By analyzing the chloroplast genes ribulose biphosphate carboxylase large chain (rbcL), maturase-K (matK), trnL and trnL-F and rpl20-rps12 intergenic spacers in 240 species from 114 genera of the Cucurbitaceae family in conjunction with a parsimony-based biogeographic analysis to determine vicarious and dispersal events, Asia was found to be the most likely center of origin for lineages in cucurbits and was gradually distributed to the rest of the continents through transoceanic dispersions (Schaefer et al., 2009).

Linguistic evidence, the endemism of most Sechium species, the morphological diversity they present, and the existence of wild populations suggest that all 11 Sechium species originated and were diversified and domesticated in Mesoamerica (Newstrom, 1990, 1991; Lira & Chiang, 1992; Lira, 1995; Lira et al., 1997b; Lira & Nee, 1999; Cross et al., 2006; Cadena-Iñiguez et al., 2008, 2011, Avendaño-Arrazate et al., 2012, 2017). In Mexico and Guatemala, the S. edule, S. mexicanum, S. compositum, S. chinantlense and S. hintonii species are most prominent; in Central America, the following species are found: S. panamensis, S. talamancensis, S. venosum, S. villosum, S. pittieri and S. tacaco. Most Central American species have high degrees of endemism, in which the altitudinal factor influences the characteristics of the ecological niche where each of them develops (Wunderlin, 1976; Lira, 1995; Lira et al., 1999; Lira & Nee, 1999). The fruits, roots and leaves of Sechium are widely used in gastronomy (Newstrom, 1990; Lira, 1995) due to the contribution of water, vitamins, sugars, minerals, fiber and carbohydrates (Aung et al., 1990; Lira, 1996; Islam et al., 2018); otherwise, fruits are also used in the industry for the elaboration of flour used in pig meal (Lalthansanga & Samanta, 2015) and as a source of endothelin, which is a powerful vasoconstrictor and mitogen that can be used as a skin-whitening agent (Okamoto et al., 2008).

S. edule. It is the most representative species of the Sechium genus due to its agricultural importance, and is known as “chayote”, which is a word originally from the Nahuatl term huizt ayotl (spiny squash), later chayotl and presently chayote. The broad morphological and genetic wealth of S. edule in southeastern Mexico and Guatemala, in addition to the presence of S. compositum, which is a highly related wild species, indicates that chayote was domesticated in the region of Chiapas-Oaxaca, Mexico (Newstrom, 1991).

Cadena-Iñiguez et al. (2011) studied eight populations of S. edule (seven cultivated and one wild) from Veracruz, Mexico. Biochemical studies have shown that the wild accession synthesizes a greater quantity of chlorophyll and cucurbitacins with respect to the cultivated populations; principal component analysis (PCA) separated chayote populations by color, texture, and secondary metabolite content. Using the same populations as S. edule, Cadena-Iñiguez et al. (2008) evaluated them via 27 morphological variables of leaves, flowers and fruits. The PCA for the first component was tilted for the variables of leaf shapes, trichomes, fruit color, flower size and color, petiole color, and vein structure; while for component two, a greater variation was obtained for the characteristics of the fruit (length, width and height). A cladistic analysis based on morphology and anatomy showed that the wild population was the most remote but most likely ancestor of cultivated chayotes. The bitter flavor of the fruits is a characteristic of the wild populations of cucurbits, in which Yuan et al. (2016) compared the cucumber, melon and watermelon genomes, thereby finding a group of genes that regulate the synthesis of cucurbitacins and that due to domestication and selection; therefore, wild populations can significantly reduce the bitterness of their fruits.

Castro et al. (2015) evaluated the morphological and biochemical diversity of seven cultured S. edule accessions in Spain. The length of the fruit had an interval of 9.2-13.1 cm, as Cadena et al. (2008) reported an upper limit interval of 3-15 cm, in which other variables such as fruit weight and seed morphometry were also affected by fruit length. However, Castro et al. (2015) found a great diversity in the color of the fruit, from light green to dark green; on the other hand, Castro et al. (2015) obtained an average of 2.27 ºBrix, which was relatively low compared to that of Cadena et al. (2011), who obtained values higher than 10 ºBrix, which is due to the presence of chayotes with yellow fruits that tend to have a slightly sweet flavor.

Chayote is an introduced crop in India, but with indices of morphological diversity. Jain et al. (2017) evaluated 36 native S. edule accessions with 18 fruit-related morphological markers and 97 polymorphic bands obtained from DAMD-type markers. The PCA results are similar to those obtained by Cadena-Iñiguez et al. (2008), given that the main components give greater variation to the characteristics of the fruit, but not so due to the morphology in f lowers and leaves, where Cadena-Iñiguez et al. (2008) found a variation. Verma et al. (2017) also found moderate genetic diversity in 74 chayote varieties in the North Eastern Hill region of India with ISSR and RAPD-like molecular markers; on the other hand, those varieties of dark green color showed the highest content of phenols and vitamin C, and are recommended for selection and hybridization.

