Genetic diversity and structure of wild populations of Carica papaya in Northern Mesoamerica inferred by nuclear microsatellites and chloroplast markers

Background and aims Few studies have evaluated the genetic structure and evolutionary history of wild varieties of important crop species. The wild papaya (Carica papaya) is a key element of early successional tropical and sub-tropical forests in Mexico, and constitutes the genetic reservoir for evolutionary potential of the species. In this study we aimed to determine how diverse and structured is the genetic variability of wild populations of C. papaya in Northern Mesoamerica. Moreover, we assessed if genetic structure and evolutionary history coincide with hypothetized (1) pre-Pleistocene events (Isthmus of Tehuantepec sinking), (2) Pleistocene refugia or (3) recent patterns.
Methods We used six nuclear and two chloroplast (cp) DNA markers to assess the genetic diversity and phylogeographical structure of 19 wild populations of C. papaya in its natural distribution in Northern Mesoamerica.
Key Results We found high genetic diversity (H_o = 0·681 for nuclear markers, and h = 0·701 for cpDNA markers) and gene flow between populations of C. papaya (migration r up to 420 km). A lack of phylogeographical structure was found with the cpDNA markers (NST < GST), whereas a recent population structure was inferred with the nuclear markers. Evidence indicates that pre-Pleistocene events or refugia did not play an important role in the genetic structuring of wild papaya.
Conclusions Because of its life history characteristics and lack of an ancient phylogeographical structure found with the cpDNA markers, we suggest that C. papaya was dispersed throughout the lowland rain forests of Mexico (along the coastal plains and foothills of Sierras). This scenario supports the hypothesis that tropical forests in Northern Mesoamerica did not experience important climate fluctuations during the Pleistocene, and that the life history of C. papaya could have promoted long-distance dispersal and rapid colonization of lowland rainforests. Moreover, the results obtained with the nuclear markers suggest recent human disturbances. The fragmentation of tropical habitats in Northern Mesoamerica appears to be the main driver of genetic structuring, and the major threat to the dispersion and survival of the species in the wild.