Caryologia. International Journal of Cytology, Cytosystematics and Cytogenetics 75(1): 89-97, 2022 Firenze University Press www.fupress.com/caryologia ISSN 0008-7114 (print) | ISSN 2165-5391 (online) | DOI: 10.36253/caryologia-1358 Caryologia International Journal of Cytology, Cytosystematics and Cytogenetics Citation: Sanjay Kumar, Asikho Kiso (2022) New reports of somatic chromo- some number and symmetric or asym- metric karyotype estimation of Sechium edule (Jacq.) Sw. (Cucurbitaceae). Caryologia 75(1): 89-97. doi: 10.36253/ caryologia-1358 Received: July 5, 2021 Accepted: March 27, 2022 Published: July 6, 2022 Copyright: © 2022 Sanjay Kumar, Asikho Kiso. This is an open access, peer- reviewed article published by Firenze University Press (http://www.fupress. com/caryologia) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All rel- evant data are within the paper and its Supporting Information files. Competing Interests: The Author(s) declare(s) no conflict of interest. ORCID SK: 0000-0003-4463-9159 New reports of somatic chromosome number and symmetric or asymmetric karyotype estimation of Sechium edule (Jacq.) Sw. (Cucurbitaceae) Sanjay Kumar1,*, Asikho Kiso2 1 Department of Botany, Banaras Hindu University, Varanasi, UP 221005, India 2 Department of Botany, Nagaland University, Lumami, Nagaland798627, India *Corresponding author. E-mail: skumar.bot@bhu.ac.in Abstract. Sechium edule (Jacq.) Sw. distributed well in Eastern Himalayan region, four- teen genotypes collected randomly from Kigwema village, Kohima (Nagaland) at an average altitude of 1538 masl (meter above sea level) with 25.61°N latitude and 94.35°E longitudes. Somatic chromosome numbers are diploid in nature. Chromosome num- bers are in agreement and comparable with earlier reports except 2n=2x=22. The chro- mosome number 2n=2x=22 could not be traced out in the present material collected for study. Two (2) new chromosome numbers 2n=2x=32 and 2n=4x=52 were recorded and expected to be diploid and tetraploid in nature respectively. It was expected that tetraploid (2n=4x=52) number of chromosome might be originated from the ear- lier reports of diploid chromosome number 2n=2x=26. Both chromosome numbers 2n=2x=32 and 2n=4x=52 are reported for the first time by the authors in the present study. Range of chromosome length was recorded in between 0.501 – 1.343. The range of chromosome length are in agreement with earlier reports of 0.700 – 0.900 approxi- mately. Range of chromosome length suggested minute (<1µm) and small (1-3µm) size of chromosomes with small differences and variations in chromosome length (CVCL). Inter-chromosomal indices (A2 and Rec), intra-chromosomal index (particularly, Steb- bin’s classification) and both inter and intra chromosomal indices (DI and GI) estimat- ed symmetric nature of the karyotype. Keywords: Sechium edule (Jacq.) Sw., mitosis, somatic chromosome number, karyo- type, inter and intra chromosomal symmetry/asymmetry estimation. INTRODUCTION Genus Sechium P. Browne was first published in a monograph on Cucur- bitaceae in 1881. Literature survey on historical background suggested that it was a monospecific genus with a single species and represented as Sechium edule (Jacq.) Sw. (Cogniaux 1881; DeDonato and Cequea 1994). Historically, genus Sechium was originally recorded from Jamaica (Browne 1756). Genus Sechium had been recorded as Sicyos edulis and Chocho edulis simultaneous- ly in initial classification (Adanson 1763). Jacquin (1788) changed the genus 90 Sanjay Kumar, Asikho Kiso Chocho into Chayota and re-designated as Chayota edu- lis. Later on, Chayota edulis re-designated as Sechium edule by Swartz (1800). At present, the genus is known as a combination of both Jacquin and Swartz i.e. Sechi- um edule (Jacq.) Sw. The most accepted term for Sechium is ‘Chayote’ worldwide. The presence of single species for the genus (mono- specific genus) had worn-out, when more species were reported for the genus by various authors from different regions during 1900s and some of them are S. edule sub spp. edule, S. edule sub spp. sylvestre, S. chinatlense, S. compositum and S. hintonii (Goldblatt1990; Singh 1990; Mercado et al. 1993; Mercado and Lira 1994). Recently, a new species called Sicyos angulatus L. for Indian flora and Sechium mexicana for Mexico have been reported respec- tively (Thakur 2016; Lira and Nee 1999). The reported species were morphologically very similar and never veri- fied for the presence of a new species in the genus. Cytologically, genus Sechium was attempted and studied for the presence of some more species, if any. Many authors reported different chromosome numbers for the genus Sechium with base chromosome number x=12, 13, 14, and 15. The base chromosome number x=11 has also been reported for the genus by Singh (1990). Genus Sechium categorized into S. edule sub spp. edule, S. edule sub spp. sylvestre, S. chinatlense, S. compositum, S. hintonii, S. mexicana and Sicyos angulatus respec- tively on the basis of earlier reports of base chromosome number. The base chromosome numbers suggest that it remains unresolved and needs thorough examina- tion cytologically. So, the present aim of the paper is to attempt and extend the information of new chromosome number count to the genus Sechium, if any. MATERIALS AND METHODS Genus Sechium is a shrub climber of Cucurbitaceae family. Fruit samples of genus were collected randomly from Kigwema village, Kohima, Nagaland (India) at an average altitude of 1538 meter above sea level (masl), latitude (25.61°N) and longitude (94.35°E). Mitosis was studied from the secondary root tips of germinating fruits. Root tips of 2-3 cm in length were pre-treated with α-bromonaphthalene at 6±2 °C for 3-4 h followed by overnight fixation (3:1 ethanol-acetic acid) and pres- ervation (70% ethanol). The root tips were hydrolyzed with 1 N HCl for 10-15 min at about 50-60 °C. The root tips were squashed in 2% acetocarmine. Three somatic chromosome preparations under 100x (emersion oil) were photographed using digital Motic BA 210 micro- scope and recorded for further analysis. Statistical Analysis Total chromosome length (µm) were measured for the genotypes with the scale bar of 10µm using ImageJ sof tware and further computation was attempted through windows MS-Excel and with the help of stand- ard formulas for inter and intra chromosomal differ- ences among the chromosome complement of genotypes (see Box 1). RESULTS AND DISCUSSION The genotypes are diploid in nature in somatic chro- mosome count except genotype 2 (Fig. 1D). Two diploids with different somatic chromosome number 2n=2x=26 (Fig. 1B) and 2n=2x=32 (Fig.1C) and a tetraploid 2n=4x=52 (Fig.1D) were recorded for the genotype. The chromosome numbers 2n=2x=32 (diploid) and 2n=4x=52 (tetraploid) are the first report for the Sechium edule. Other somatic chromosome numbers are in agreement and comparable with earlier reports except 2n=2x=22 which could not be traced out in the present study mate- rials (Sugiura 1940; Sobti and Singh 1961; Giusti et al. 1978; Mercado and Lira 1994). The presence of differences, if any, in cultivated or wild forms of the Sechium, possibly could have been originated through the chromosomal evolutionary fac- tors in due course of time with the help of primary, sec- ondary, agmatoploidy, symploidy, dysploidy or pseudoa- neuploidy evolutionary factors and needs to be verified through cytological and molecular techniques. Similarly, Sechium edule suggested ploidy nature 2n=4x=52 (genotype 2) of the species. Ploidy is not reported earlier in the Sechium edule and hence, the first report for the species. The findings of ploidy nature in Sechium suggested towards the whole genome content change, diversification, evolutionary changes