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Visiting Bitter Gourd (Momordica charantia) from a Breeding Perspective: A Review
Plant Breed. Biotech. 2020;8:211-225
Published online September 1, 2020
© 2020 Korean Society of Breeding Science.

Hari Kesh1 , Prashant Kaushik2,3*

1Department of Genetics and Plant Breeding, CCS Haryana Agricultural University, Hisar 125004, India
2Institute of Conservation and Improvement of Valencian Agrodiversity, Polytechnic University of Valencia, Valencia 46022, Spain
3Nagano University, 1088 Komaki, Ueda, Nagano 386-0031, Japan
Corresponding author: Prashant Kaushik, prakau@doctor.upv.es, Tel: +34-963-877-244, Fax: +34-963-877-000
Received March 31, 2020; Revised May 24, 2020; Accepted July 15, 2020.
This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Bitter gourd is an important vegetable of the family Cucurbitaceae, cultivated mainly in humid and subtropical Asia. Bitter gourd is a vegetable with immense health benefits due to the presence of medicinal compounds such as charantin, vicine, and polypeptide-p, which play essential roles in lessening blood glucose levels. Moreover, bitter gourd fruits are particularly rich in vitamin C, minerals, and carotenes. Here, an effort has been made to critically evaluate the extent of achievements during the enhancement and enactment of bitter gourd breeding programs with the use of latest technologies. Broadening the genetic base of cultivated bitter gourd varieties as a result of enrichment of existing resources by using wild species in breeding programs. Practical seed production technological know-how along with the use of the MS system (male sterility)/chemical-induced sterility procedure is nonetheless vital to cope with market demands. Superior yielding bitter gourd hybrids combining early maturity and resistance to biotic and abiotic stresses are regularly needed to cope with the challenge of bitter gourd production.
Keywords : Bitter gourd, Breeding, Genetic diversity, Genomics, Heterosis, Molecular breeding, Mutation breeding
INTRODUCTION

Momordica is one of the largest genera in the Cucur-bitaceae family. The genus includes 59 species distributed widely in Africa and Asia (de Wilde and Duyfjes 2002; Schaefer and Renner 2010). Out of 59 species, 34 species are dioecious, and 22 are monoecious, while the remaining three are not clear whether they are dioecious or monoe-cious (Schaefer and Renner 2010). Two monoecious viz. M. charantia L. and M. balsamina L. and four dioecious viz. M. dioica, M. sahyadrica, M. cochinchinensis and M. subangulata of genus Momordica (Joseph 2005). Momordica charantia is cultivated as a vegetable as well as a medicinal plant in Asia and Africa (Degner 1947; Walters and Decker Walters 1988). However, the recommended region of do-mestication for bitter gourd lies in eastern Asia especially in Southern China or India (Walters and Decker-Walters 1988; Robinson and Decker-Walters 1997; Marr et al. 2004), Fiji as well as the South Pacific (Smith 1981) along with southwestern India (Joseph 2005). The first history claimed the growing of M. charantia in China in 1370 CE (Yang and Walters 1992). Bitter gourd is an abundant source of phenolic compounds (Kaushik et al. 2015). Nevertheless, there aren’t any archaeological accounts of M. charantia continues to be in China (Yen 1977; Marr et al. 2004). Small-fruited or wild cultivated crops with medicinal uses are quoted in Ayurvedic texts by Indo Aryan cultures (Decker-Walters 1999) indicating old growing of bitter gourd in India. Recent scientific studies proved that M. charantia originated from Africa (Schaefer and Renner 2010).

The domestication was believed to arrive from Africa to Brazil with the slave trade then dispersed to Central America (Ames 1939; Marr et al. 2004). Wild relatives of bitter gourd are green to dark blue, spiny, brief and very bitter and abundantly present in Northeastern India (Gaikwad et al. 2008). Based on historical, scientific studies (Chakravarty 1990; Miniraj et al. 1993; Walters and Decker-Walters 1988) along with molecular analyses (Dey et al. 2006; Singh et al. 2007; Gaikwad et al. 2008), eastern India (Orissa, West Bengal, Assam, Jharkhand, and Bihar) may be con-sidered as the primary center of diversity. Diversity an-alysis based on simple sequence repeat markers among Asian collections of bitter gourd landraces and hybrids showed that South Asia is the center of domestication of bitter gourd (Dhillon et al. 2016). M. charantia is a com-mercial crop grown in India, China, Japan, Malaysia, Thailand, Philippines, Australia, Tropical Africa and South America. Here, an effort has been made to critically evalu-ate the extent of achievements during the enhancement and enactment of bitter gourd breeding programs with the use of latest technologies.

