Database

A database with geographic locations indicating the presence of C. papaya L. in various sites in Mexico was developed in the Excel spreadsheet for Windows (version 10), according to ^{Scheldeman and van Zonneveld (2010)}. Individuals of wild and cultivated papaya were georeferenced in the states of Guerrero, Oaxaca, Veracruz, Tabasco and Chiapas. The information was collected with field trips in the period from 2013 to 2016, using the Garmin^® portable geographic location system, model GPSMAP 64s. The morphological identification of the wild and cultivated papaya was made based on the botanical descriptions of ^{Moreno (1980)}. Also, 20 georeflection data of papayas present in several states of Mexico were included (^{Hernández, 2013}).

Potential distribution model

The MaxEnt program version 3.3.3 (^{Phillips et al., 2017}) was used to develop the analyzes according to the type of papaya: wild and cultivated (races, varieties and hybrids). In total, 390 georeferenced sites with presence of wild populations (208) and cultivars (182) were used in the analyzes. The number of sites analyzed with presence of the species (390) was considered as a sufficient sample size, since ^{Baldwin (2009)} indicates that the gain in prediction of geographical distribution is acceptable with a sample size greater than 50 georeferenced sites of the species under study.

The 22 climatic variables were used as predictors (Table 1), of which 19 were spatial resolution bioclimatic of 0.5 arc minutes and were obtained from the WorldClim database (www.worldclim.org). The digital elevation data (DEM, with 90 m resolution) was obtained from the CGIAR-CSI (http://srtm.csi.cgiar.org). The layers in vector format of land use and vegetation were taken from the National Commission for the Knowledge and Use of Biodiversity (^{CONABIO, 1998}), and the soil moisture regime of ^{Maples (1992)}. The last three variables used were considered because they are important factors of the physical space in which the species lives (^{Challenger and Caballero, 1998}). Agricultural and livestock areas were also discarded in the estimation of potential distribution.

Potential distribution of wild papaya

The wild papaya is distributed in the rainforest of Mexico, and is located mainly in the secondary vegetation of the humid and sub-humid tropical forest (^{Paz and Vázquez, 1998}). In addition to the bioclimatic factors, its distribution in Central and South America has been favored by the abundant amount of seeds produced by the plants and the prolonged viability of the seeds (^{Paul and Duarte, 2011}).

In this analysis, the potential distribution, based on the registry of 208 individuals from wild populations, generated two zones of high probability of finding this type of papayas: 1) The southeast of the Gulf of Mexico, including the state of Veracruz, Tabasco, Campeche and a part to the north of the state of Chiapas, mainly. In these areas the habitat quality for the species is low to moderate (Figure 1). The ecological zone is humid tropical, predominantly humid warm climate, average annual temperature greater than 22 ºC and temperature of the coldest month greater than 18 ºC. The precipitation of the driest month is less than 60 mm, rainfall in summer and percentage of winter rain greater than 10.2% of the annual total, average annual rainfall in a range of 1 500 to 2 000 mm per year; and 2) The Pacific coasts of the state of Chiapas have a moderate to high habitat quality. The ecological zone corresponds to humid tropical, where the climate is warm subhumid with average annual temperature greater than 22 ºC and temperature of the coldest month greater than 18 ºC.

Figure 1 Potential distribution of wild populations of Carica papaya L. High distribution areas are indicated in red, with gradual decrease in probability to a low level of green. The white areas indicate non-potential areas. 

The analyzes also indicated the existence of potential zones of smaller size on the coast of Oaxaca (15 172.14 km²); similarly, in the Costa Chica, Costa Grande, Centro, Norte and Tierra Caliente regions of Guerrero, which together comprise 3 171.91 km²; also in Yucatan (2 487.66 km²). However, in these areas the habitat quality for the species was classified as low.

Finally, smaller isolated areas were located in San Luis Potosi (954.89 km²), Tamaulipas (170.22 km²), Colima (120.03 km²), and Jalisco (82.16 km²), where characteristics of the subhumid tropic also prevail.

The total area of potential distribution for wild specimens was 114 546.5 km². The largest potential area was located (Table 2) in three states that contributed 69.1%: Chiapas (25%), Veracruz (27.6%) and Campeche (16.5%). In Chiapas, ^{Chavez and Nuñez (2016}) report the distribution of wild papaya in the regions bordering Tabasco, and point out the effects caused by agriculture and livestock on the structure of the vegetation where the wild papaya survives.

On the other hand, the zones of high probability of geographical distribution of wild papaya populations, defined in this study, are located partially in the priority regions of biodiversity conservation of Mexico published by ^{Arriaga et al. (2000)}, as with other groups that have been analyzed for potential distribution (^{Villaseñor and Téllez, 2004}), so it is important to validate the potential distributions with observations in the field. In Oaxaca, the areas of potential distribution of wild papaya coincide with priority region number 130 (Sierra del Norte de Oaxaca-Mixe) and 132 (Jungle Zoque-La Sepultura); in Veracruz with the priority regions (RP) 124 (Wetlands of Papaloapan) and 131 (Sierra de los Tuxtlas-Laguna del Ostion); in Chiapas with RP 133 (El Triunfo-La Encrucijada-Palo Blanco) and 135 (Tacana-Boquerón); in Tabasco with RP 142 (El Manzanillal), 143 (Lagunas de Catazaja-Emiliano Zapata) and 144 (Marshes of Centla).