and spe- ciation in the genus. It could be correlated that a new species might be established from the pre-existing spe- cies through reproductive or genetic isolation from the progenitors. The speciation through evolution and diver- sification required various events of primary (deletion, duplication, inversion, and translocation), secondary (fusion, fission, rearrangements) and disploid (ascending or descending) alterations of chromosome numbers. In the present paper, origin, diversification, genetic isola- tion or possibility of interbreeding between and among Sechium needed to be explored (Fig. 1A – P). In past, few reports are available on the origin and evolution of cultivated cucurbits and suggested the Mex- ico, Central America and Guatemala as the centre of 91Chromosome number and karyotype study of Sechium edule (Jacq.) Sw. from India variation for the crop. Earlier, Sechium edule was con- sidered mono-specific (genus with single species) and native to New World, but now it includes as many as eight species and cultivated throughout tropical and sub- tropical regions of the world but not explored extensively (Newstrom 1990). At present, first report on new chro- mosome number gives a hope for the presence of some more species in the genus, Sechium. Statistical analysis results on genotype chromosomes presented in Table 1. Total chromosome length (ΣTCL) or chromosome volume (CV) was recorded maximum for the genotype 3 (29.618) which is very close to the genotype 2 with 2n=4x=52 (28.884) but the somatic chromosome number differs in both suggested the dif- ferences in the size of the chromosomes (Martonfiova, 2013). Chromosome length range (CLR) was recorded in between 0.501 – 1.343 for the present genotypes. Seven genotypes have chromosome length more than 1 µm which indicates the heterogeneity of chromosome length for chromosome complement. Two types, minute and small size chromosomes were recorded based on the classification minute (<1µm), small (1-3µm), medium (3-5µm) and large (>5µm) suggested by Kutarekar and Wanjari (1983). Earlier, range of chromosome length was reported in between 0.700 – 0.900 for Sechium edule. The range of chromosome length are in agreement with earlier reports approximately and comparable (Sanjappa 1979; Cadena- iniguez et al. 2007). Value of relative chromatin (VRC) was recorded high and indicated towards the heterochromatic nature of genotypes. High heterochromatic nature could be cor- related with the less advanced type of karyotype with more number of metacentric chromosomes alongwith small differences in size of largest and smallest chromo- some of karyotype (Beevy and Kuriachan 1996). Coefficient of variation of chromosome length (CVCL) was recorded high for each chromosome and hence Σ CVCL for genotypes indicated variation in the chromosome length of karyotype complement (Thakur and Sinha 1973). A2 value was computed for each chromosome and recorded near to zero and summation value ΣA2 for each genotype presented. A2 value close to zero indicates the conservation of chromosome size in the karyotype with low variation among the chromosome length and asym- metry remains the constant i.e. A2, approximately, indi- cated towards the symmetric nature of the karyotype (Carvalheira et al.1991). Similarly, Rec index value ranges from 0-100. The value was recorded for individual chromosome in karyo- type and summation value Σ Rec for the each genotype presented is high. High value for Rec index suggested the maximum resemblance among the chromosomes with symmetric nature of the karyotype. Rec index value measures the resemblance between the chromosomes and the average degree of symmetry over the whole kar- yotype (Huziwara 1962). Rec index = 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇∑𝑇𝑇𝑒𝑒𝑒𝑒𝑒𝑒𝑇𝑇ℎ𝑇𝑇𝑜𝑜𝑒𝑒𝑇𝑇𝑜𝑜ℎ𝑜𝑜ℎ𝑟𝑟𝑇𝑇𝑟𝑟𝑇𝑇𝑟𝑟𝑇𝑇𝑟𝑟𝑒𝑒 ÷ 𝐿𝐿𝑇𝑇𝑒𝑒𝑒𝑒𝑒𝑒𝑟𝑟𝑇𝑇𝑜𝑜ℎ𝑟𝑟𝑇𝑇𝑟𝑟𝑇𝑇𝑟𝑟𝑇𝑇𝑟𝑟𝑒𝑒 𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑇𝑒𝑒𝑇𝑇𝑟𝑟𝑇𝑇𝑒𝑒𝑟𝑟𝑇𝑇𝑜𝑜𝑜𝑜ℎ𝑟𝑟𝑇𝑇𝑟𝑟𝑇𝑇𝑟𝑟𝑇𝑇𝑟𝑟𝑒𝑒𝑟𝑟 x 100 (Greilhuber and Speta 1976) A2 index = 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆ℎ𝑆𝑆𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑆𝑆𝑟𝑟𝑆𝑆ℎ 𝑀𝑀𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆ℎ𝑆𝑆𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑆𝑆𝑟𝑟𝑆𝑆ℎ (Romero – Zarco1986) Coefficient of Variation (CVCL) = 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆ℎ𝑆𝑆𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑆𝑆𝑟𝑟𝑆𝑆ℎ 𝑀𝑀𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆ℎ𝑆𝑆𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑟𝑟𝑆𝑆𝑆𝑆𝑟𝑟𝑆𝑆ℎ x 100 (Lavania and Srivastava 1999; Paszko 2006) Disparity Index (DI) = 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿ℎ𝑟𝑟𝐿𝐿𝑟𝑟𝐿𝐿𝐿𝐿𝐿𝐿𝑟𝑟𝐿𝐿 − 𝑆𝑆ℎ𝐿𝐿𝑟𝑟𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿ℎ𝑟𝑟𝐿𝐿𝑟𝑟𝐿𝐿𝐿𝐿𝐿𝐿𝑟𝑟𝐿𝐿 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿ℎ𝑟𝑟𝐿𝐿𝑟𝑟𝐿𝐿𝐿𝐿𝐿𝐿𝑟𝑟𝐿𝐿 + 𝑆𝑆ℎ𝐿𝐿𝑟𝑟𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿ℎ𝑟𝑟𝐿𝐿𝑟𝑟𝐿𝐿𝐿𝐿𝐿𝐿𝑟𝑟𝐿𝐿 x 100 (Mohanty et al. 1991) Value of Relative Chromatin (VRC) = ∑ Total Length of chromosome / n (Dutta and Bandyopadhyaya 2014) where n=somatic chromosome count Gradient Index (GI) = 𝑆𝑆ℎ𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜ℎ𝑜𝑜𝑜𝑜𝑟𝑟𝑜𝑜𝑜𝑜𝑜𝑜𝑟𝑟𝑜𝑜 𝐿𝐿𝑜𝑜𝐿𝐿𝐿𝐿𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜ℎ𝑜𝑜𝑜𝑜𝑟𝑟𝑜𝑜𝑜𝑜𝑜𝑜𝑟𝑟𝑜𝑜 x 100 (Lavania and Srivastava 1992) Chromosome volume = πr2h where h = total length of chromosome (Toijam et al. 2013) Box 1 92 Sanjay Kumar, Asikho Kiso C A F E D B Figure 1. Somatic chromosome count, Sechium edule (Jacq.) Sw.; A) G1, 2n=2x=26; B) G2, 2n=2x=26; C) G2, 2n=2x=32; D) G2, 2n=4x=52; E) G3, 2n=2x=30; F) G4, 2n=2x=26. 93Chromosome number and karyotype study of Sechium edule (Jacq.) Sw. from India I J H G K L Figure 1 (continued). Somatic chromosome count, Sechium edule (Jacq.)Sw.; G) G5, 2n=2x=28; H) G6, 2n=2x=28; I) G7, 2n=2x=28; J) G8, 2n=4x=24; K) G9, 2n=2x=24; L) G10, 2n=2x=28. 94 Sanjay Kumar, Asikho Kiso Intra chromosomal asymmetry index could contain more number of acrocentric or telocentric chromosomes than the metacentric and submetacentric chromosome which could be the result of change in position of cen- tromere. The change in centromere position brings the rearrangement in the chromosomes and may lead to increase in karyotype asymmetry percent. Intra chromosomal asymmetry depends on exact identification of the centromere and the chromosomal morphology but not only the chromosome size. The extreme symmetry (ideal karyotype A) or asymmetry (ideal karyotype C) of karyotype is meager in nature (Stebbin 1971). However,the present analysis indicates an extreme symmetric karyotype (1A) among the genotypes except genotypes G2 and G11 and may be classified as ideal kar- yotype B of Stebbin’s classification and suggested that kar- yotypes of the two genotypes deviated from symmetric to asymmetric and are in agreement with the hypothesis of Stebbins classification (1971). According to the hypothesis asymmetric karyotypes are being originated from the sym- metrical karyotypes over a period of time and due course of evolution. Similar work has been reported earlier and in agreement that primitive members with symmetrical kary- otypes give rise to advance members with the asymmetri- cal karyotype (Levitzky 1931; Kumar and Kumar 2014). N O P M Figure 1 (continued). Somatic chromosome count, Sechium edule (Jacq.) Sw.; M) G11, 2n=2x=26; N) G12, 2n=2x=30; O) G13, 2n=2x=26; P) G14, 2n=2x=28. 