Improvement of Fertility and Herbage Yield by Selection within Two-Allele Populations of Tetraploid Alfalfa1
- T. W. Pfeiffer and
- E. T. Bingham2
Two tetraploid alfalfa (Medicago sativa L.) clones were obtained by chromosome doubling two cultivated diploid clones. The doubled plants were selfed and progeny from within each group were sibbed for four generations to develop two populations of tetraploid alfalfa, HG2 and W315, with a maximum of two alleles per locus. Selection was based upon winter survival and differential fertility in the self and first two sib generations. In the third and fourth sib generations visual selection for vigor was practiced. Inbreeding increased from F = 0.33 to F = 0.54. Crosses between the two populations, and with two different male sterile testers, were made with the initial doubled plants and the second and fourth sib generations. Crosses between the advanced generations of the two populations yielded significantly more forage in progeny yield tests than the cross of the initial doubled plants. For the HG2 population progeny from crosses to male sterile testers by the advanced generations yielded significantly more than progeny from similar test crosses using the initial parent. An increase in cross fertility with advancing generations occurred in the same population. Forage yields of clonal rows were two to three fold greater for the fourth sib generation than for the initial doubled plant in the HG2 population. Changes in the W315 population over generations were small and variable but occurred while the level of inbreeding was increasing in the population. Allelic heterozygosity could not increase in these populations. No new dominant alleles were added to mask unfavorable recessives in a linkage block. Assuming that interlocus interactions are absent, improvement must have come from the accumulation of favorable alleles already present due to recombination within linkage blocks.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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