_??_1992 The Japan Mendel Society Cytologia 57: 289-293, 1992 Taxonomic Significance of Karyomorphology in lpomoea spp. Sangeeta Sinha and Shards Nand Sharma Dep. rtment of Botany, Patna University, Patna-800005, India Accepted December 5, 1991 Ipomoea L. is taxonomically a very difficult genus of the family Convolvulaceae. The spp. are ubiquitous confined mainly to the Tropics. They are herbaceous with climbing and trailing habits. On the basis of floral characters, Hooker (1885) divided the Indian Ipomoea spp. into six sub-genera, namely Calonyction, Quamoclit, Pharbitis, Aniseia, Batatas and Euipomoea. Hal lier (1893) revised the genus on the basis of pollen morphology and separated Merremia and Operculina from Ipomoea on this ground, non-spinous in Merremia and Operculina and spinous in Ipomoea. It is worth mentioning that Merremia and Operculina spp. were included within the sub-genus Euipomoea by Hooker (loc. cit.). There are only a few reports on the karyomorphology of Ipomoea spp. (Rao 1947, Sharma and Chatterji 1957, Sharma and Datta 1958, Nakajima 1963, Sampathkumar 1970, 1979). Therefore the present studies have been undertaken with a view to assessing as to how far the data from the critical karyotypic analyses support/contradict the concepts of taxonomic de limitations within Ipomoea spp. Materials and methods The following collections were made for the present studies: Ipomoea quamoclit Linn. (sub-genus, Quamoclit), I. hederacea Jacq. (sub-genus, Pharbitis), I. batatas Lamk. (sub-genus, Batatas), I. aquatica Forsk., I. palmata Forsk., I. pilosa Sweet., I. vitifolia Sweet. syn. Mer remia vitifolia Hall., I. sinuata Orteg. syn. Merremia dissecta Hall., I. turpethum Br. syn. Operculina turpethum Manso. (all the last six spp. under sub-genus Euipomoea). I. carnea Jacq. has also been described in detail in the present work although it has not been listed in either of the sub-genera of Hooker. While the somatic tissue of I. palmata, I. carnea, I. batatas and I. aquatica was obtained from adventitious roots of natural populations, in the rest of the species it was obtained from the seeds germinated on moist filter papers kept in petridishes. The healthy root-tips were pretreated in saturated solution of para-dichlorobenzene at 12-18•Ž for 3hr and fixed in Car noy's fluid (6:3:1-ethyl alcohol: acetic acid: chloroform). They were then squashed in aceto carmine following the normal procedure. Observations The chromosomes are small and appear to be highly variable, ranging from 1.04 to 4.58ƒÊ m. They have been classified as long, medium and short ones. They are given below in Table 1. The karyomorphological details have been given below in Table 2. The somatic number of all the taxa is 2n=30 except in I. batatas where 2n=90. The preponderence of 15 as the gametic number (Federov 1969) throughout the members suggests that the basic number for the genus is x=15. However, Love and Love (1961) have been of 290 Sangeeta Sinha and Sharda Nand Sharma Cytologia 57 the opinion that the primary basic number for the family Convolvulaceae is x=5, while x=15 represents a secondarily derived number. Thus 2n=90 as observed in I. batatas indicates its hexaploid nature. Table 1. Classification of chromosomes of Ipomoea spp. on the basis of length inƒÊm Table 2. Details of the somatic chromosomes of ten different species of Ipomoea M=Median, Sm=Sub-median, St=Satellite. A review of Table 2 reveals that certain features are common to all the ten taxa. These are: (1) The presence of median and sub-median type of chromosomes. (2) The frequency of median chromosome is higher than the submedian ones. (3) B, D and E types of chromosome are common in all the taxa examined. These cytological observations suggest Ipomoea to be a natural assemblage of species. 1992 Taxonomic Significance of Karyomorphology in Ipomoea spp . 291 The maximum and minimum chromatin length are 78 .99ƒÊm (I. vitifolia) and 55.44ƒÊm (I. pilosa). All other taxa come within these extremes (Table 2) except for I. batatas (a poly ploid) where it has been found to be 134.38ƒÊm. Contrary to this, the T. F. % is highest in I. quamoclit (47.18 %) and lowest in I . aquatica (42.18 %). Table 2 reveals that the grouping of taxa based on T. F. % values does not correlate with the grouping based on total chromatin length. It thus appears that the various kinds of aberrative forces have played vital roles in the evolutionary diversification of the snecies . The secondary constrictions constitute an important feature of chromosome for working out species relationships. Sharma and Datta (1958) and Sampathkumar (1979) have reported the occurrence of secondary constrictions in all these species, but in the present investigation, the satellited chromo somes have been observed to occur only in seven out of the ten species. The SAT chro mosomes are two pairs each in I. cornea (Fig. 2b), I. turpethum (Fig. 6b), I. aquatica (Fig. lb) and I. batatas (Fig. l0b) and only one pair/complement in species of I. sinuata (Fig. 4b), I. palmata (Fig. 7b) and I. pilosa (Fig. 3b). However, for estimating the relation ship between species, centromeric indices are more important than secondary constric tions. The latter ones may be lost or gained during the course of evolution (Lewis and John 1963). Figs. la-10a. Somatic metaphase chromosomes in Ipomoea spp. la, I. aquatica-2n=30 (•~705). The karyotypic formula of I. batatas 1B 2a, I. carnea-2n=30 (•~750). 