Several cross-pollinations were made between Solanum scabrum (2n = 6x = 72) and S. nigrum (2n = 6x = 72), S. americanum (2n = 2x = 24), and a C3 amphiploid of the cross S. villosum (2n = 4x = 48) × S. americanum (2n = 2x = 24) with a view to determine their phyletic relationship. It was found that the crosses between S. nigrum and S. scabrum, and S. scabrum and a "big-fruited form" were accomplished easily, whereas the cross S. scabrum × S. americanum was successful only when S. scabrum, the higher chromosomal form, was used as the maternal parent. The hybrids (F1) exhibited moderate pollen fertility and vigorous growth, and set fruits and seeds abundantly, even though meiotic abnormalities were encountered in most of the pollen mother cells. From the chromosome pairing behaviour in the hybrids, it is concluded that S. americanum is the diploid ancestor of hexaploid S. scabrum and S. nigrum. The latter two species might have evolved from the sterile triploid hybrids of S. villosum × S. americanum by spontaneous amphiploidy. Key words: Solanum, phylogenetics. polyploidy.
The black nightshade, Solanum nigrum and related species form a polyploid complex based on x=12 chromosomes (Stebbins and Paddock 1949). This polyploid series of closely related forms constitute a taxonomically "difficult" species complex. However , several earlier workers reported that S. nigrum has 3 cytotypes (Bhaduri 1933, Magoon et al. 1962, Tandon and Rao 1966, Edmonds 1977a) and these to be diploids (n=12) , tetraploids (n=24) and hexaploids (n=36). Recent experimental studies have established beyond doubt that the diploids with small bluish-black berries are S. americanum Mill ., while the tetraploids with orange-red ber ries are S. villosum. The binomial S. nigrum is retained with the hexaploid form which bears purplish-black berries. The "garden huckleberry" another species of S. nigrum complex attracted great deal of attention, since its origin and interrelationship with other species of the complex is obscure (Schilling 1981). This tribasic species is morphologically distinguishable from other species by its large purplish-black berries and large ovate leaves. Also this species is cultivated for its large fruits that are used for cooking and commercially as a source of food colouring (Francis and Harborne 1966). In the past there was confusion regarding nomenclature of this taxon. Linnaeus treated this as S. nigrum L. var. guineense L. Until recently, its correct name was thought to be S. melanocerasum All. Now it is established that S. guineense (L.) Mill., S. intrusum Soria, S. melanocerasum All. and S. memphiticum Mart. are synonyms of S. scabrum Mill. (Henderson 1974, Edmonds 1979). In the past there was only a single report by Henderson (1974) on natural crossing between S. scabrum and S. nod(orum Jacq. (=S. americanum), but he did not provide details of chromo some pairing. Since chromosome pairing behaviour in species-hybrids is of immense value in determining the relationships and origin of many plant species (see Goodspeed 1934, Sax 1935, de Wet and Harlan 1972), in the present investigation an attempt has been made to understanding the species relationships with the help of pairing behaviour of chromosomes in the species hybrids. Materials and methods The plant materials used in the present investigation are S. scabrum Mill. and S. ameri canum Mill. The seeds of former were obtained from Dr. J. M. Edmonds, Department of Agricultural and Forest Sciences, University of Oxford, United Kingdom. A stock of S. americanum raised from local collections maintained in the Department of Botany, Bharathidasan University, Tiruchirapalli, Tamil Nadu. Reciprocal cross-pollinations were made between the two species. Cytogenetic analyses were conducted on pollen mother cells undergoing meiosis. Young flower buds were fixed in Carnoy's fluid (6 parts ethanol; 3 parts chloroform; 1 part acetic acid) for 30 minutes. They were then transferred to propionic alcohol (1:3); the propionic acid
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