GENENTIC DIVERSITY

The importance of genetic diversity is well recognized in providing the basic raw material for selection and hy-bridization program of plant breeding and crop improve-ment. The evolution of transgressive segregants in the hybridization program largely depends on the availability of diverse germplasm in the gene pool of a crop. A great level of diversity is present in both cultivated and wild relatives of Momordica genus for several important traits. Although interspecific crosses were not successful between these two species (Joseph 2005), the formation of bivalents during the normal meiotic cycle suggested that these two species are phylogenetically similar (Singh 1990). Germplasm which leads to partial fertility upon hybridization comes under the secondary gene pool. Gene pool 2 includes M. dioica, M. sahyadrica, M. subangulata subsp. renigera, and M. cochinchinensis. Except for two viz., M. cochinchinensis × M. dioica and M. cochinchinensis × M. sahyadrica, a suc-cessful, viable seed setting was observed between all pos-sible combinations of dioecious species (Bharathi 2010).

A lower seed setting and germination were observed when a diploid species was used as a female parent while a proper seed setting was reported when a tetraploid species was used as a female parent. M. subangulata subsp. renigera, a tetraploid species, showed high fruit setting when crossed with diploid species M. sahyadrica and M. dioica while in the reverse cross of M. sahyadrica and M. dioica with M. subangulata subsp. renigera, the fruit setting was comparatively low (Brar et al. 2019; Behera et al. 2020). Tertiary gene pool includes species which are distantly related to each other and gene exchange between them is impossible except by using some special techniques such as embryo rescue, somatic hybridization, bridging cross and genetic engineering. Tertiary gene pool includes M. cymbalaria, which did not show any seed setting when hybridized with the rest of the species of the Momordica genus (Behera et al. 2020). M. charantia shows a tremendous amount of diversity for various morphological and quality traits. Fruits of wild type plants are generally smaller in size, pointed at both ends and produced grey to black seeds while cultivated plants have fruits of different shapes and sizes with lengths up to 60 cm and produced large brown seeds (Walters and Decker-Walters 1988; Yang and Walters 1992). Significant variation was observed for earliness-related traits by Dey et al. (2009) and Khan et al. (2015). Diversity for ascorbic acid (60.20 mg to 122.07 mg/100 g) among bitter gourd accessions was observed in Dey et al. (2005). Chemical and nutritional composition study of dif-ferent colour and size of bitter gourd varieties revealed that the highest carbohydrate and fiber contents were found in the light green small types (8.22 and 1.21 g) (Krishnendu and Nandini 2016).

The critical traits for bitter gourd are earliness, number of fruits per plant, fruit length, fruit girth, number of female flowers per plant, number of days taken for first female flower emergence, fruit weight, fruit colour, bitterness, fruit yield per plant, disease and insect resistance etc. Light brown seed coat colour is recessive to deep brown (Srivastava and Nath 1972; Ram et al. 2006); large seed size is re-cessive to small seed size (Srivastava and Nath 1972); white coloured epicarp is recessive to eco-friendly (Suribabu et al. 1986; Vahab 1989); spiny fruit is dominant over sleek (Vahab 1989); Green fruit colour is dominant over yellow (Hu et al. 2002) and is managed by 2 genes (Liu et al. 2005). The bitterness of fruit is a monogenic characteristic, with much more bitterness is dominant over less (Suribabu et al. 1986; Dalamu et al. 2012). The monogenic inheri-tance was described for fruit colour, fruit lustre, fruit area system, and stigma colour, but a digenic form of inheri-tance was suggested for seed colour (Kole et al. 2012). Srivastava and Nath (1972) discovered immature fruit colour in bitter gourd was managed by one nuclear gene without any cytoplasmic element involved. The light green colours were most likely influenced by incomplete domin-ance or maybe modifier genes (Hu et al. 2002). The con-stant perturbation in fruit length indicated its quantitative inheritance, and over 4 genes had been reported to be en-gaged in managing this particular trait (Kumari et al. 2015). The quite short fruit length is partly dominant, and cur-viness and tubercles of fruits in bitter gourd are, governed by a single pair of a nuclear gene as well as the tubercles trait (Kumari et al. 2015). Expression of gynoecious trait in the bitter gourd was discussed by Iwamoto and Ishida (2006) as partly dominating in Japanese germplasms of bitter gourd while in Indian germplasms of bitter gourd Ram et al. (2006) observed gynoecy (gy1) is, in fact, recessive to monoecy. Whereas, the genetic analysis by Kim and Kim (1990) showed that a small fruit size was partially domin-ant to large fruit size. Complementary epistasis and domin-ance × dominance gene interaction were important deter-minants of fruit yield in bitter gourd (Singh and Ram 2005). Inheritance of resistance to melon fruit fly indicates that fruit fly resistance is dominant over susceptibility (Tewatia and Dhankhar 1996).

