CHROMOSOME HOMOLOGY IN SOME INTERCONTINENTAL HYBRIDS IN HIBISCUS SECT. FURCARIA

Ten kinds of interspecific hybrids were obtained involving the following species: H. surattensis L. (2x, genome constitution BB), H. sudanensis Hochr. (2x, GG), and H. rostellatus Guill. and Perr. (4x, GGHH) from Africa; H. furcatus Roxb. non Willd. (8x) from India and Ceylon; H. furcellatus Lam. and H. bifurcatus Cav. (both 4x, PPQQ) from South America; and H. heterophyllus Vent. (6x) from Australia. Chromosome pairing in pollen mother cells (PMC's) at metaphase I in the 4x hybrids H. bifurcatus-rostellatus and H. furcellatus-rostellatus indicated that the parents have one genome in common (Q = G or H). Hibiscus furcatus was shown earlier to have a B genome; hybrids of H. surattensis-sudanensis F, X furcatus were hexaploid, having received an unreduced gamete from their hybrid parent, and had approximately 36 II, 36 I in PMC's. The genome formula of H. furcatus may therefore be designated BBGGWWZZ. The hybrid H. rostellatus-furcatus (BGGHWZ) confirmed that H. furcatus has a G genome in common with H. rostellatus; pairing of the other three genomes was inconsistent, as was that in H. rostellatus-heterophyllus. Some samples of the latter approached 36 II, 36 I, expected if H. heterophyllus were GGHHJJ; other samples had less pairing. Hibiscus furcatus-heterophyllus hybrids apparently arose from unreduced gametes of H. heterophyllus and originated as decaploids rather than heptaploids; chromosome number was unstable in PMC's. Nevertheless, multivalents, especially trivalents, were frequent enough to suggest that H. furcatus and H. heterophyllus share G genomes. On the other hand, an 8x H. bifurcatus-furcatus hybrid, which apparently arose from an unreduced gamete of H. bifurcatus, had a low multivalent frequency. Hybrids were obtained of H. heterophyllus X sudanensis and H. surattensis-sudanensis X heterophyllus, but the plants were weak and were not analyzed cytologically. We suggest that the New World, African, Indian, and Australian genomes which retain a considerable degree of homology (G or H or both) were distributed by land prior to separation of the southern continents by continental drift. MANY PLANT GROUPS with primarily austral distributions on two or more continents have been recognized (Good, 1927; Camp, 1947). It has long been suspected that such distributions reflect former land connections betweeni southern continents (Good, 1947). Revival of interest in, and new evidence for, the Wegnerian theory of continental drift (Bullard, Everett, and Smith, 1965; Vine, 1966; Hurley, 1968; Gilluly, 1969; Orowan, 1969; Dietz and Sproll, 1970; MlcElhinny and Luck, 1970) lend credence to the idea that the primary diversification and dispersal of such groups occurred prior to final severing of the land masses that made up the ancient supercontinent of Gondwanaland (Hawkes and Smith, 1965; Fryxell, 1965). 1 Received for publication 19 August 1970. This research was supported by Atomic Energy Commission Contract No. AT-(40-1)-3348. The authors wish to acknowledge the help of Terry Ashley, J. K. Shull, Barbara B. Smith, D. A. Wise, and Karla G. Qazilbash, who participated in the intensive hybridization program during the winter of 1968-69 which produced many informative new hybrids, including the ones described here. Special thanks are extended to E. E. Register, without whose expert care many of the hybrids would not have survived, and to F. D. Wilson and P. A. Fryxell for many stimulating discussions. Several taxa in the Malvaceae have this type of distribution, among them several groups of Hibiscus species (Camp, 1947), Cienfuyosia (Fryxell. 1967), Radyera (Fryxell and Hashmi, personal communication), and Gossypium. Of these, only Gossypium has been thoroughly studied cytogenetically (see Anon., 1968 for a recent summary of geographical distributions and genome relationships). The diploid and polyploid species of Hibiscus L. with the base number x = 18, which comprise sect. Furcaria DC. emend. Hochr., afford a striking example of a group with a Gondwanaland distribution. Chromosome pairing has been analyzed in a number of interspecific hybrids in sect. Furcaria (Menzel and Wilson, 1969a; Menzel and A\'1artin, in press). The genome homologies thus established show that a minimum of seven, and perhaps 15 or more, different genome groups exist among the 26 species which have been available for study. Tentative genome formulae have been assigned to 15 species with the geographical relationships shown in Table 1. In addition, it has been shown that octoploid H. furcatus Roxb. non Willd. from India and Ceylon has one B and 3 non-B genomes, that octoploid