The genetic stability of partially inbred lines of maize after several generations of reproduction

Abstract

communication) has noted that S2 lines are relatively uniform phenotypically, while being more vigorous than highly inbred lines. In an earlier study, Loeffel (1971) suggested that it may be feasible to use S2 or S3 parental lines for hybrid seed production. The use of S2 lines to produce hybrids would have several advantages: (1) lower seed costs for the farmers because of the increased productivity of the S2 lines compared with highly inbred lines. (2) S2 lines would be less sensitive to environmental effects because of their greater heterogeneity. (3) Some good combining lines, which may normally be discarded because of poor seed or pollen production after extensive inbreeding, could be utilized at the S2 level because of increased vigor. Although the use of S2 lines has many advantages, there are also some potential disadvantages: (1) the hybrid progeny will have variability for plant and ear traits. (2) Off-type plants may be more difficult to detect in seed production fields. (3) S2 lines will be more difficult to manipulate than homozygous lines (i.e., conversion to cytoplasmic male sterility, introduction of specific genes by backcrossing). (4) The genetic heterogeneity within S2 lines may result in genetic changes because of drift during line maintenance, which may also cause changes in the hybrids. This study was initiated: (1) to determine if measurable changes of one or more traits have occurred in S2 maize lines after several 3 generations of maintenance (i.e., reproduction). (2) To obtain information for the number of plants needed in the maintenancce of an S2 line, such that genetic changes will not occur because of finite sampling. (3) To determine if mild selection during the maintenance of S2 lines can be used to influence some favorable changes in plant and ear traits. (4) If changes have occurred in the S2 lines, to assess the expression of these changes in single crosses. 4 LITERATURE REVIEW The Use of Heterogeneous Lines in Hybrids The widespread use of single-cross maize hybrids since the early 1960s has increased the importance of the performance of the inbred lines per se. Today's maize hybrids in the U.S. are usually produced by intercrossing two highly inbred lines selected for their good combining ability and performance per se. However, some studies have suggested the potential use of partially inbred lines for the development of hybrids. Davis (1934) was one of the first to explore the potential use of heterogeneous lines in the production of hybrid maize. His study involved using S2 (F = 0.75) lines that had been derived from local cultivars on the basis of their combining ability. These lines were subsequently crossed to a widely grown cultivar, 'Yuco Torre', and these crosses showed a significantly greater yielding ability than did the widely grown open-pollinated varieties. The further inbreeding of the S2 lines to the generation produced no yield advantage over the S2 lines when evaluated in hybrid combination with a tester. The potential use of lines as an effective method to i...