HETEROSIS BREEDING

Degree of heterosis depends on the genetic divergence between parents involved in the cross, their modes of reproduction, traits to be studied and the plant develop-mental stage. This hypothesis suggests that heterozygosity at individual loci as leading to heterosis. Heterosis is main-ly utilizing dominance variation (Kumar et al. 2020). The exploitation of heterosis is more feasible in bitter gourd due to its cross-pollinated nature. The crucial points for the exploitation of heterosis are the identification of superior and divergent inbred lines, information on the combining ability of inbred lines and production of hybrid based on the pedigree and combining ability of inbred lines. It will be hugely beneficial to the breeder if a correlation could exist between the genetic distances of hybrid parents and the yield obtained from their respective cross or hybrid. Molecular markers can be used for the prediction of heterosis based on genetic diversity of parental lines as was demonstrated in several other crops, such as rice (Xie et al. 2014). The preferable parameters to satisfy the consumer demands are soft fruits with a smaller number of seeds, minimized ridges, uniform green colour, high vigour, a more significant number of female flowers, good fruit setting and high yield (Al-Mamun et al. 2015). However, consumer preference is varied depending on the region.

Light green, medium-long, spindle fruits are preferred in Vietnam, while long cylindrical and smooth fruit and light-green are preferred in Thailand. South Asian consumers like small-to-medium size, dark-green, spindle-shaped fruits with spiny exterior surfaces. In Taiwan and India, white fruit is used in soups (Dhillon et al. 2016). A lot of variabilities were noticed for vegetative and fruit figures in bitter gourd. Due to the presence of large variability, mono-ecious characteristics, convenient and conspicuous flowers along with a substantial selection of seeds per fruit, bitter gourd work as a prospective crop for the exploitation of heterosis (Thangamani and Pugalendhi 2013). As bitter gourd does not show or very negligible inbreeding depres-sion, the homozygous inbred lines are developed after 6 to 7 generations of continuous selfing. Before making the crosses between inbred lines directly, information regarding the general and specific combining ability will help the breeder in getting the superior hybrids (Haripriya 1991). High general combining ability effects for different charac-teristics may help identify better parents with favourable alleles for different components of yield (Acharya et al. 2019). Thus, a substantial general combining capability of the parents appears to be a dependable criterion for the prediction of certain combining ability (Brar and Sidhu 1977). In bitter gourd, heterosis continues to be noticed for days to the very first female flowering, fruit per yield and vine per vine (Vahab 1989), vine measurements, fruit mea-surements, as well as yield per vine (Devdass 1993), fruit fat as well as yield per vine (Richard et al. 1995), vine length, days to first female flowering, fruit length and fruit number (Celine and Sirohi 1996), fruit flesh thickness and fruits per vine (Jadhav et al. 2009), node at which first female flower appears and fruit length (Thangmani and Pugalendhi 2013), days to first harvest and number of fruits per plants (Table 1) (Al-Mamun et al. 2015).

Table 1 . Review of literature on magnitude of heterosis (%) over the mid parent, and better parent values for yields and its component traits in bitter gourd.

