Abstract

Key words

Citrullus colocynthis – C. vulgaris - wide hybridisation – cross compatibility

Introduction

Indian hot desert occupies an area of 0.28 million square kilometer situated mostly in the states of Rajasthan, Gujarat and Haryana. Inspite of abundant sunshine the principle-limiting factor for cultivation in this region is water. Tumba (Citrullus colocynthis Schrad.) is an unexploited perennial creeper growing wild in hot Indian arid zone. Being a perennial creeper with luxuriant growth, its soil binding capacity is of considerable significance. It has a potential to yield around one million tones of the oil rich seeds from the arid districts of Rajasthan. It is an important constituent of Ayurvedic system of medicine. Citrulline is extracted from the fruit pulp, which is internationally marketed by Sigma and Emerc. Cucurbitaceae is a difficult family for the study of specific and generic segregation as excess of parental zygotes indicates operation of powerful restriction to recombination in interspecific crosses (Adams 1967). Therefore, present study was undertaken to investigate the crossability, pollen fertility and cytomorphology of the F₁ hybrids of a cross between C. colocynthis and interspecific tumba derivatives (established lines obtained through hybridization between C. vulgaris and C. colocynthis).

Materials and Method

In order to obtain F₁ hybrids crosses were attempted between C. colocynthis cv. GP 177 and interspecific tumba derivative viz., TD 7 [pedigree: local matira (C. vulgaris) x local tumba (C. colocynthis) 20-0-6-5-1] in both directions at Agricultural Research Station, Durgapura, Jaipur, Rajasthan, India. To calculate pollen fertility anthers from freshly opened flowers were dusted on the micro-slides and released in acetocarmine (1%) mixed with a little glycerol and mounted using a cover slip. Deeply stained and well-developed pollen grains were counted as fertile. Meiotic studies were carried out in young flower buds of appropriate size fixing in Cornoy’s Fluid (1:3:6) with a drop of ferric chloride (5%). Buds were fixed for 24 hours and then transferred to the alcohol (70%) for storage until studies were made by the acetocarmine (1%) squash technique.

Results And Discussions

Crossability

Only one fruit (392 seeds) was obtained from straight cross (out of twenty one buds pollinated), whereas two fruits (1195 seeds) were obtained from the reciprocal cross (out of twenty-four buds pollinated). Results of crossability revealed that crosses involving C. colocynthis as female parent had lower percent of fruit setting (4.76%) than the reciprocals (8.33%). The species studied in present investigation exhibited different degree of female cross compatibility (Sain and Joshi 2002).

Pollen fertility

The pollen fertility in hybrid combination was lower than in parental genotypes (GP 177 = 97.5% and TD 7 = 95.6%) with uniform pollen grain (Table 1). Singh (1978) reported 93-95 per cent pollen fertility in C. colocynthis and C. vulgaris. Pollen fertility of F₁ hybrids involving C. colocynthis as female parent was lower (30.1%) than the reciprocal (80.2%). Such sterility promotes out crossing and selection of chromosomally balanced genotypes by sieving out unbalanced gametes. Hence, it might be possible to select some useful gene combinations that have arisen from introgression or chromosomal exchange.

Morphology

The F₁ plants of the hybrid resembled female parent for many morphological attributes. The hybrid exhibited intermediary values between the parents for leaf area, days to opening of female flower, 1000 seed weight and oil content. While for fruit circumference, number of fruits per plant, fresh fruit weight, seed weight per fruit and number of seeds per fruit, hybrid was inferior to either of the parent (Table 1). Pulp taste and seed colour was similar to female parent. In reciprocals leaf area increased due to increase in number of sub-lobes over better parent. As regards quantitative attributes viz., fresh fruit weight, seed weight per fruit, percent seed testa and oil content, hybrid depicted intermediary values whereas for number of fruits per plant and number of seeds per fruit, it was inferior to either of the parent. For days to opening of female flower, fruit circumference, seed colour and 1000 seed weight, hybrid was similar to female parent (Table 1). Pulp taste was intermediate. The flowers of reciprocal F₁ hybrids were normal whereas those of hybrids C. colocynthis x C. vulgaris exhibited deformed anthers and flower shedding which resulted in reduced seed setting and delayed maturity of fruits. Smartt (1970) and Patel et al. (1998) also reported such reciprocal differences in F₁ hybrids of species crosses. Crosses involving C. colocynthis as female parent exhibited higher rate of seed abortion than their reciprocals.

