials is suggested. Utilization of unique tetraploid hybrids of interspecific origin like Ligenioides, Racemusta and Robarbica are proposed for use in breeding for resistance to coffee berry disease, insects and nematodes on the basis of their cytoplasmic genetic endowments. INTRODUCTION Coffee is perhaps the single Third World commodity that has attained distinction in world trade of being the second largest traded commodity. While this is a position of pride, coffee trade in the recent times has suffered serious setbacks due to global surpluses (Van der Vossen, 2000), causing consternation in the small grower sector all over the world, as the cost of production could not be realized from the proceeds of sale. In this situation, reducing the cost of cultivation is most important. Present paper briefly reflects on the Arabica coffee breeding programmes and discusses the possibility of evolving materials that are likely to be less demanding by way of crop husbandry, on the basis of available evidence pointing to the possible compilospecies nature of C. arabica. 740

2 What is a Compilospecies? Harlan and De Wet (1963) defined compilospecies as a species that is capable of annexing the genetic endowments of related species, sometimes to the extent of driving the species being annexed out of existence. This definition implies that compilospecies are aggressively adaptable. The cultivation history of C. arabica is a pointer indicating the wide adaptation of this species. Thus, even though C. arabica is a native of the Southwestern highlands of Ethiopia, it survived the hostile climate of Yemen where the Arabs who controlled the world coffee trade for over three centruries took it. Arabica not only survived but flourished in the various colonies of almost all European powers spread across the globe from Far East, South East Asia and Latin America and the Oceania besides the home continent of Africa contributing to the present day crisis caused by over production. Origin of Coffea arabica Various early studies indicated the possible involvement of C. eugenioides and C. canephora, C. congensis, C. stenophylla, C. dewevrei or C. liberica as the parent species in the origin of C. arabica (Carvalho et al., 1969; Narasimhaswamy, 1962; Narasimhaswamy and Vishveshwara, 1967; Ram and Sreenivasan, 1981; Reddy et al., 1984; Charrier and Berthaud, 1985; Eskes, 1989). Molecular marker studies confirmed that C. eugenioides or a closely related sub-species is the most likely female progenitor of C. arabica on the basis of organellar DNA analysis (Berthou et al., 1983; Lashermes et al., 1996b) and C. canephora or a sub-species is strongly suggested to be the probable male contributor on the basis of DNA markers (Lashermes et al., 1995, 1996a, 1997, 1999; Orozco-Castillo et al., 1996). C. eugenioides and C. congensis were considered to have contributed genomes to C. arabica on the basis of genomic in situ hybridization analysis (Raina et al., 1998). RAPD and microsatellite marker studies also indicated that many other species of Coffea possess DNA that is homologous/ homeologous that of C. arabica (Combes et al., 2000; Ram and Sreenath, 2000a, b). Ligenioides, an amphiploid derived from an F 1 hybrid of C. liberica and C. eugenioides by natural doubling of chromosomes is observed to have large genetic similarity with Hibrido de Timor (Ram et al., 2000, 2002). Considering the diverse origin of these two interspecific hybrids it was inferred that C. liberica and C. eugenioides also carry substantial genomic homology with C. canephora and C. arabica the parents of HDT. Arabica was shown to have inherited the rust resistance genes of C. liberica (S H 3) and C. canephora (S H 6,7,8,9) (Rodrigues et al., 1975; Eskes, 1989; Bettencourt et al., 1992) and support this inference. The foregoing discourse indicates that C. arabica has the capacity to assimilate genes from many species of Coffea diploid gene pool and thus can be classified as a compilospecies that has emerged in a contemporary climatic situation from a species complex with low adaptive and survival value in the location of its origin, the Southwestern high lands of Ethiopia. A recent origin in the quaternary period was suggested for C. arabica on the basis of ribosomal DNA (rdna) sequence data (Lashermes et al., 1995, 1997) and its distribution outside the area of distribution of diploid species on the basis of Pleistocene events (Lashermes et al., 1999). Free intercrossing of different species of Coffea to form reasonably fertile hybrids was reported (Narasimhaswamy and Vishveshwara, 1967; Carvalho and Monaco, 1967; Charrier, 1978; Louarn, 1993). Cases of natural doubling of chromosomes in a hybrid of C. liberica x C. eugenioides and spontaneous occurrence of tetraploids in the descendants of the 741