CharactersMid parentBetter parentReferences
Vine length1.66 to 23.37Sirohi and Choudhary (1978)
2.10 to 22.30Singh and Joshi (1980)
4.26 to 57.81Chaudhari and Kale (1991b)
0.99 to 20.981.41 to 4.21Munshi and Sirohi (1993)
‒8.31 to 29.81‒24.51 to 2.20Yadav et al. (2009)
‒42.56 to 111.74‒45.10 to 49.08Talukdar et al. (2010)
‒6.00 to 15.90Behera et al. (2009)
‒34.60 to 30.72Talekar et al. (2013)
‒32.57 to 57.48Verma and Singh (2014)
Number of primary branches/vine7.80 to 37.00Singh and Joshi (1980)
3.30 to 25.85Singh et al. (1997)
‒30.46 to 251.77‒11.21 to 293.65Yadav et al. (2009)
‒26.23 to 13.70Talekar et al. (2013)
‒47.37 to 182.35‒50.00 to 140.00Talukdar et al. (2010)
Internodal length‒21.92 to 20.66‒34.34 to 16.44Yadav et al. (2009)
‒21.13 to 20.00Talekar et al. (2013)
‒39.31 to 22.36‒42.53 to 19.16Talukdar et al. (2010)
Days to opening of the first male flower‒8.29 to ‒26.8Singh et al. (2000)
‒1.09 to 12.09‒7.43 to 8.51Yadav et al. (2009)
‒27.22 to 9.80‒29.79 to 7.33Talukdar et al. (2010)
‒17.83 to 10.64Talekar et al. (2013)
Days to opening of the first female flower54.00 to 66.00Lawande and Patil (1989)
‒15.72 to 7.48Ram et al. (1997)
‒0.12 to ‒6.50Singh et al. (2001)
‒0.22 to ‒29.51Kandasamy (2015)
‒17.65 to 3.08‒23.00 to ‒2.22Yadav et al. (2009)
‒19.34 to 14.47Talekar et al. (2013)
‒18.46 to 10.77‒23.89 to 3.94Talukdar et al. (2010)
‒9.17 to 34.86Verma and Singh (2014)
Days to first fruit harvest‒0.42 to 15.06Sing et al. (1997)
‒6.19 to ‒22.20Singh et al. (2000)
Chaubey and Ram (2004)
‒26.82 to ‒17.90Behera et al. (2009)
‒1.4 to ‒26.3Al-Mamun et al. (2015)
‒17.27 to 11.20Talekar et al. (2013)
‒11.91 to 4.66‒16.42 to ‒0.14Talukdar et al. (2010)
‒19.80 to 3.60Dey et al. (2010)
‒19.33 to 7.69Verma and Singh (2014)
Number of fruits per plant0.86 to 44.440.39 to 35.02Munshi and Sirohi (1993)
‒66.67 to 30.61Ram et al. (1997)
0.0 to 104.7Al-Mamun et al. (2015)
‒32.23 to 22.99Talekar et al. (2013)
74.12 to 100.23Dey et al. (2010)
39.86 to 32.43Verma and Singh (2014)
Fruit length0.90 to 17.75Khattra et al. (1994)
1.40 to 25.46Singh et al. (2001)
31.27 to 37.00Behera et al. (2009)
‒37.7 to 6.8Al-Mamun et al. (2015)
1.43 to 39.79Kandasamy (2015)
‒37.57 to 4.87‒30.95 to 20.76Yadav et al. (2009)
‒18.17 to 15.01‒20.24 to 12.38Talukdar et al. (2010)
‒38.08 to 39.11Talekar et al. (2013)
4.65 to 13.80Dey et al. (2010)
29.72 to 17.99Verma and Singh (2014)
Fruit diameter‒11.7 to 13.4Al-Mamun et al. (2015)
1.07 to 25.34Kandasamy (2015)
20.90 to 37.60Behera et al. (2009)
‒24.19 to 23.88‒22.73 to 39.53Yadav et al. (2009)
2.30 to 10.50Dey et al. (2010)
‒5.58 to 7.09‒8.37 to 4.95Talukdar et al. (2010)
‒36.57 to 41.08Verma and Singh (2014)
Fruit weight5.00 to 14.50Dey et al. (2010)
‒18.60 to 9.38Verma and Singh (2014)
26.75 to 46.38Behera et al. (2009)
‒10.41 to 29.97‒29.17 to 28.67Talukdar et al. (2010)
Flesh thicknessRanpise et al. (1992)
6.13 to 16.261.47 to 6.27Celine and Sirohi (1996)
‒17.11 to 29.66‒25.19 to 26.87Mallikarjunarao et al. (2018)
Fruit yield per plant4.35 to 64.28Khattra et al. (1994)
19.7 to 102.0Al-Mamun et al. (2015)
132.00 to 142.20Behera et al. (2009)
‒8.35 to 113.01‒22.80 to 94.25Talukdar et al. (2010)
0.38 to 60.38Kandasamy (2015)
‒34.75 to 23.10‒10.29 to 58.51Yadav et al. (2009)
‒30.22 to 136.43‒43.64 to 98.75Mallikarjunarao et al. (2018)
‒58.96 to 51.14Talekar et al. (2013)
38.22 to 97.49Dey et al. (2010)
‒39.41 to 35.23Verma and Singh (2014)
Vitamin C content (mg/100 g)‒11.60 to 29.03‒16.15 to 24.75Mallikarjunarao et al. (2018)
‒33.33 to 18.52Kumar and Pathak (2018)
Iron content (mg/100 g)‒23.43 to 52.34‒34.62 to 36.97Mallikarjunarao et al. (2018)
TSS (°Bri×)‒23.50 to 27.37‒34.44 to 20.18Mallikarjunarao et al. (2018)
Carotene content (mg/100 g)‒79.61 to 52.92Kumar and Pathak (2018)
Total sugar (g/100 g)‒57.73 to 81.13Kumar and Pathak (2018)
Reducing sugar (g/100 g)‒59.07 to 79.77Kumar and Pathak (2018)

