Taxonomy, Distribution and Host Range of Cuscuta Species Across Kenyan Ecosystems
Masanga, Joel Okoyo
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A largely unreported and ever-expanding infestation of African ecosystems by dodders (Cuscuta spp.) portends for severe threats to food security and economic livelihoods on the African continent. Dodders are noxious parasitic weeds that colonize numerous host plants, and entirely depend on them for support and nourishment. This parasitism is detrimental to host plants, since they are deprived of water, nutrients, and photosynthates. Over 200 dodder species have been identified worldwide, some natively endemic, while others are invasive in new localities. The successful spread of dodder is attributed to its ability to parasitize an array of angiosperms under different environmental conditions. In Africa, little is known or reported regarding the diversity of dodder species, their host range and whether these parasites can affect essential crops. Moreover, distribution patterns of these weeds under current and future environmental conditions are unknown. Therefore, this study hypothesized that different dodder species have invaded Kenyan ecosystems and that their geographical distribution and success in colonizing plants is influenced by a combination of climatic and environmental factors, coupled with high genetic variability and distinct population structures. Using a combination of taxonomic and genomics tools, dodder accessions collected across Kenya were characterized. Further, integrated phylogeographic and population genetics analyses, targeting genes from the chloroplast and nuclear genomes, were used to determine genetic variability and population structure of the dodder accessions to reveal factors that have shaped their successful colonization of current localities. These analyses revealed presence of 3 dodder species, namely Cuscuta kilimanjari, Cuscuta campestris, and Cuscuta reflexa, in Kenya. In addition, population genetics profiling for C. reflexa and C. campestris revealed that these species had high genetic variation, evidenced by high numbers of haplotypes, gene diversities and heterozygosity (0.2-0.66 and 0-0.6 in C. campestris and C. reflexa, respectively). Moreover, these species had distinct lineages, corresponding to Kenya's eastern and western regions, with evidence of isolation by distance. A comprehensive list of host plants, from sampling efforts and literature review, was compiled to reveal dodder's host range. Additionally, species distribution modelling (SDM), based on the Maximum Entropy (MaxEnt) algorithm, was used to determine dodder's distribution patterns targeting climatic and vegetation variables. Results revealed highly suitable dodder habitats across Eastern Africa. Crucially, some of the localities identified were important locations for high-value commercial agriculture. Histological analysis of the host-parasite interface was performed to determine the ability of C. reflexa to parasitize three economically-important crops, namely mango (Mangifera indica) coffee (Coffea arabica), and tea (Camellia sinensis). Experimental results proved that C. reflexa could parasitize all three crops. Interestingly, mango exhibited post-attachment resistance response to dodder, which occurred through secretion of a sap-like substance that arrested parasitism. These results are applicable in policy development among governments and related agencies and will guide targeted approaches aimed at eradicating current dodder populations and prevent new introductions.