However, there are potential distribution areas, determined in this study, that are not included in any region with conservation priority; for example, in Los Tuxtlas, Veracruz, ^{Chávez et al. (2014)} highlight that wild papaya populations survive in fragments of vegetation with less genetic diversity than populations present in areas of continuous vegetation; they also report that agricultural and livestock activities cause isolation of populations and put at risk the persistence of natural populations in their ecological niche. For example, in Costa Rica the papaya that develops in areas with disturbance and secondary vegetation, can present a genetic bottleneck (^{Brown et al., 2012}). This indicates that the potential distribution areas of wild papaya in Mexico, which are not included in federal protection zones, are exposed to anthropocentric activities that could alter the structure of their genetic reservoir.

Potential distribution of cultivars

It is important to note that the distribution of cultivated papaya has been favored with agricultural management and the economic benefits obtained; In this sense, agronomic technology has focused on increasing the yield of fruit and promoting the best environmental conditions for the development of plants (^{Guzmán et al., 2010}). In this way, the varieties of papaya, which have emerged from a process of genetic improvement, are established as a commercial crop in five leading Mexican states: Oaxaca, Chiapas, Colima, Veracruz and Michoacán (^{SIAP, 2016}). In addition to varieties, over time farmers have developed some native varieties that propagate in home gardens or on agricultural land (^{Paz and Vázquez, 1998}). For example, the types known as ‘papaya zapote’, ‘papaya oreja de mico’, ‘papaya amarilla’, ‘papaya de monte’, are present in home gardens in communities of Yucatán, Campeche and Tabasco, given their food use (^{Mariaca, 2012}). Inclusively, ^{Moo et al. (2017)} indicate that in Mexico there is a process of substitution of native varieties for varieties arising from genetic improvement; for example, by the Maradol variety.

According to the results of the present study, papaya cultivation has potential in 15 states, but the largest area was located in Chiapas (25.7%), Veracruz (17.5%) and Guerrero (16.7%), which together accounted for almost 60% of the total surface (Figure 2). These zones have distinctive characteristics of a humid tropical ecological zone, with a predominant humid warm climate, an annual average temperature higher than 22 ºC and a temperature of the coldest month higher than 18 ºC. The precipitation of the driest month is less than 60 mm, rainfall in summer, amount of winter rain greater than 10.2% of the annual total and average annual rainfall is in the range of 1 500 to 2 000 mm per year.

Figure 2 Potential distribution of cultivars of Carica papaya L. High distribution areas are indicated in red color, with gradual decrease of the probability to a low level of green color. The white areas indicate non-potential areas. The black dots are the geo-referencing sites. 

Other potential areas of the crop, of smaller size and with low ecological quality were located in the states of Baja California, Sonora, Sinaloa, Nayarit, Jalisco, San Luis Potosi and Michoacan (image not presented) as a whole, these areas added 11.8% of the total potential distribution area of papaya cultivars (Table 2).

Climatic variables and potential distribution

Of the 22 climatic and soil variables, used as predictors in the MaxEnt model of geographic distribution of wild populations and cultivars, eight were the most important to explain the distribution of Carica papaya in Mexico (Table 3). The variables annual average temperature (BIO01), average minimum temperature of the coldest period (BIO06), annual precipitation (BIO12) and precipitation of the rainiest four-month period (BIO16) were common predictors of the distribution of cultivated and wild papaya. In relation to this, ^{Scheldeman et al. (2007)} determined that C. papaya is better adapted in high temperature niches (24.2 ºC in average) with little seasonal variation, and high rainfall (1 830 mm). On the other hand, ^{Bradie and Leung (2017)} establish that temperature and precipitation are environmental variables that contribute the most in the distribution of terrestrial plant species.

In the case of wild papayas, the variables that contributed more than 75% of their distribution were: average annual temperature (BIO01, with 20.1%), average minimum temperature of the coldest month (BIO06, 33%), average temperature of the colder four-month period (Bio11, 10.7%) and humidity regime (BIO21, 13.5%). In general, it is notorious that the distribution of wild papaya is influenced by the low temperature, the soil moisture regime and the level of rainfall.

For papaya cultivars, the variables of greatest importance for their distribution were: average minimum temperature of the coldest month (BIO6, with 41.4%), precipitation of the rainiest four-month period (BIO16, 19%), annual rainfall (BIO12, 11 %) and precipitation of the coldest four-month period (BIO19, 5.1%). This indicates that the distribution of papayas grown in the potential regions will be influenced, in greater magnitude, by the level of rainfall and low temperatures.

In the present study, the average minimum temperature of the coldest period (BIO 6) was the variable that most participated in the definition of potential areas in both wild populations and cultivars. In addition, the climatic variables that contributed substantially to the distribution of the wild were BIO01 (average annual temperature) and BIO21 (soil moisture regime), while for the cultivars were BIO16 (precipitation of the rainiest four-month period) and BIO12 (rainfall annual).

The temperature influences the germination of papaya seeds, when in the soil it oscillates between 29.5 ºC and 29.8 ºC during the summer (^{Saran et al., 2016}). The growth of the plant in height and diameter of the stem is adequate with temperatures above 20 ºC (^{Allan, 2002}). Temperatures below 10 °C and above 35 °C are related to fruit and flower development disorders (^{Workneh et al., 2012}).

The moisture deficit in the soil triggers the mechanism of osmotic adjustment in the tissue of papaya seedlings and the arrest of growth (^{Mahouachi et al., 2006}). Such moisture deficiencies in the soil, in combination with high temperature (> 37 °C), have a negative impact on the photosynthetic process (^{Liu et al., 2014}). This affects even the wild types of papaya (^{Díaz et al., 2014}).

In addition, it was notorious that rainfall was a relevant bioclimatic variable in the potential distribution of papaya cultivars. For its cultivation the availability of water is important, given that the plants produce 1.8 to 2.8 kg of fresh fruit per cubic meter of water applied (^{Carr, 2014}). However, excess water in the soil, for at least one day, causes problems due to lack of oxygen in the root system (^{Thani et al., 2016}).