CHANGES IN THE GENETIC DIVERSITY OF COTTON IN THE USA FROM 1970 TO 1995

Despite concern about genetic vulnerability, little is known about the recent changes in the genetic diversity of upland cotton (Gossypium h irsutum L.). The objectives of this study were to determine the trends in the genetic diversity of upland cotton in the USA during the last 25 yr and to investigate probable causes for these changes. Two estima tes of genetic diversity, coefficient of parentage (r(p)) and field un iformity (r(r); r(p) weighted by the proportion of the hectarage occup ied), were made for cultivars occupying over 1% of the hectarage withi n a region (Southeast, South-central, Southwest, and West) from 1970 t o 1995 at 5-yr intervals. An average of 17 (range = 8-46) cultivars ac counted for 97% of the cotton hectarage within a region. Regional r(p) values were relatively stable at 0.12 to 0.15 from 1970 to 1990 and t hen sharply increased to 0.20 in 1995. Higher r(p) values in commonly grown cultivars than in released cultivars indicated that much of the genetic diversity in cotton remains unused by growers. Field uniformit y (r(f)) remained at about 0.30 for all regions during the past 25 yr because increases in r(p) were matched with an increase in the number of cultivars grown and/or a decline in the proportion of the area plan ted to any single cultivar. The frequent use of several parents for th e creation of new cultivars and the planting of only a small portion o f the available cultivars has led to a high level of genetic uniformit y.