Cytology

In this hybrid of cross GP 177 x TD 7 majority of chromosome configurations was 1^IV + 9^II (17.8%). Quadrivalents (0-2) and trivalents (0-1) were observed in 28.9 per cent and 31.2 per cent of pollen mother cells (Table 2), respectively. The average chromosome association per PMC was 0.31^IV + 0.31^III + 8.96^II + 1.91^I. Regular distribution of chromosomes was recorded in 41.2 per cent of cells. Amongst abnormalities maximum (23.5%) was due to laggards at anaphase I followed by unequal distribution of chromosomes (17.6%). Bridge-fragment was observed in 3 out of 51 PMCs examined. The F₁ hybrid of reciprocal cross i.e. TD 7 x GP 177 was characterized by chromosome association 11 bivalents in majority (28.0%). Quadrivalents (0-2) and trivalents (0-1) were recorded in 30.9 per cent and 18.0 per cent of PMCs, respectively (Table 2). Average drawn for hybrid was 0.33^IV + 0.18^III + 9.38^II + 1.36^I (Table 2). Normal distribution of chromosomes at anaphase I was exhibited by 90.9 per cent of cells. While 6.8 per cent cells were noticed with laggard at anaphase I and II. The hybrids involving C. vulgaris as female parent exhibited normal bivalent formations (11^II) in majority of PMCs. Univalent mean per PMC was higher in crosses involving C. colocynthis as female parent and bivalent mean in the crosses involving interspecific tumba derivative (Table 2). These results are in agreement with the reports of Singh and Yadava (1984) and Sain et al. (2002).

The results of meiotic analysis of chromosome association in the F₁ hybrids showed formation of 8.96^II bivalents. This indicated on an average out of 11 pairs of chromosome around 9 pairs were able to associate normally This indicated the possibility of high recombination and thereby genetic introgression of desirable traits through hybridization between C. colocynths and established interspecific tumba derivatives. Results of cytological analysis further confirmed the reciprocal differences observed in crossability, pollen fertility and morphological characters of the F₁ hybrid. Results of present study have clearly revealed that involving interpecific tumba derivative as seed parent, superior recombinants with higher frequency could be isolated. Present trend indicated that interspecific derivatives with broader genetic base, large seeds, thin testa and with low seed-pulp ratio have enough scope for increasing the productivity under arid conditions. Intensive hybridization among selected interspecific derivatives and superior recombinants at different stages of advancement would help in releasing cryptic variability and providing useful segregants with higher test weight and thinner testa.

Table 1: Mean values for quantitative, qualitative attributes and pollen fertility of GP 177, TD 7, and their F₁ hybrids

Table 2: Chromosome configuration at diakinesis/ metaphase I in the hybrids of cross C. colocynthis cv. GP177 x Interspecific derivative cv. TD 7 and reciprocal

References

1. Adams, N.W. 1967 Basis of yield component compensation in crop plants with special reference to field bean Phaseolus vulgaris. Crop Sci.,7: 505-510.

2. Patel, O.P., Verma, R.C., Upadhyaya, S.N., Kulmi, G.S. and Singh, C.B.1998. Morphological studies on intergeneric and interspecific hybrids of some grain legumes. Indian. J.

3. Sain, R.S., Joshi, P. and Divakar Sastry, E.V. 2002. cytoenetic analysis of interspecific hybrids in genus Citrullus (Cucurbitaceae). Euphytica., 128: 205-210.

4. Sain, R.S. and Joshi, P. 2002. Cross compatibiltiy between wild and cultivated species of Citrullus. Indian J. Genet., 62(2): 171-172.

5. Singh, A.K. 1978. Cytogenetics of semi-arid plants III. A natural interspecific hybrid of Cucurbitaceae (Citrullus colocynthis x Citrullus vulgaris Schrad). Cytologia., 43: 569-574.

6. Singh, A.K. and Yadava, K.S. 1984. Cytogenetics of Cucumis III. An analysis of interspecific hybrids and phytogenetic implications.Plant Syst. Evol., 47: 237-252.

7. Smartt, J. 1970. Interspecific hybridization between cultivated American species of the genus Phaseolus. Euphytica., 19: 480-489.