Abdelnour & Rocha (2008) evaluated the biochemical diversity with isoenzymes in 42 S. edule accessions native to Costa Rica, and found that 57% of the 14 loci were polymorphic; 24 accessions were heterozygous for a single locus, and the others had polymorphisms at two or three loci. Avendaño-Arrazate et al. (2012) reported 59.8% polymorphism in 27 cultivated S. edule populations and three wild isoenzyme populations; populations of S. edule with dark green fruits and spines were found to have the highest observed and expected heterozygous values; even so, the observed average heterozygosity was low (0.05). The highest genetic variation was found by isoenzymatic analysis in materials with dark green fruits. Therefore, isoenzymes are currently not recommended for genetic diversity studies since their expression is regulated by the environment and the results may be distorted.

The recalcitrant nature of S. edule seeds prevents their timely conservation, making it difficult to obtain archaeological records that allow delving into themes related to their origin. However, through ethno-historical and linguistic techniques (Guevara et al., 2014), it has been determined that the Aztecs used and cultivated chayote (Newstrom, 1991; Lira, 1996). Since it is the Sechium species with the highest degree of domestication in a region with high ecogeographic and cultural diversity such as Mexico, varietal complexes have been formed within the species that differ primarily in the physiological, chemical and anatomical characteristics related to the fruits (Cadena-Iñiguez & Arévalo-Galarza, 2011). The highest degree of observed heterozygosity was found in materials with an intense green color and with spines, which are prominent characteristics of wild materials (Avendaño-Arrazate et al., 2012).

The fruits of the wild populations of S. edule tend to have a bitter flavor and high concentrations of cucurbitacins, and are rich in ethyl acetate to capture free radicals (Cadena-Iñiguez et al., 2011; Riviello-Flores et al., 2018). These chemical compounds have anti-neoplastic properties and apoptosis inducers in cancer cells, and can inhibit the metastatic process of lung cancer (Shukla et al., 2016). The alcoholic extracts of the varietal complexes of chayote and their improved genotypes H-378 (S. edule amarus silvestrys × S. edule var. virens levis 290) and Perla negra (S. edule var. nigrum minor × S. edule var. amarus silvestrys) (Cadena-Iñiguez et al., 2013a) have shown positive results in cancerous cell lines such as L929 (mice lung fibrosarcoma), HeLa (human cervical carcinoma) and P-388 (mice leukemia macrophages), with the peculiarity of having minimal negative effects on normal cells (Monroy-Vázquez et al., 2009; Cadena-Iñiguez et al., 2013b; Aguiñiga-Sánchez et al., 2015, 2017; Salazar-Aguilar et al., 2017).

The alcoholic extracts of S. edule decrease levels of agents related to coronary heart disease and significantly increase high-density cholesterol levels (Neeraja et al., 2015). The dried fruit powder of S. edule var. nigrum spinosum has significant effects in reducing oxidative stress, which helps combat metabolic syndrome problems in older adults (Rosado-Pérez et al., 2019); otherwise, alcoholic extracts of S. edule from roots and fruits have reducing effects on blood pressure (Lombardo-Earl et al., 2014; Fauziah et al., 2019); they also act as regulators of blood glucose levels (Maity et al., 2013; Martiar, 2017), protectors of the gastrointestinal mucosa (Sateesh et al., 2012), and have antibacterial activity against the gram-negative Escherichia coli ATCC 8739, Salmonella typhimurium ATCC 3224 and Shigella flexneri ATCC 12022 (Kalpana et al., 2013); just to cite some examples of its many applications.

Concerning breeding to increase the content of cucurbitacins, which confer bitter flavor both to leaves and to fruits, two genes, bi-1, and Bt-1 have been found to be involved. From a set of 2416 pairs of SSR primers developed from the sequencing of the cucumber genome (Cucumis sativus L.) (Ren et al., 2009), Zhang et al. (2013) used QTL analysis to study 149 RIL´s produced by the crossing of the inbred lines 9110Gt (P1) and 9930P (P2) of cucumber to detect molecular markers linked to bitter f lavor in fruits and leaves. In this way, it can possibly be detected that the bi (bi-1) and Bt (Bt-1) genes are linked and confer bitter flavour to the aforementioned structures. These genes were found on chromosomes 5 and 6 of the cucumber and could have an application to be detected in other genera of the family since it belongs to the same botanical family. The classical breeding methods, based on selection and hybridization between the species S. compositum, S. chinantlense, and varietal complexes of S. edule, have produced improved varieties and hybrids for human consumption and pharmaceutical potential due to their high content of cucurbitacin (Cadena-Iñiguez et al., 2013a; Avendaño-Arrazate et al., 2014).

