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Natural and Mass Selection Estimates of Relative Fitness for the Soybean rps Gene¹

Relative-fitness estimates were obtained for the rps gene that controls susceptibility to phytophthora rot in the soybean [Glycine max (L.) Merr.] Composites of resistant and susceptible varieties carrying phenotypic markers were used. With natural selection under epiphytotic conditions, rpsrps was 75% as reproductive as RpsRps in 2 years and only 35% as reproductive in another year. Mass selection through sieving for larger seed further reduced the relative fitness of rpsrps. The estimates were applied to a theoretical bulk-hybrid population segregating for Rps and rps to show the probable effects of natural and mass selection in increasing the proportion of resistant plants.

Key Words: Glycine max (L.) Merr. • Phytophthora rot resistance • Phytophthora megasperma Drechs. var. sojae Hildeb. • Soybean composites

² Research Scientists in soybean breeding and pathology, resepectively. The technical assistance of Mr. R. D. Walker and Mr. C. G. Fitzios is acknowledged.

Received for publication June 9, 1971.