Genetic Similarity Indices for Ancestral Cotton Cultivars and their Impact on Genetic Diversity Estimates of Modern Cultivars

The accuracy of pedigree‐derived genetic‐distance estimates depends on the availability of breeding records and on the validity of a number of assumptions. Pedigree analysis for cotton ( Gossypium hirsutum L.) indicates a wide genetic base, which conflicts with other types of distance estimates indicating a narrow genetic base. The objectives of this study were to determine genetic similarity indices from agronomic and morphological traits for ancestral cotton cultivars and to determine their impact on the coefficient of parentage (CP) of recent cultivars. Twelve ancestral and two recent cultivars were grown at three locations (Clayton, NC; Florence, SC; and Stoneville, MS) during 1995 and evaluated for agronomic and fiber properties. Multivariate analysis of agronomic and fiber properties was used to create genetic similarity indices ( s ). There was little agreement between CP and s . Several ancestral cultivars developed from the earliest (pre‐1800) introductions into the USA were very similar to the most recent (post‐1900) introductions from Mexico. This suggested that all the original Mexican introductions were genetically very similar. Changing the assumptions in pedigree analysis to assume the original introductions were related by the average s among the ancestral cultivars ( s = 0.38) decreased the level of diversity in frequently grown modern cultivars from CP = 0.16 to CP = 0.46 but had little effect on the relative ranking of cultivars. This occurred because effects of possible erroneous pedigree assumptions in distant generations became diluted. The high degree of similarity among ancestral cultivars suggests that pedigree‐derived genetic distance estimates have overestimated the genetic diversity among today's cultivars but not relative relationships.

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