3a, 1. pilosa-2n=30 (Sm, St)+1C (Sm, St)+1D (Sm)+IE (M)+ (•~705). 4a, I. sinuata-2n=30 (•~705). 5a, I. 2E (Sm)+3F (M)+4F (Sm)+25G (M)+7G quamoclit-2n=30 (•~300). 6a, I. turpethum-2n (Sm) clearly indicates its allohexaploid na =30 (•~700). 7a, I. palmata-2n=30 (•~705). 8a, I. ture. This contention is amply supported by hederacea-2n=30 (•~705). 9a, I. vitifolia-2n=30= Thompson (1951) who reported that the sweet (•~300). 10a, I. batatas-2n=90 (•~700). potato is of comparatively recent origin derived from amphidiploidy of a tetraploid (2n=60) and a diploid (2n=30) species. It is further evidenced by the fact that the plants of I. batatas generally do not produce flowers, and even if they do, seed setting is rare. Thus marked sterility of flowers has been attributed to high ploidy level. So far as the species interrelationship is concerned, the T. F. % values of the three non spinous pollen bearing taxa namely, I. vitifolia (=Merremia vitifolia), I. sinuata (=M. dissecta) and I. turpethum (=Operculina turpethum) indicate various degrees of overlapping relationships with the spinous pollen bearing counterparts but not among themselves (Table 2). Similarly, in respect of their total chromatin lengths, while I. sinuata (70.96ƒÊm) and I. turpethum (70.33ƒÊ m) come quite close to each other, I. vitifolia (78.99ƒÊm) stands widely apart. Moreover, the shortest chromosomes of G type have been observed in I. turpethum and I. sinuata (bearing non-spinous pollen) and in other spinous pollen bearing taxa but they are completely absent in I. vitifolia (bearing non-spinous pollen). Thus the results of the present karyomorphological studies fail to support the idea of supraspecific categorization of the species of Ipomoea L. as done by Hooker (loc. cit.). All the more, the present results also fail to agree with the paly notaxonomic delimitations of Hallier (loc. cit.) within the genus Ipomoea. 292 Sangeeta Sinha and Sharda Nand Sharma Cytologia 57 However, the critical karyotypic analyses have clearly indicated a pattern of close relation ship among the different species of Ipomoea L. under reference and that too in a reticulate manner, suggesting thereby that they form a compact natural assemblage within the genus. Figs. 1b-6b. Karyotype in Ipomoea spp. 1b, I. Figs. 7b-l0b. Karyotype in Ipomoea spp. 7b, I. aquatica (•~1050). 2b, I. carnea (•~1100). 3b, I. palmata (•~1050). 8b, I. hederacea (•~1050). 9b, pilosa (•~1050). 4b, I. sinuata (•~1050). 5b, I. I. vitifolia (•~500). 10b, I. batatas (•~1600). quamoclit (•~500). 6b, I. turpethum (•~1050). Summary In all, ten Ipomoea spp. have been karyomorphologically analysed. The data from the present studies analysed in terms of chromosome number, absolute chromatin length, cen tromeric index, T. F. % and representative chromosome types tend to indicate the various degrees of overlap establishing close relationships between the species. The genus Ipomoea has been suggested to be a natural assemblage showing a reticulate evolution. The splitting of Ipomoea spp. into distinct subgenera on morphological ground (Hooker 1885) or into sep arate genera on the basis of pollen exine character (Hallier 1893) does not find support from karyomorphology which is a better biological character. Acknowledgement Our sincere thanks are due to Prof. R. P. Sinha for going through the paper critically. Thanks are also due to the UGC, New Delhi for financial assistance. References Federov, An. A. (Ed). 1969. Chromosome Number of Flowering Plants. Academy of Sciences of U. S. S. R. Hallier, H. 1893a. Versuch einer naturlichen Cliederung der Convolvulaceen auf morphologischer and anat omischer Grundlage. Englar, Bot. Jahrb. 16: 453-591. Hooker, J. D. 1885. The Flora of British India. 4: 196-216. L. Reeve & Co. Ltd., U. K. Lewis, K. R. and John, B. 1963. Chromosome Marker. J. & A. Churchill Ltd. London, W. I. 1992 Taxonomic Significance of Karyomorphology in Ipomoea spp. 293 Love, A. and Love, D. 1961. Chromosome numbers of Central and North West European plant species. Opera Botanica Lund. 5: Nakajima, G. 1963. Karyotype of the genus Ipomoea. Cytologia 28: 351-359. Rao, N. S. 1947. Chromosome studies in the genus Ipomoea. Curr. Sci. 16: 156. Sampathkumar, R. 1970. Karyotype analysis in some South Indian Convolvulaceae. J.