MUTATION BREEDING

Mutation may occur spontaneously or can be induced artificially. Artificial mutations can be induced by physical mutagens such as X-rays, y-rays and neutrons, and by chemical mutagens such as ethyl methanesulfonate (EMS) (Ahloowalia and Malugzgnslia 2001). Physical mutagens are used more frequently than chemical mutagens, and among physical mutagens, y-rays are used more commonly than X-rays (Beyaz and Yildiz 2017). Mutation breeding has become a powerful technique for developing novel plant genotypes (Penna et al. 2012). Mutation screening is the selection of individuals from a sizeable mutated popu-lation that meet specific selection criteria, and mutant con-firmation is re-evaluating (Oladosu et al. 2016). A Bitter gourd landrace MC 013 was treated with gamma radiation and developed a new cultivar MDU 1 which possesses improvement for yield, long greenish white fruits, toler-ance to pumpkin beetle, fruit fly and leaf spot diseases (Rajasekharan and Shanmugavelu 1984). Similarly, a white fruited type mutant variety Pusa Do Mausami was developed from a natural population of green fruited variety Pusa Do Mausami variety (Behera et al. 2010).

INTERSPECIFIC HYBRIDIZATION

Inter-specific and intra-specific hybridization have es-sential roles in tracing the genomic relationship and im-proving crops by transferring desirable agronomic characters and some specific traits such as disease, pests and stress resistance from wild relatives to cultivated ones (Bowely and Taylor 1987). In Momordica, studies were carried out on inter-specific hybridization to establish phylogenetic relationships (Vahab 1989). Crossability studies among different species of Momordica such as M. dioca, M. cochinchinensis and M. subtangulata subsp. renigera were reported by Bharathi et al. (2011). The hybrids were ob-served as intermediate for ovary colour, fruit shape and colour, and the number of seeds and distribution of trichomes on stem and leaves (Hassena and Suhara 2012). High cross-ability and pollen fertility were observed between inter-varietal cross (M. charantia var. charantia × M. charantia var. muricata).

In contrast, low crossability and moderate pollen fertility were observed in the inter-specific cross (M. charantia × M. balsamina). Similarly, Rathod et al. (2019) attempted inter-specific crossing for the characterization of plant morphology, pollen-pistil compatibility cytology and molecular relationships among parents and hybrids. Both direct and reciprocal crosses showed an approximate 90% cross-ability with a good percentage of pollen viability.

An interspecific cross resulted in a hybrid between M. charantia variety Pusa Aushadi (female) and M. balsamina (male) variety Pusa were selfed and backcrossed with Pusa Aushadi and M. balsamina. Inheritance studies showed that a single dominant gene governed fruit tubercle, dis-continuous ridges and green colour of fruit, seed coat colour was governed by semi-dominant gene, and gynoecia was governed by a single recessive gene (Annual report IARI-2018-19, India, www.iari.res.in). Crossability per-centage among interspecific crosses in the bitter gourd is presented in Table 2.

Table 2 . Crossability percentage among interspecific crosses in bitter gourd.

Inter-specific combinations% crossabilityReferences
M. charantia var. charantia × M. charntia var. muricata100%Hassena and Suhara (2012)
M. charntia var. muricata × M. charantia var. charantia88.89%
M. charantia var. charantia × M. charntia var. muricata97%Bharathi et al. (2012)
M. charntia var. muricata × M. charantia var. charantia85%
M. charntia var. muricata × M. balsamina6%
M. dioca × M. sahyadrica75%
M. dioca × M. subangulata subsp. renigera53%
M. dioca × M. cochinchinensis60%
M. subangulata subsp. renigera × M. dioca65%
M. subangulata subsp. renigera × M. sahyadrica84%
M. subangulata subsp. renigera × M. cochinchinensis2%
M. sahyadrica × M. subangulata subsp. renigera50%
M. sahyadrica × M. dioca81%
M. sahyadrica × M. cochinchinensis65%
M. cochinchinensis × M. subangulata subsp. renigera5%
M. charantia var. charantia × M. charntia var. muricata90.98%Rathod et al. (2019)
M. charntia var. muricata × M. charantia var. charantia84.43%

