Relationships, origin, and diversity of Galápagos tomatoes: implications for the conservation of natural populations

Abstract

Endemic Galápagos tomatoes (Lycopersicon cheesmanii) are of great value for cultivated tomato (L. esculentum) breeding, and therefore their conservation is of significance. Although within L. cheesmanii there is heterogeneity for many traits and formal infraspecific classification is not justified, here we distinguish three forms, without taxonomic significance, of L. cheesmanii that are of interest to breeders because of their distinctive morphology and habitat preferences: L. cheesmanii 'short' (one- to two-pinnate leaves, short internodes, and coastal habitats), L. cheesmanii 'long' (one- to two-pinnate leaves, long internodes, and inland habitats), and L. cheesmanii forma minor (three- to four-pinnate leaves, short internodes, and coastal habitats). In a recent survey of tomato populations in the Galápagos Islands, we found that several populations of L. cheesmanii reported 30-50 years earlier had disappeared, mostly as a consequence of human activity. In addition, a previously unreported invasive wild red-fruited form, which we named L. esculentum 'Gal cer,' was found on the island of Santa Cruz. The total diversity (estimated with amplified fragment length polymorphisms [AFLPs]) within L. cheesmanii (H(T) = 0.051) is almost as high as that for the mainland wild species L. pimpinellifolium (H(T) = 0.072). Lycopersicon esculentum 'Gal cer,' on the other hand, has a much lower diversity (H(T) = 0.014). Comparison of AFLP fragments shared by L. esculentum 'Gal cer' with other species showed that it is closely related to weedy tomato L. esculentum var. cerasiforme and, therefore, likely of recent origin. Genetic differentiation among the three native L. cheesmanii forms is low (G(ST) = 0.235), indicating that they share a common genetic background. Nonetheless, L. cheesmanii 'short' is about twice as diverse as L. cheesmanii 'long' or L. cheesmanii f. minor. UPGMA cluster and principal components analysis distinguish four groups within Eulycopersicon: L. pimpinellifolium, cultivated L. esculentum, L. esculentum var. cerasiforme including L. esculentum 'Gal cer,' and L. cheesmanii. The geographic distance and genetic distance in the wild forms of Galápagos tomatoes were not correlated. Apart from the pressure of humans, some native L. cheesmanii populations, especially L. cheesmanii 'long,' might be displaced by invasive L. esculentum 'Gal cer' because they share a similar habitat. We did not find evidence of intercrossing of L. cheesmanii with introduced L. esculentum, but occasional hybridization that contributes to loss of genetic integrity of L. cheesmanii cannot be ruled out. Establishment of reserves of L. cheesmanii to protect this species from introduced herbivorous animals and from hybridization with L. esculentum 'Gal cer' would help to conserve L. cheesmanii. Furthermore, accessions collected by C. M. Rick and others in the 1950s-1970s and now stored in germplasm banks could be used to reinstate some extinct populations.

Use of microsatellites for evaluation of genetic diversity in cherry tomato

“…cerasiforme and S. pimpinellifolium, respectively), indicating little genetic differentiation among the accessions of the populations assessed, despite the high phenotypic differentiation thereof. These estimates were low compared with those reported by Nuez et al (2004) for 6 microsatellite markers in S. pimpinellifolium with Fst = 0.17; even above these values, the Fst of this research was found, indicating that the molecular markers used allow the detection of population structure and have a high power to differentiate groups and accessions of cherry tomato.…”

A New Family of Ty1-copia-Like Retrotransposons Originated in the Tomato Genome by a Recent Horizontal Transfer Event

“…The total genomic DNAs were digested with EcoRI. The phylogenetic tree on the left is from Nuez et al (2004). The direction of DNA migration on the gel is indicated by the dashed arrow.…”

Genetic Diversity in Tomato (Solanum lycopersicum) and Its Wild Relatives

“…The endemic species S. cheesmanii shows a reduction of its population due to human activity. Differentiation within S. cheesmanii was also observed (Nuez, Prohens et al 2004) as well as hybridization with the two introduced species S. lycopersicum and S. pimpinellifolium (Darwin, Knapp et al 2003). …”

“…Efforts on in situ conservation and participatory approaches as proposed by Jarvis, Brown et al (2008) and (Thomas, Dawson et al 2011) could be very useful to maintain the adaptive potential of tomato genetic resources. Nuez and colleagues proposed to use S. cheesmanii accessions now stored in germplasm banks to reinstate some extinct populations in Galapagos Islands (Nuez et al 2004). This could help avoiding the present paradox: the more knowledge we gain on tomato diversity and its evolutive history, the less available those genetic resources are available in the wild.…”