PHENETIC ANALYSIS AND THE PHYLOGENY OF THE DIPLOID SPECIES OF GOSSYPIUM L. (MALVACEAE).

Studies of interspecific relationships, evolution, phylogeny, and the systematics of Gossypium are numerous. Phillips (1966) recently summarized the cytogenetic data upon which current views are based, and Fryxell (1969) presented a brief account of the systematics of the genus that is in accord with these data. Distributional maps are also available (Anonymous, 1968, Figs. 2, 3). Recent papers by Cherry et al. (1970) and by Johnson and Thein (1970) have utilized data on the electrophoretic analysis of seed proteins to assess species relationships in Gossypium and to draw phyletic conclusions. The two papers, although supporting one another in broad outline, do not agree in many specific details, and consequently reach certain different conclusions. For example, Johnson and Thein show G. armourianum and G. harknessii to have very similar patterns of protein bands, whereas Cherry et al. find them to be distinctly different. Conversely, the latter authors find G. anomalum and G. triphyllum similar, while the former authors find them to be different. In view of these discrepancies, it is difficult to assign significance to the seed protein data until the basis for these discrepancies can be clarified. Cytogenetic data can be uniquely valuable in providing evolutionary understanding, by demonstrating such things as aneuploid and polyploid series, and by tracing complex patterns of genome relationships among amphidiploids and their relatives. None of these phenomena occur, however, in the diploid species of Gossypium that are under consideration here. In Gossypium the cytogenetic data from which phylogenetic interpretations can be derived concern principally the degree of chromosome pairing in interspecific hybrids. Cytogenetic data have limitations. Certain hybrids have not been or cannot be produced, or are not viable even with such aids as embryo culture. Consequently, the data that can be applied to the question of species relationships have numerous omissions. Cytogenetic data, bearing on species relationships, may be gotten from: chromosome pairing behavior at meiotic metaphase I; chiasmata frequency at metaphase I; pachytene pairing behavior; or genetic segregation data in advanced hybrid generations. Data on pachytene pairing are not conveniently accessible in Gossypium. Genetic segregation data of diploid interspecific hybrids have been reported for only a few crosses (Saunders, 1961; Endrizzi and Brown, 1968), and in any case are only available for those species pairs that will produce fertile hybrids. Consequently, the available evidence is confined largely to data derived from studies of meiotic metaphase-specifically, measurements of univalent and bivalent frequencies and of chiasmata frequencies. These metrics are relatively insensitive measures of affinity. This insensitivity gives rise to the possibility of misleading conclusions about species relationships. Poisson et al. (1969) have noted other shortcomings of this kind of data in assessing species relationships in Gossypium. The assessment of evolutionary affinity from seed protein data is subject to several criticisms. The assumption that protein