BIOTECHNOLOGY

Traditional breeding techniques for any crop improve-ment is time-consuming. Due to the increasing importance of bitter gourd, the improvement and the development of new varieties are necessary, which could be done through the modern application of biotechnology (Sinha et al. 2019). The application of transgenic technology through gene manipulation opens the way for producing genotypes carrying important traits such as resistance against biotic and abiotic stresses. Plant tissue cultures are used for plant breeding, commercial production and basic biological research (Agarwal 2015). Types of explants, media com-position, growth conditions, genotypes and physiological conditions of the explants affect callus induction and plant regeneration. Wang et al. (2008) established in vitro plant regeneration method from cotyledon node. Overall, it was determined that age of seedlings, along with the different combination of hormones, results differently in the in vitro morphogenesis of M. charantia. In this direction, the effect of various growth regulators was studied by Malik et al. (2007) on callogenesis and organogenesis of M. charantia. It was determined that the leaf explants produced a maxi-mum callus percentage as compared to stem and cotyledons.

Among the generally used approaches of gene transfer, the Agrobacterium-mediated gene transfer strategy is seen as most effective for the healthy integration of genes into the host plant genome. Agrobacterium-mediated b-glucu-ronidase gene expression was recognized in explants of immature cotyledonary nodes in M. charantia (Sikdar et al. 2005) as well as in the leaf disc (Thriruvengadam et al. 2012).

GYNOCISM IN BITTER GOURD

Gynoecious lines have a better genetic combining ability, and gynoecious × monoecious hybrids mature early with higher yield potential (Dey et al. 2010). Thus, gynoecious lines can easily be utilized for the hybrid seed generation in bitter gourd as it stays away from the mechanical emas-culation and pollination. The main sex type in the bitter gourd is monoecious; however, gynoecious sex type has been reported from India, China and Japan (Behera et al. 2006). For the very first time, bitter gourd crops with an entire phrase of gynoecious flowering habit (only pistillate blossoms on a plant) have been put in 3 gynoecious col-lections, namely Gy23, Gy63 and Gy263B (Ram et al. 2002). And then, many populations with an extremely high pro-portion (more than 90 %) of pistillate blossoms have been designed (Ram et al. 2002b). 2 gynoecious lines known as DBGy-202 and DBGy-201 are isolated from its vivid rela-tives M. charantia var. muricata L. (Behera et al. 2006). These lines have been recognized for the inheritance of development and gynoecia of hybrid cars (Behera et al. 2009). They hold great potential in future breeding plans for the improvement of earliness and yield in bitter gourd (Varalakshmi et al. 2014). Gynoecium is actually under the command of an individual recessive gene (gy 1) (Ram et al. 2006; Behera et al. 2009) whereas Iwamoto and Ishida (2006) found that gynoecious sex expression in bitter gourd is partly dominant. In this direction, 2 pairs of genes were also determined by Cui et al. (2018). Moreover, the sub-sequent decades (including F1) utilizing gynoecious as a single parent showed an extremely high percent of pistillate (female) blossoms (Ram et al. 2002; Behera et al. 2006; Iwamoto and Ishida 2006).

In Okinawa, a gynoecious line (OHB61-5) was identified, which was supposed to be a spontaneous mutant. This line, when crossed with OHB95-1A (monoecy) and F2 pro-genies studied, indicating that the trait was governed by a single recessive gene (Matsumura et al. 2014). High yielding hybrids between gynoecious × monoecious have recently become available (Behra et al. 2009). For example, the popular hybrid bitter gourd (VNR 28), after transplanting in the last week of March in Chhattisgarh, India in 2012, produced only pistillate flowers continuously from the first week of May through the end of May. Still, staminate flowers did not appear until the end of May (Dhillon et al. 2017).

Similarly, Kumaken BP1, an early high yielding hybrid of bitter gourd in Japan developed using a gynoecious inbred line as the seed parent, needs a pollenizer for more initial fruit setting (Iwamoto et al. 2009). Thus, adapted monoecious lines when blended with gynoecious × mono-ecious hybrids that produce pistillate flowers earlier than staminate flowers (Dhillon et al. 2017).