S. compositum. Its geographic distribution is from Chiapas in Mexico to Central American countries such as Guatemala. It can develop up to an altitude of 2100 m, allowing it to grow in humid environments, such as mesophyll and tropical forests. Newstrom (1990, 1991) described it as the wild species closest to S. edule, despite having different chromosome numbers [(S. compositum n = 14) (Lira et al., 2009) (S. edule n = 12, 13 and 14) (Palacios, 1987; Mercado et al., 1993; De Donato & Cequea, 1994)] and because it has the singularity of not having an apical fissure, which is a characteristic that can be used as an object of selection (Lira, 1995) or isolation.

S. chinantlense. This is limited to northern Oaxaca, Mexico, with an altitude below 800 m. It develops in high or medium evergreen forests, and in mesophyll and deciduous forests. S. chinantlense is related to S. edule and S. compositum, which is why its conservation is necessary to understand the evolutionary processes of cultivated species (Lira, 1995; Lira et al., 1999).

Taxonomic studies of palynology by Lira et al. (1994) showed that the qualitative physical characteristics of the pollen grains (thorny and covered grains) of S. chinantlense belong to the Siciynae subtribe and share certain characteristics with the rest of the Sechium species; however, the absence of an analysis of variance makes it difficult to observe the presence/absence of significant differences.

S. hintonii. It has a restricted distribution in Temascaltepec (Mexico State), and in the southern state of Guerrero (Mexico). It thrives in a limited altitude range, fluctuating between 1300 and 1500 m in deciduous tropical forests and live oak forests (Lira, 1995). Recently, it has been collected from the mesophyll forest of Veracruz at an altitude of 1340 m in the germplasm bank of the Grupo Interdisciplinario de Investigación en Sechium edule en México (GISeM).

S. mexicanum. This species is endemic to Mexico and is found in the states of Hidalgo, Puebla, Querétaro and Veracruz. It develops at altitudes of 900-2300 m in mesophyll, oak, and pine forests (Lira & Nee, 1999). Molecular studies suggest that this species should be relocated to another genus, and Sicyos seems to be the closest (Cross et al., 2006; Sebastian et al., 2012).

S. tacaco. This species is totally restricted throughout the Talamanca mountain range, Costa Rica, at altitudes of 1000-1700 m, and is locally known as “tacaco” (Wunderlin, 1976). There exist cultivated and semi-cultivated populations of this species in San José, Costa Rica, where their fruits are exploited as vegetables to make regional dishes (Lira, 1995). Morales (1994) studied the vegetative and reproductive morphology of S. tacaco, and found similarities with S. edule in the anatomical structure of plant organs, multicellular trichomes, structure of floral nectaries, and the presence of anomocytic stomata in vegetative and reproductive organs. Lira (1995) emphasized the low morphological diversity of S. tacaco fruits, except for the presence/absence of spines and the amount of fiber; however, in a recent study, Monge & Loría (2017) described morphologically five varieties of S. tacaco in geographically distant localities, although at similar average altitudes of 1100 m, thereby finding diversity and significant differences in the characteristics of the fruit (weight, length, width). The most important findings of Monge & Loría (2017) were found in the report of tacaco fruits with 6-7 complete longitudinal sutures and 2-5 incomplete longitudinal sutures, which provided new information to elucidate evolutionary processes at the level of species and Sechium genus.

S. panamense. This species is endemic to Panama, where it develops at altitudes that fluctuate between 1500 and 3000 m. Its botanical description is only based on the four samples collected, so it is necessary to intensify its search to find the variation and possible hybridizations that will contribute to its study. Its adaptation to cold climates makes it ideal to become incorporated into genetic improvement programs, which would help induce its search and research (Lira, 1995).

S. talamancense. Also known as “chayotillo” and “tacaquillo”, S. talamancensis is endemic as S. tacaco to the Talamanca mountain range in Costa Rica. It is found in the cloud (mesophyll) forest at altitudes of 2400 to 3200 m. Due to the low temperatures where it thrives, it could be useful in the transfer of genes to improve frost-resistant cultivated materials (Lira, 1995).

S. venosum. This species is endemic to the Caribbean coast of Costa Rica and is very similar to S. hintonii in terms of its pendular inflorescences. It adapts perfectly to conditions with a high degree of moisture (Lira, 1995), suggesting that its genetic structure is resistant to attack by fungal diseases, which are the main problems in cultivated species of Sechium (Olguín-Hernández et al., 2013) and one of the most significant approaches in the genetic improvement of species with agricultural importance (Newstrom, 1990; Moncayo-Pérez et al., 2020).

S. villosum. This species is endemic to Costa Rica, and thrives in disturbed environments of tropical or mesophyll forests, at altitudes of 1500-2000 m. Similar to S. venosum, it develops in humid environments, which could be a source of genes for resistance to fungal-related diseases (Lira, 1995).

S. pittieri. This species is distributed from Nicaragua to Costa Rica, where it can be consumed as a vegetable. Its phenotypic and genetic plasticity allows it to adapt in diverse environments with a broad altitudinal range that covers up to 2500 m, which can be synonymous with S. pittieri, possibly having a wide range of genetic diversity, and can also be used as S. talamancensis and S. panamensis to improve cultivated species (Lira, 1995).<