95Chromosome number and karyotype study of Sechium edule (Jacq.) Sw. from India The presence of asymmetric karyotype could be the result of chromosome structural changes particu- larly centric fusion or fission which leads to symploid or agmatoploid chromosome rearrangements in due course of plant species evolution. The centric fusion and fission could also be suggested as cause of frequent disploidy or pseudoaneuploidy among plant species (Eroglu et al. 2013). Both dispersion index (DI) and gradient index (GI) are considered as combination of inter-intra chromosom- al index and used for the evaluation of karyotype sym- metry. Both represents the nature of evolutionary process occurring or occurred in genus or species and indicates the trend of evolution had taken place in genus, species or cytotypes (Lavania and Srivastava 1992). Comparatively, lower DI value especially below 30 and higher GI value more than 30 suggested symmetri- cal nature of the karyotype for the genotypes except G2 with 2n=32 and 52 and supports Stebbins hypothesis. Both DI and GI showed high degree of symmetry which may lead to the lesser degree of chromosomal variation and evolution (Stebbins 1971). At present, 2n=2x=32 and 2n=4x=52 chromosome numbers were not reported earlier and, hence first report in the present paper. New chromosome number may suggest towards the whole genome content change, diversification, evolution and speciation in the genus. A very few or negligible reports are available on genus Sechium from India (Sanwal et al. 2008; Kapoor et al. 2014; Jain et al. 2015; Jain et al. 2017). Genus Sehium remains very poorly known cytologically, therefore, prop- er chromosome count is important for understanding the interrelationship among different Sechium species. CONCLUSION Somatic chromosome number and kar yomor- phometric estimations are in the agreement of earlier reports except two new reports of chromosome number in genotype 2 of the Sechium edule. ACKNOWLEDGEMENT Author, SK is thankful to AK for collection of mate- rial and squash preparation of the Sechium edule (Jacq.) Sw. The authors gratefully acknowledged the Nagaland Table 1 Karyotype symmetry/asymmetry estimation of Sechiumedule. G en ot yp e (s ) BCN (2x) ΣTCL or CV Interchromosomal index Intrachromosomal index Inter+Intra chromosomal index CLR VRC ΣCVCL ΣA2 ΣRec Largest/Smallest Chromosome ratio Arm ratio proportion Stebbin’s classification DI GI G1 2n=2X=26 16.733 0.501-0.764 0.643 815.648 3.145 84.225 1.52 < 2:1 1A 20.79 65.575 G2 2n=2X=26 20.919 0.583-1.043 0.804 414.048 4.132 77.128 1.78 < 2:1 1A 28.29 55.896 G2 2n=2X=32 18.099 0.295-0.855 0.565 752.794 7.517 66.138 3.13 > 2:1 1B 70.289 31.929 G2 2n=4X=52 28.884 0.342-0.803 0.555 802.814 8.850 69.098 2.34 > 2:1 1A 40.174 42.643 G3 2n=2X=30 29.618 0.696-1.343 0.987 467.969 4.669 73.496 1.92 < 2:1 1A 31.731 51.824 G4 2n=2X=26 22.890 0.642-1.113 0.880 417.482 4.162 79.029 1.73 < 2:1 1A 26.837 57.681 G5 2n=2X=28 23.470 0.628-1.086 0.838 408.486 4.071 77.177 1.72 < 2:1 1A 26.721 57.826 G6 2n=2X=28 26.090 0.651-1.209 0.931 447.191 4.458 77.043 1.85 < 2:1 1A 30.00 53.846 G7 2n=2X=28 20.320 0.583-0.876 0.725 312.523 3.115 82.819 1.50 < 2:1 1A 20.082 66.552 G8 2n=2X=24 16.274 0.521-0.904 0.678 305.383 3.042 76.735 1.73 < 2:1 1A 26.877 57.632 G9 2n=2X=24 16.899 0.541-0.863 0.704 272.606 2.713 80.907 1.54 < 2:1 1A 22.934 62.688 G10 2n=2X=28 19.276 0.591-0.805 0.688 279.181 3.612 85.503 1.36 < 2:1 1A 15.329 73.416 G11 2n=2X=26 23.784 0.600-1.202 0.914 454.258 4.528 76.089 2.00 > 2:1 1B 33.407 49.916 G12 2n=2X=30 26.747 0.620-1.097 0.891 484.374 4.829 81.258 1.76 < 2:1 1A 27.781 56.517 G13 2n=2X=26 20.009 0.568-0.946 0.769 291.900 2.901 81.349 1.66 < 2:1 1A 24.966 87.925 G14 2n=2X=28 21.956 0.603-0.939 0.784 322.102 4.983 83.46 1.55 < 2:1 1A 21.789 64.217 BCN, Basic Chromosome Number; ΣTCL= Summation of total chromosome length; CV= chromosome volume; CLR=Chromosome length range; VRC=value of relative chromatin; ΣCVCL=Summation coefficient of variation in chromosome length; DI=dispersion index; GI=gradient index. 96 Sanjay Kumar, Asikho Kiso University and Banaras Hindu University for providing the instrumental facilities to analyze the samples. 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