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Response to Inbreeding in Alfalfa Populations Derived from Single Clones¹

Spring growth and plant heights at each of three harvest times in each of 2 years were studied in progenies from three clones each in ‘MSA-W4’ and ‘MSB-W4’ alfalfa (Medicago sativa L.). The generations from five of the clones were S₀, S₁, S₁-SYN-1, S₃, and S₃-SYN-1. One of the clones from MSB-W4 had the S₄-SYN-1 generation, but not the S₁-SYN-1. The data were analyzed by multiple regression on the coefficients of inbreeding, by Gallais's genetic model, and by a newly developed genetic model for autotetraploid populations.

Except for first harvest height in MSB-W4, additive and digenic genetic effects were responsible for more than half the total genetic variation among and within families. Trigenlc or quadrigenic effects for individual families were frequently significant, but significant average trigenic or quadrigenic effects were rare. Deviations from neither genetic model were significant, but better fits were obtained with my model and regression on the coefficients of inbreeding than with Gallais's model.

Large differences due to parental clone were observed in the response to inbreeding—two clones showed almost no response and others were drastically reduced in vigor by the S₃ or S₃-SYN-1 generations. The frequent presence of an inbred generation with greater vigor than a less inbred, previous generation was attributed to unavoidable selection during development of the inbred progenies.

Key Words: Coefficient of inbreeding • Genetic models • Autotetraploids • Medicago sativa L.

² Research agronomist, U.S. Regional Pasture Research Laboratory, ARS-USDA, University Park, PA 16802.

Received for publication September 18, 1975.