The evolutionary and genetic aspects of the process of domestication have been studied intensively in temperate cereal crops and pulses and are fairly well understood for those crops . In contrast, we have a much less comprehensive knowledge of domestication in perennial crops such as tree fruits and nuts , even though these species make up an important component of human nutrition throughout the world . It has been suggested that the study of tree crops may reveal fundamental differences in the process of domestication and its genetic consequences . The fact that tree crops have long lifespans and that they are often asexually propagated limits the number of generations that have passed since their first cultivation by humans and the degree of evolution that has occurred under human selection in these taxa . Clement described five stages in the domestication process: 1) wild , 2) incidentally co-evolved , 3) incipiently domesticated , 4) semi-domesticated , and 5) domesticated . In species that are still in the early stages of domestication , cultivated and wild-type individuals will coexist in close proximity in the region of origin, where we can study how traits have responded to anthropogenic selection in the face of continued gene flow between wild and improved types. Some of our best-known examples of plants in the early stages of domestication come from Mesoamerica, where native crop species are frequently cultivated in traditional agroecosystems such as home gardens . We expect fruit and seed characteristics of tree crops to change in predictable ways under anthropogenic selection during the domestication process, 25 liter pot leading to a type of domestication syndrome for edible fruits .
Compared to their wild relatives, edible fruits of domesticated taxa tend to be larger and sweeter or higher in oil content . The ratio of edible product to waste, or “economic ratio” , goes up. We also expect to see a reduction in toxic compounds that confer defense against natural enemies at the cost of palatability . In addition to changing mean trait values, anthropogenic selection could have contradictory effects on the variance of traits. Due to selection bottlenecks, we expect neutral variation to decline with cultivation in domesticated species . For particular genes under strong directional selection, the reduction in genetic variation may be even more dramatic . However, if distinct cultivated varieties are selected for contrasting phenotypes, then phenotypic variance will be higher in cultivated plants . Some even have used changes in the variance and shape of the distribution of traits as an indicator of the degree of domestication shown by populations of indigenous fruit trees . Chrysophyllum cainito L. , commonly known as caimito or star apple, is highly desired throughout the tropics for its value as anornamental tree and its production of large, edible fruits. Caimito is presently cultivated throughout the Caribbean, Central America, and parts of South America as well as in Southeast Asia . Various hypotheses have been suggested for its geographic and taxonomic origin, as well as its center of domestication . Historical accounts made by plant explorers in the 16th and 17th centuries mention C. cainito as occurring in the islands of the Lesser and Greater Antilles as well as on the mainland and islands of the Bay of Panama . Patrick Brown mentions C. cainito as being cultivated all over the island of Jamaica and that it “thrives with very little care” . Seemann reports the presence of both wild and cultivated caimito in the Isthmus of Panama. Our preliminary molecular genetic results show much greater overall diversity of the species in Panama compared to northern Central America, Mexico, and the Caribbean, and support the hypothesis that C. cainito was domesticated in Panama.
In contemporary Panama, C. cainito is widely distributed as a natural part of tropical lowland forest. In fact, this species is promoted as a native tree for environmentally sensitive reforestation efforts, for example, by the Proyecto de Reforestación con Especies Nativas . The forest plot network that bisects the Panamanian Isthmus documents the presence of C. cainito at many sites on the Pacific side of central Panama . In the wild, C. cainito is found at low density, does not produce fruit until the tree reaches the canopy of the forest, maintains few branches below 10 m, and produces ripe fruits over only a short time interval; taken together, these aspects complicate the study of this species in wild populations. Cultivated caimito is a common component of Panamanian home gardens and small ranchos. Usually only one or two individuals are planted per property, as fruits are not sold on a commercial scale. While an owner will occasionally report that a tree “arrived on its own,” presumably dispersed by a bird or mammal, usually trees in gardens have been planted from seeds selected from a fruit, usually a fruit provided by a friend or neighbor . Sometimes plants have been purchased as seedlings from a nursery. During the Canal Zone era , C. cainito was planted horticulturally as a street tree in some communities within and surrounding Panama City. Thus the cultivation of C. cainito trees in Panama has included a combination of individual selection via fruits and seeds, with some relatively small-scale central production and distribution of plants. In addition, human migration between the Antilles and Panama may have resulted in the importation of caimito seeds from the Antilles. Our goal was to test the hypothesis that cultivated individuals of C. cainito show signs of a domestication syndrome: Distinctive fruit and seed characteristics that are typically associated with domestication relative to wild phenotypes growing in the same region. As a putative region of domestication for this species and an area with extensive populations in protected forests, Panama is the ideal place for such a study. We compared fresh collections from wild and cultivated trees in central Panama for a range of morphological and fruit quality traits to infer how human selection has influenced these traits. We then used Discriminant Analysis to investigate the relative importance of various traits for distinguishing cultivated fruits from wild fruits. In this paper, we also present information on the variance in traits, correlations among traits, and variation in trait values from year to year.We chose to focus our collections in central Panama, where caimito is cultivated in both rural and urban communities set within the context of wild populations in protected forests. Central Panama includes extensive areas of semi-deciduous, tropical forest, primarily associated with the Panama Canal watershed. There is a natural rainfall gradient of 1,750–3,000 mm per year from the Pacific to the Caribbean side of the isthmus, with a pronounced dry season .
