CHROMOSOME NUMBERS AND PHYLOGENY IN MELAMPODIUM (COMPOSITAE)

The haploid chromosome numbers of n = 9, 10, 11, 12, 18, 20, 23, 25 i 1, 27, 30, and 33 have been reported by various authors from 26 of the 37 recognized species of Melampodium. A chromosomal survey of 375 plants from 275 different populations suggests that the recorded numbers are stable within the genus and that infraspecific euploidy and aneuploidy are uncommon. These chromosome numbers can be arranged numerically, with morphological and limited cytogenetic substantiation, into four euploid series of x = 9, 10, 11, and 12. Of these four groups of species, the x = 10 series is the largest and morphologically most diverse. This consideration, along with additional evidence from the morphology of sterile disc ovaries, suggests that x = 10 is the ancestral chromosomal base in Melampodium. A comparison of morphological and cytological data from the closely related genera, Acanthospermum and Lecocarpus, indicates that the latter are probably on a common base of x = 11. Present day distributional patterns of all three genera support the hypothesis that x = 10 is the ancestral base for the entire complex. THE GENUS Melampodium L. is recognized currently to contain 37 species that are distributed mainly throughout Mexico and Central America (Stuessy, 1968, 1970a, 1971b). Chromosome numbers in the genus have been reported by several workers (see Stuessy, 1971a, for a review) and a remarkable diversity of haploid numbers has been discovered: n = 9, 10, 11, 12, 18, 20, 23, 25 4? 1, 27, 30, and 33. This chromosomal series has the widest range among all 24 genera (Stuessy, unpublished) of the subtribe Melampodiinae, and is one of the widest within the entire family (Darlington and Wylie, 1955; Cave, 1958-65; Ornduff, 1967-69; Moore, 1970). The purpose of this paper is to synthesize the published chromosomal information in conjunction with morphological and cytogenetic data and to interpret phylogenetic relationships both within Melampodium and among it and two allied genera, Acanthospermum Schrank and Lecocarpus Dcne. Before phyletic inferences based on chromosome numbers can be advanced, however, the limits of infraspecific variation must be understood clearly. This point is well illustrated in Claytonia viriinica L. (Portulacaceae), in which wide-ranging aneuploidy is rampant even among plants within a single population (Lewis, 1962, 1970a, b; Lewis, Oliver, and Suda, 1967). Phyletic judgments based on only a few counts from such chromosomally unstable taxa would be very misleading. Consequently, the various species in Melampodium have been surveyed in order to reveal the extent of infraspecific chromosomal variability. ' Received for publication 10 February 1971. Appreciation is expressed to Drs. E. F. Paddock, B. L. Turner, and S. B. Young for constructive criticism of the final manuscript. Chromosome counts have been reported for 26 of the 37 species of Melampedium (Table 1). Some species, such as M. americanum, M. cinereum, M. divaricatum, and M. leucanthum, are known chromosomally from many plants in populations throughout their distribution, while other taxa are known from only one plant (e.g., M. costaricense, M. cupulatum, and M. lon3 icorne). Most of the species, however, have been counted from at least several populations. and the chromosome numbers appear mostly uniform and stable within a single taxon. Infraspecific aneuploidy and euploidy are known only in a few species of Melampodium (M. cinereum, M. dicoelocarpum, M. leucanthum, M. paniculatum, and M. perfcliatum). These cases are acknowledged in assessing the evolutionary relationships. Of the 11 uncounted species, four are known only from isolated type localities. It is hoped that cytological material from the remaining se-en species will be obtained in the near future. Particularly interesting will be material of M. bibracteatum S. Wats. and M. repens Sesse & Moc., which together comprise the section Bibractiaria Stuessy,2 the only section of the genus with no chromosome counts available. Cytological collections of both these taxa have been made previously, but no satisfactory counts have been obtained. In spite of these gaps, it is believed that sufficient cytological data now exist to warrant discussing probable evolutionary relationships within the genus. 2 The three new sectional names in Melampodium used in this paper are not validly published here. The rationale for introducing these names, however, is to allow for unequivocal correlation of the present discussions with the new sections that are to be validated in a forthcoming revision of the genus.