Abstract
Endemic flora of the Galapagos Islands has adapted to thrive in harsh environmental conditions. The wild tomato species from the Galapagos Islands, Solanum cheesmaniae and S. galapagense, are tolerant to various stresses, and can be crossed with cultivated tomato. However, information about genetic diversity and relationships within and between populations is necessary to use these resources efficiently in plant breeding. In this study, we analyzed 3,974 polymorphic SNP markers, obtained through the genotyping-by-sequencing technique, DArTseq, to elucidate the genetic diversity and population structure of 67 accessions of Galapagos tomatoes (compared to two S. lycopersicum varieties and one S. pimpinellifolium accession). Two clustering methods, Principal Component Analysis and STRUCTURE, showed clear distinction between the two species and a subdivision in the S. cheesmaniae group corresponding to geographical origin and age of the islands. High genetic variation among the accessions within each species was suggested by the AMOVA. High diversity in the S. cheesmaniae group and its correlation with the islands of origin were also suggested. This indicates a possible influence of the movement of the islands, from west to east, on the gene flow. Additionally, the absence of S. galapagense populations in the eastern islands points to the species divergence occurring after the eastern islands became isolated. Based on these results, it can be concluded that the population structure of the Galapagos tomatoes collection partially explains the evolutionary history of both species, knowledge that facilitates exploitation of their genetic potential for the identification of novel alleles contributing to stress tolerance.
Highlights
Biodiversity in the Galapagos Islands has inspired theories of adaptation and evolution, and increased our understanding of processes of population divergence and speciation (Darwin, 1859)
The one S. pimpinellifolium accession was added to compare the Galapagos tomatoes to another wild tomato, while two S. lycopersicum varieties were added as references: Heinz 1706 is the variety for which the reference genome sequence was completed
When the data were separated for the individual species and the same parameters were used for single nucleotide polymorphism (SNP) filtering, 2,820 SNPs were obtained for S. cheesmaniae, 1,448 SNPs for S. galapagense, and 3,905 SNPs for S. lycopersicum
Summary
Biodiversity in the Galapagos Islands has inspired theories of adaptation and evolution, and increased our understanding of processes of population divergence and speciation (Darwin, 1859). The islands were formed at a volcanic hotspot in the Nazca Plate, which is moving east at approximately 59 km per million. Galapagos Tomatoes Diversity and Population Structure years (Geist et al, 2014). The age of the islands can be estimated by their current distance from the hotspot: the western islands are millions of years younger than the eastern ones (Geist et al, 2014). The eastward movement of the Nazca Plate causes its subduction beneath the South American plate, isolating previously interconnected islands and causing their eventual disappearance from east to west (Christie et al, 1992). The isolation of the islands and constantly changing environmental conditions have allowed the adaptation and divergence of many species, differing morphologically and genetically from one island to the other (Romagosa et al, 2013)
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