Chromosomal modification is often a major factor in the evolutionary diversification of a large, rapidly evolving genus. Pectis, the largest and most widespread genus in the New World tribe Tageteae (Compositae), has radiated into a variety of habitats during its evolutionary history. Until relatively recently, however, little has been known of the role of chromosomal change in the evolution of the genus. Pectis was cytologically unknown until the studies of Raven and Kyhos (1961) and of Turner, Ellison, and King (1961). Subsequently, additional counts have been reported for several taxa (Table 1). To date, however, only a few of the species have been studied cytologically, and these only from a few populations. Of the four large genera of the Tageteae, Pectis has been the most poorly represented in cytological studies. This investigation is one of a series of studies of the evolution and taxonomy of Pectis. The genus reaches its greatest diversity in the warmer areas of North America, and it is from this region that most of the collections upon which this study is based were made. Through an examination of the chromosomes of as many species as possible, and from numerous populations of wide-ranging species, information regarding the variability of chromosomes in Pectis can be better correlated with the evolutionary history and systematics of the genus.



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