For transgressive segregants form, NBGH-167 cultivar was isolated. The lines development through progeny row method takes four consecutive generations of selfing for obtaining complete gynoecious expression (Jadhav et al. 2018). For the maintenance of gynoecious line, the induction of male flower is essential. For the effective induction staminate flower, the gynoecious lines were treated with silver nitrate, silver thiosulphate and Gibberellic acid. Silver nitrate (AgNO3) performs well as compared to GA3 and silver thiosulphate [Ag(S2O2)2] for induction of male flowers in gynoecious line (Jadhav et al. 2018). Silver nitrate inhibits the synthesis of ethylene and thus induce staminate flower (Krishnamoorthy 1975; Saha and Behera 2015).

Identification of molecular marker associated with gynoecy trait is of great importance to determine the cost-effective hybrid seed production (Gaikwad et al. 2014). In this direc-tion, using the F2 population from a linkage map was constructed in bitter gourd, and five single nucleotide poly-morphism (SNP) loci were found linked to gynoecy. One of the SNP markers GTFL-1 was located at 5.46 cM distance (Matsumura et al. 2014). Similarly, gy-1, a gynoecious gene flanked by bTP_54865, and TP_54890 markers were mapped on LG 12 in the bitter gourd genetic map (Gangadhara Rao et al. 2018).

MOLECULAR BREEDING

Traditional plant breeding techniques are primarily de-pending on the phenotypic screening of germplasm lines. These techniques require a more extended period of time for diversity analysis, identification of desirable parents and took a long time for the improvement of essential traits. Genetic diversity grounded on quantitative characteristics has been completed in a bitter gourd by Mishra et al. (1998) and Ram et al. (2000). But genetic diversity among dif-ferent lines can also be done by molecular markers. Mole-cular markers are independent of environmental conditions and show a higher level of polymorphism. The initial study on diversity analysis in bitter gourd was done by Dey et al. (2006). They evaluated 38 bitter gourd lines, including commercial cultivars collected from different parts of India using RAPD primers. Out of 116 primers, 29 were found to be polymorphic and informative.

The primary reason for mismatch might be that the majority of the quantitative traits are actually managed by a big selection of genes and are highly affected by locations (Dey et al. 2006). Ferriol et al. (2003) additionally did not find some correlation between morphological and RAPD characterization in strawberry. The inter-simple sequence repeat (ISSR) markers had been implemented to expose polymorphism among thirty-eight lines of M. charantia collected from various agro-ecological zones of India. The dendrogram analysis showed that 38 genotypes were grouped into 2 major groups with 36 genotypes in one group and 2 genotypes in the second group. RAPD markers were used by Rathod et al. (2008) for determining the genetic re-lationships among twenty genotypes of M. charantia. The genotypes were divided into two clusters, cluster A with one genotype and cluster B with 19 genotypes. Cluster B was further divided into two sub-clusters B1 (one genotype and B2 (18 genotypes). The RAPD and ISSR marker (Dey et al. 2006; Singh et al. 2007) used for diversity analysis could not provide complete insights into the cultigens examined (Gaikwad et al. 2008). The discriminating power of AFLP marker is generally higher than that of RAPD and ISSR (Vos et al. 1995; Powell et al. 1996) due to high polymorphic nature, broad distribution throughout the genome and high multiplex ratio (Milbourne et al. 1997).

The genetic relationships among 38 bitter gourd lines originating from different geographical regions were studied by Gaikwad et al. (2008). The results indicated that these lines could be used directly as parents in hybridization or as germplasm for selection to improve economically important trait (Fazio et al. 2003; Fan et al. 2006; Gao et al. 2010). The genotypes were divided into three main clusters and six sub-groups using RAPD markers and three main clusters and seven subgroups based on ISSR markers. The characterization by high polymorphism, simple sequence repeats (SSRs) are ideal genetic markers and have gained signifi-cant importance in plant breeding (Akkaya et al. 1992; Morgante and Olivieri 1992; Gupta et al. 1996; Peakall et al. 1998). Guang-guang et al. (2013) studied genetic di-versity and relationship among 50 bitter gourd varieties using 16 pairs of SSR primers. The genotypes were classi-fied into 6 groups using UPGMA methods (Saxena et al. 2014). Since marker-assisted selection (MAS) is useful in the breeding of crops, genetic mapping of agronomically important traits directly contribute to the breeding pro-grams. The first genetic map in bitter gourd was developed Kole et al. (2012) in the F2 population between Taiwan White × CBM12 using AFLP markers. Subsequently, Wang and Xiang (2013) developed a linkage map in F2 population of Z-1-4 × 189-4-1 with the use of different kinds of mole-cular markers such as simple sequence repeat (SSR), AFLP, and sequence-related amplified polymorphism (SRAP). A list of QTLs identified for important traits in bitter gourd (Table 3).

