Physiology of Cotton PDF by James McD. Stewart, Derrick Oosterhuis, James J. Heitholt, Jack Mauney

TABLE OF CONTENTS

THE ORIGIN AND EVOLUTION OF GOSSYPIUM

INTRODUCTION
The genus Gossypium has a long history of taxonomic and evolutionary study. Much of this attention has been stimulated by the fact that the genus includes four domesticated species, the New World allopolyploids G. hirsutum and G. barbadense (2n = 52), and the Old World diploids G. arboreum and G. herbaceum (2n = 26). These cultivated species embody considerable genetic diversity, but this diversity is dwarfed by that included in the genus as a whole, whose 50 species have an aggregate geographic range that encompasses most tropical and subtropical regions of the world.

A remarkable morphological diversification accompanied the global radiation of Gossypium in response to the demands of particular ecological settings and selective environments. Plant habit, for example, ranges from fire-adapted, herbaceous perennials in northwest Australia to small trees in southwest Mexico that “escape” the dry season by dropping their leaves. Corolla colors embrace a rainbow of mauves and pinks (“Sturt”s Desert Rose”, G. sturtianum, is the official floral emblem of the Northern Territory, Australia), whites and pale yellows (Mexico, Africa- Arabia) and even a deep sulphur-yellow (G. tomentosum from Hawaii). Seed coverings range from nearly glabrous (e.g., G. klotzschianum and G. davidsonii), to short, stiff, dense brown hairs that aid in wind-dispersal (G. australe, G. nelsonii), to long, fine white fibers that characterize highly improved forms of the four cultivated species. There are even seeds that produce fat bodies to facilitate ant-dispersal (section Grandicalyx cottons from NW Australia). Much of this morphological diversity is lucidly detailed in Fryxell (1979), and need not be belabored here. Perhaps it is worthwhile, though, to express the truism that the morphological and ecological breadth must have parallels in physiological and chemical diversity. The wild species of cotton, consequently, represent an ample genetic repository for exploitation.

Although these wild species remain a largely untapped genetic resource, examples abound of their productive inclusion in breeding programs (e.g., Meyer, 1974, 1975; Fryxell, 1976; Narayanan et al., 1984; Niles and Feaster, 1984; Meredith, 1991). Further utilization of the many wild relatives of the cultivated cottons requires first that we understand their biology and relationships. This understanding grows from a combination of basic plant exploration, detailed taxonomic investigations, and phylogenetic studies designed to incorporate what is known about the biology of species into an evolutionary perspective.

Against this backdrop, it seems entirely appropriate to start a book on agronomic Gossypium with a review of what is known about wild and agronomically primitive Gossypium. This is the intent of the present chapter. Specifically, we first discuss the evolutionary origin of the cotton genus, and then focus on the taxonomy and diversification of Gossypium itself. This is followed by a synopsis of the biogeography of the genus and what is known about the origin of its many species. Particular attention is focused on the evolution of the New World allopolyploids, including the lineage to which G. hirsutum and G. barbadense belong.

Finally, we discuss the development of the cultivated cottons, from their original domestication by aboriginal cultivators through the various stages of their progressive agronomic refinement.