Wild individuals of Chrysophyllum cainito are distributed at low density throughout the forestsof the Canal Zone, particularly on the Pacific side of the continental divide, where they are found in old-growth as well as second-growth forests . For the purpose of our sample, we defined trees as wild if they were in natural forest habitat and if, to the best of our knowledge, people did not plant them. We used surveys and the knowledge of local indigenous people to locate over 150 adult wild trees, but only a percentage of these produced fruit in any given year. We included in our study every wild tree from which we could obtain enough fruits, including trees from Parque Nacional Soberanía, Old Gamboa Road, Venta de Cruces, Parque Nacional Camino de Cruces, and Parque Natural Metropolitano . Sample sizes differ among variables and years and are provided below; our sample sizes increased from 2006 to 2008 as we found new individuals and added new sites. We defined trees as cultivated if they were obviously planted either alongside rural dwellings or as horticultural plantings in more urbanized areas. Whenever possible, we interviewed owners to obtain direct information about who planted the tree; we did not use trees whose origin was ambiguous. Chrysophyllum cainito is primarily propagated by seed, apparently exclusively so in Panama. We roughly matched the sample sizes and locations of our cultivated sample to the wild tree sample, 25 liter plant pot including trees from the communities of Gamboa, Chilibre, Chilibrillo, Paraíso, Ciudad del Saber, Los Rios, Burunga, and Balboa . From among the available trees, we chose our cultivated sample based primarily on access, as it was not always possible to get permission from owners to collect fruits.We collected five to thirty fruits per tree, and of these, five or three were randomly selected for analysis. We completed measurements on fruits almost always within several hours of collection, and always within three days. For each fruit, we took a photograph, noted color, measured fresh weight, length and diameter, and toughness using a penetrometer . We then cut the fruit open and measured the thickness of the exocarp, or rind . We extracted fluid from the fruit pulp by passing it through a fine filter and assessed sugar content with a hand-held refractometer . We counted the number of seeds and measured mean seed mass . One wild tree produced no healthy mature seeds, decreasing the sample size for seed mass to 28. Fruit shape is expressed as diameter/length, and proportion allocation to exocarp as /diameter.To compare wild and cultivated classes, we used nested ANOVA with trees nested within class , and samples nested within tree. Nested ANOVA also allowed us to test for significant variation among parent trees within the classes. We used Levene’s test to test for a difference between cultivated and wild trees in the variance of each trait. For any trait that showed heterogeneity of variance, P-values from the ANOVAwere suspect. Therefore, for the subset of traits that tested significant for Levene’s test, we performed Welch’s t-test assuming unequal variances on the comparison between wild and cultivated fruits—but this test could not be used to test for significant variance among trees within classes. Chemistry data from 2006 were analyzed separately from the 2008 data. While the geographical logistics of this system did not allow for the sampling of truly independent but comparable regions, our sample in 2008 was large enough to support separating the data into Northern and Southern sub-regions. When the analyses were done separately on these sub-regions, we obtained essentially the same patterns of difference between wild and cultivated classes for the various traits. Therefore, only the combined data are presented here. Pairwise correlation coefficients were calculated among traits in the 2008 sample and among chemical traits in the 2006 sample . Correlations between fruit and leaf phenolics could not be estimated because they came from different sets of trees. We used Discriminant Analysis to test whether the classes of wild and cultivated trees could be distinguished from each other, and what traits contribute to this discrimination. From the 2008 dataset, we included the following variables: Fruit mass, fruit diameter/length, exocarp proportion, toughness, seed number, seed mass, and sugar concentration . We left out the most redundant variables: Fruit length, diameter, and absolute exocarp width. We used forward stepwise variable selection to choose variables that significantly contributed to the model , then ran the model. For all trees in the sample that produced fruits both in 2007 and in 2008 , we calculated correlation coefficients between the mean trait value in 2007 and the mean trait value in 2008.We found many traits that differ significantly between wild and cultivated fruits of C. cainito. Fruits from cultivated genotypes were larger and had more pulp and less exocarp, resulting in a higher “economic ratio” sensu Clement . Fruit pulp had significantly higher concentrations of sugar, was less acidic, and had lower concentrations of phenolics. Seeds were bigger and more numerous and were less defended with phenolics in cultivated than in wild fruits. Historical accounts mention caimito already being cultivated in Panama in the mid-1500 s . Although these data are scant, we can infer that caimito has been utilized as a fruit crop for at least 450 years and possibly much longer. Caimito trees may come into bearing within about 15 years, allowing us to estimate that a minimum of 30 generations or possible rounds of human-mediated selection have occurred to date. Interestingly, early historical accounts mention the presence of large and small fruited types , suggesting that substantial selection may have already occurred by that time. The sugar concentration in C. cainito is extremely high. In 2008 we measured a mean °Brix of 14.9, with a maximum value of 28.