Abstract
To sustain epidemiological studies on coconut lethal yellowing disease (CLYD), a devastating disease in Africa caused by a phytoplasma, we developed a multilocus sequence typing (MLST) scheme for "Candidatus Phytoplasma palmicola" based on eight housekeeping genes. At the continental level, eight different sequence types were identified among 132 "Candidatus Phytoplasma palmicola"-infected coconuts collected in Ghana, Nigeria, and Mozambique, where CLYD epidemics are still very active. "Candidatus Phytoplasma palmicola" appeared to be a bacterium that is subject to strong bottlenecks, reducing the fixation of positively selected beneficial mutations into the bacterial population. This phenomenon, as well as a limited plant host range, might explain the observed country-specific distribution of the eight haplotypes. As an alternative means to increase fitness, bacteria can also undergo genetic exchange; however, no evidence for such recombination events was found for "Candidatus Phytoplasma palmicola." The implications for CLYD epidemiology and prophylactic control are discussed. The usefulness of seven housekeeping genes to investigate the genetic diversity in the genus "Candidatus Phytoplasma" is underlined.IMPORTANCE Coconut is an important crop for both industry and small stakeholders in many intertropical countries. Phytoplasma-associated lethal yellowing-like diseases have become one of the major pests that limit coconut cultivation as they have emerged in different parts of the world. We developed a multilocus sequence typing scheme (MLST) for tracking epidemics of "Ca Phytoplasma palmicola," which is responsible for coconut lethal yellowing disease (CLYD) on the African continent. MLST analysis applied to diseased coconut samples collected in western and eastern African countries also showed the existence of three distinct populations of "Ca Phytoplasma palmicola" with low intrapopulation diversity. The reasons for the observed strong geographic patterns remain to be established but could result from the lethality of CLYD and the dominance of short-distance insect-mediated transmission.
Highlights
To sustain epidemiological studies on coconut lethal yellowing disease (CLYD), a devastating disease in Africa caused by a phytoplasma, we developed a multilocus sequence typing (MLST) scheme for “Candidatus Phytoplasma palmicola” based on eight housekeeping genes
DnaC, gyrB, leuS, lpd, secA, recA, rsmI, and rplV, were selected to investigate the genetic diversity among 132 samples of “Ca. Phytoplasma palmicola” originating from three of the main African countries affected by this phytoplasma, i.e., Ghana, Nigeria, and Mozambique
The PCR products observed on agarose gels each showed a unique and clear DNA band from 553 to 983 bp depending on the target gene (Table 1)
Summary
To sustain epidemiological studies on coconut lethal yellowing disease (CLYD), a devastating disease in Africa caused by a phytoplasma, we developed a multilocus sequence typing (MLST) scheme for “Candidatus Phytoplasma palmicola” based on eight housekeeping genes. Molecular epidemiology requires easy-to-use tools that can sufficiently discriminate at a population level Both multilocus sequence typing (MLST) and multilocus variable number tandem repeat (VNTR) analysis (MLVA) have been developed to fulfill this objective and were applied to the surveillance of human, animal, and plant. Metagenomic approaches through NGS have opened new opportunities, and 17 additional phytoplasma genome draft sequences have been deposited in GenBank (NCBI) Such genome-wide sequence analyses have provided easy access to genetic markers, allowed unbiased definitions of species boundaries in the genus “Candidatus Phytoplasma,” and have revealed the horizontal transfer of potential mobile units and effectors [14, 15]
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