3 interspecific hybrids of C. liberica x C. eugenioides, C. excelsa x C. eugenioides and C. racemosa x C. canephora were also reported (Narasimhaswamy, 1962; Narasimhaswamy and Vishveshwara, 1967; Reddy et al., 1991). These phenomena are important in theorizing the origin of C. arabica as many of these tetraploids resemble C. arabica and some intercross with C. arabica giving rise to moderately fertile hybrids (Ram et al., 2002). These events point to the possible participation of more than two species in the evolution of C. arabica either at the time of its birth or subsequently by the inheritance of their characters through interspecific hybridization. Genetic conversion reported in the interspecific hybrids of C. liberica dewevrei x C. pseudozanguebariae (Ky et al., 2000) is a significant contribution towards understanding the evolution of C. arabica. Given the present knowledge that C. arabica is of recent origin by interspecific hybridization and allopolyploidy in the vegetation flux of Pleistocene, the following hypothesis appears to be appropriate. Prior to Pleistocene, all species of Coffea were probably diploid and occupied a focus area in what is present equatorial East Africa. From this focus, the young genus would have started undergoing the various processes of organic evolution such as migration, adaptation and different modes of isolation (Stebbins, 1950) to give rise to the presently recognized species. Molecular evidence points to the possible differentiation of the main clusters of evolution in the genus Coffea before the appearance of C. arabica (Berthou et al., 1983). At this point of time in evolution, the violent events of Pleistocene would have been experienced by these evolving species, some of which probably came to exist together in environments that are not very conducive for their survival (such as Southeastern Sudan and Southwestern Ethiopia). Thus, a new trend of evolution through interspecific hybridization would have got initiated and followed by polyploidy to give rise to diverse tetraploids whose interbreeding in the new habitat would have resulted in the formation of C. arabica. The great variability of Ethiopian Arabicas recorded by the earlier investigators (Monaco, 1965; Sylvain, 1955) and confirmed by the modern molecular studies (Lashermes et al., 1995, 1996c) is consistent with the possibility of C. arabica being a compilospecies. Implications for Breeding C. arabica s being a compilospecies has important implications for the methodology of breeding this very important species that is sustaining the economies of many developing countries. Several genes of C. arabica were shown to follow a disomic inheritance as is typical of all diploid species (Krug and Carvalho, 1951; Carvalho et al., 1991). This behaviour probably holds good to the extent of selection and breeding utilizing the many biotypes of C. arabica without crossing the boundaries of this species. However, this could not be so on account of the relatively narrow genetic base of C. arabica and possible limited variability concerning the traits of interest such as pest and disease resistance. Prospecting for resistant types revealed the presence and variability of these traits in the various species of diploid constellation and the spontaneous tetraploid interspecific hybrids prompted the temptation to transfer these characters from diploid species to C. arabica. Recently, tetrasomic inheritance of characters, using molecular markers, was reported in Arabusta hybrids (Lashermes et al., 2000). This is positive evidence that C. arabica would not behave like a diploid when interspecific hybrids are involved in breeding. The foregoing discussion indicates that simple methods of plant breeding are not adequate for breeding C. arabica as it manifests diverse behaviour in different genetic contexts. The following model is suggested to take care of the compilospecies behaviour. 742

4 Breeding Strategies At present, Arabicoid germplasm carrying diverse traits of interest from the species of the secondary and tertiary gene pools are available in various countries where research efforts are devoted towards the development of interspecific hybrids. It is important that these be deployed in breeding programmes to derive types suitable for niche environments. Indian experience with three Robusta-Arabica hybrids is that when pure Arabicas are to be improved by involving them it is best to exploit the F 1 hybrids or the immediate next generation derived from selected F 1 individuals to counteract the effects of modifier and epistatic genes and loss of target genes through genetic conversions. This was advocated by other breeders also (Charrier, 1982; Van der Vossen, 1985). Besides the above, in India, there are novel tetraploid hybrids that can be of potential application in breeding Arabicas resistant to diseases and pests (Ram et al., 2004, in preparation). Thus, Ligenioides was found to cross freely with a variety of Arabicas to give rise to fertile hybrids. Ligenioides manifested considerable resistance to CLR and white stem borer (Ram et al., 2002; Ram, 2003). F 1 hybrids of Ligenioides x HDT manifest high resistance to CLR, improved bean size and cup quality (Ram et al., 2004, in preparation). In F 2, segregation towards susceptibility was recorded. Thus, exploiting F 1 hybrid by vegetative selection is envisaged for this material. Another interesting material is a hybrid derived from the cross of C. racemosa and C. canephora. F 1 hybrids of these species are diploid (2n = 22) and highly sterile (Ram et al., 1981). However, a few open pollination derived seed gave rise to Arabica-like tetraploid plants (Reddy et al., 1991), which are also self-compatible. It is observed that this hybrid also crosses freely with Arabicas and Arabicoids to yield hybrids. These are being studied and are a potential source of resistance to CLR, nematodes, leaf miner and possibly abiotic stress. Several tetraploids were identified in the open pollination derived progenies of C. excelsa x C. eugenioides. These plants are a potential source of resistance against a variety of adversaries. These are now being studied for their possible utility in Arabica breeding. Another interesting possibility of the use of these novel materials is in the context of breeding for resistance against coffee berry disease (CBD) that is presently endemic in the African countries. Most of the Arabica coffee grown in these countries is of the varieties derived from Kents and Bourbon. These varieties are pure Arabicas without any introgression of genes from diploid species. Considering the possible maternal ancestry of C. eugenioides in the evolution of C. arabica, all pure Arabicas are expected to carry a large cytoplasmic contribution from this species. Thus, the very first appearance of CBD on C. eugenioides and its subsequent spread to C. arabica (Mogk, 1975; Van der Vossen, 1985) reflect a case of cytoplasmic uniformity rendering the coffee plant varieties vulnerable to CBD pathogen very similar to that of T-cytoplasm and Southern blight of Corn (Levins III, 1990). Novel Indian coffee hybrids carry the cytoplasmic components of C. liberica (in Ligenioides), C. racemosa (in Racemusta) and C. canephora (in Robarbica) and offer a new alternative in breeding for CBD resistance. From the foregoing discourse, it is evident that tetraploids can be obtained from the hybridization of various diploid species of Erythrocoffea (Canephoroid group) and Pachycoffea (Liberio-Excelsoid group) with several Mozambicoffea (C. eugenioides and related species). These tetraploids offer considerable natural protection against adversaries like diseases (CLR and CBD), pests (nematodes and insects) and abiotic stress by integrating them in the Arabica breeding programmes. Inherent resistance of the materials emanating from such breeding programmes renders Arabica cultivation relatively less demanding and more attractive. 743

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