Changes in the Genetic Diversity of Cotton in the USA from 1970 to 1995

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Abstract
Despite concern about genetic vulnerability, little is known about the recent changes in the genetic diversity of upland cotton (Gossypium hirsutum 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 estimates of genetic diversity, coefficient of parentage (rp) and field uniformity (ri; rp weighted by the proportion of the hectarage occupied), were made for cultivars occupying over 1% of the hectarage within a region (Southeast, South‐central, Southwest, and West) from 1970 to 1995 at 5‐yr intervals. An average of 17 (range = 8–46) cultivars accounted for 97% of the cotton hectarage within a region. Regional rp values were relatively stable at 0.12 to 0.15 from 1970 to 1990 and then sharply increased to 0.20 in 1995. Higher rp values in commonly grown cultivars than in released cultivars indicated that much of the genetic diversity in cotton remains unused by growers. Field uniformity (ri) remained at about 0.30 for all regions during the past 25 yr because increases in rp were matched with an increase in the number of cultivars grown and/or a decline in the proportion of the area planted to any single cultivar. The frequent use of several parents for the creation of new cultivars and the planting of only a small portion of the available cultivars has led to a high level of genetic uniformity.

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