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PremiseParasitic plants and their hosts are model systems for studying genetic variation in species interactions across environments. The parasitic plant Striga hermonthica (witchweed) attacks a range of cereal crop hosts in Africa and exhibits substantial variation in performance on different host species. Some of this variation is due to local adaptation, but the genetic basis of specialization on certain hosts is unknown.MethodsTo identify genomic regions that are strongly differentiated between parasites attacking different host species, we present an alignment-free analysis of S. hermonthica population diversity using whole genome sequencing (WGS) data for 68 individuals from western Kenya. We validate our findings with germination experiments and analyses based on a de novo assembled draft genome.ResultsReference-free and reference-based analyses suggest that only a small portion of the S. hermonthica genome is strongly differentiated by host species in populations from western Kenya. Analysis of host-associated k-mers implicated genes involved in development of the parasite haustorium (a specialized structure used to establish vascular connections with host roots) and a potential role of chemocyanins in molecular host-parasitic plant interactions. Conversely, no phenotypic or genomic evidence was observed suggesting host-specific selection on parasite response to strigolactones, hormones exuded by host roots and required for parasite germination.ConclusionsThis study demonstrates the utility of WGS for plant species with large, complex genomes and no available reference. Contrasting with theory emphasizing the role of early recognition loci for host specificity, our findings support host-specific selection on later interaction stages, recurring each generation after homogenizing gene flow.

Pumpkins (Cucurbita spp.) are among most neglected and underutilized crops cultivated for food and medicine. The major constraint to pumpkin production is lack of genetically improved seeds. The current study was aimed at evaluating the genetic diversity of pumpkins from eight counties in western Kenya using five SSR markers. Seeds were extracted from pumpkin fruits, dried and planted on plastic trays for 4 weeks. DNA was isolated from young leaves using CTAB method and amplified. The samples were genotyped using an ABI 3730 genetic analyzer and the allelic data analyzed using Power Marker V 3.25, DARwin V 6.0.12 and GenAIEx V 6.41software. The five SSR loci were polymorphic with a total of 33 alleles and a mean PIC value of 0.534. The gene diversity and observed heterozygosity was 0.796-0.329 and 0.967-0.164, respectively. Most of genetic variations were found within and among individual samples rather than among counties, with samples of some counties having private alleles. Based on the inbreeding coefficient (F), there was outbreeding in pumpkins from Kakamega county (F = - 0.282) and inbreeding in pumpkins from Kisii, Bungoma and Nyamira counties (F = 0.500, 0.409 and 0.286 respectively). The findings of this study suggest that genetic variation and distribution of pumpkins in western Kenya was due to monocropping and intercropping farming systems, trading of pumpkins in markets and exchange of seeds among local farmers rather than geographical and climatic differences.