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Effect of Selection for Self-fertility and Self-sterility in Alfalfa and Related Characters¹

Two cycles of selection for self-fertillty in alfalfa (Medicago sativa L.) doubled self-fertility, as measured by the percentage of ovules developing into seed after selling, and they increased cross-fertility slightly, as measured by the percentage of ovules developing into seed after crossing to unrelated tester plants. Selection for self-sterility reduced self-fertility by more than threefourths and cross-fertility by one-third to one-half. Based on 40 plants representing a wide range of self-fertility, correlations between self- and cross-fertility were 0.5 or 0.6, depending on the tester used in crossing. Selection for self-fertility increased the abundance of pollen, whereas selection for self-sterility decreased it. Both self and cross-fertillty were highly correlated with abundance of pollen. Number of ovules per flower and the size of ovules before pollination changed only slightly in response to selection and neither was correlated with self or cross-fertility.

By 96 hours after pollination, the ovules had become separated by size into two distinct groups. The smaller ovules, representing one group, either were not fertilized after pollination or aborted very soon after fertilization. The larger ovules, representing the other group, most probably were fertilized, but some aborted before producing seed. The most pronounced difference in the distribution after selling and crossing was the proportion of ovules in the two groups. Ovule growth occurred mostly at random within the ovary, but after selling, there was evidence that basal ovules were fertilized slightly less often than ovules nearer the style, suggesting a weak self-incompatibility system.

Key Words: Medicago sativa L. • Plant breeding methods • Seed production in alfalfa

² Research geneticist and agronomists, respectively, ARS, USDA, Raleigh, NC 27607.

Received for publication December 7, 1974.