Table 3 . List of QTLs identified for important traits in bitter gourd.

TraitsNumber of QTLsLinkage groupReferences
Fruit colour1LG 7Kole et al. (2012)
Seed colour2LG 3
Fruit length2LG 2 and LG 7
Fruit diameter1LG 1
Fruit weight1LG 1
Fruit number4LG 1, LG 2 and LG 5
Fruit yield4LG 1, LG 2 and LG 3
Sex ratio (male:female)9LG 9, LG 13, LG 14 and LG 16Gangadhara Rao et al. (2018)
Days to first pistillate flower appearance8LG 3, LG 5, LG 14 and LG 16
The node at first pistillate flower appearance5LG 5, LG 9 and LG 14
Gynoecy1LG 12
Female flower ratios3LG 4, LG 5 and LG 9Wang and Xiang (2013)
First female flower node3LG 4, LG 5 and LG 9
Fruit length4LG 1, LG 2, LG 5 and LG 9
Fruit diameter5LG 1, LG 9 and LG 11
Flesh thickness2LG 1
Fruit shape5LG 4, LG 5, LG 9 and LG 11
Fruit pedicel length3LG 4, LG 8 and LG 9
Fruit pedicel length ratios5LG 4, LG 6 and LG 8
Fruit weight4LG 4, LG 5, LG 6 and LG 12
Fruit numbers per plant3LG 1 and LG 5
Yield per plant2LG 5 and LG 9
Stem diameter2LG 2 and LG 4
Internode length2LG 2 and LG 5
Gynoecy2MC01Cui et al. (2018)
First female flower node2MC01
Female flower number2MC01
Fruit wart1MC04
Width of ridge1MC10
Hue angle1MC10
Lightness variable1MC10
Bitterness3LG 3 and LG 4Shang et al. (2020)

BITTER GOURD GENOME

The bitter gourd draft genome was generated with the inbred line, OHB3-1, by employing Illumina sequencing with an estimated size of 339 Mb (Urasaki et al. 2017). Overall, 45,859 protein-coding genes were determined, and based on the synteny bitter gourd was determined to be more closely related to watermelon (Citrullus lanatus) as compared to cucumber or melon. Interestingly, it was determined that trypsin-inhibitor and ribosome-inactivating genes were notable characteristics in the bitter gourd genome (Yilmaz and Khawar 2020). These genes provide medicinal benefits properties to the bitter gourd. The bitter gourd genome is further assembled to chromosome-level, and is also available with a maximum contig N50 close to 10 Mb. The new version of bitter gourd genome based on the Nanopore long-read assembler was published with a claim of the complete assembly of the Cucurbitaceae family (Matsumura and Urasaki 2020). Based on the preference at a particular location phenotypic variation exists in the bitter gourd cultivars, e.g., in South-East Asia, small size fruits with bitter taste are preferable. Based on the genome as-sembly, it was formulated that the South Asian varietal group diverged from the progenitors around 6000 years ago (Matsumura et al. 2019).

CONCLUSIONS AND FUTURE PROSPECTS

Bitter gourd produces large flowers and the techniques used for the hybrid development are well established in the bitter gourd and is similar to the ones for melon and cucumber. Further inbreeding or selfing is performed for several generations to produce uniformity in the inbreds before their crossing to develop them into hybrids. More-over, there is no significant inbreeding depression in the bitter gourd; therefore, inbreds are regularly maintained by selfing. Further inbred testing is performed for their com-bining ability via employing several matting designs Line by Tester analysis, diallel analyses, etc.

It is further based on the obtained information regarding the general and specific combining abilities, most promising parents are chosen for the hybrid production. With the availability of bitter gourd genome and detailed genetic map, the selection for heterosis related alleles can be easily achieved in the bitter gourd improvement program.

Moreover, to exploit heterosis in bitter gourd, it should not be only for yield but in line with the interest of other important traits such as those related to climate change and also a mix of traits like insect pest and disease resistance. In this direction, crop wild relatives (CWR) can play an im-portant role as they are the storehouse of important genes desired for the improvement of yield as well as stress tolerance. Bitter gourd is especially vital to countries like in South East Asia where the population is still poor and cannot afford the medications for the treatment of their chronic illness. There is considerable reliance on the plants with therapeutic potential, and bitter gourd has been proved as a cure for many chronic diseases. We hope the use of bitter gourd likewise will increase in the western world and feature some significant research roads which ought to be organized. Failure to generate and provide a qualified amount of hybrid seed, specifically of general public bred hybrids warrants urgent notice.

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