5. An "Oenothera" or Multiple Translocation Method of Establishing Homozygous Lines.--A method by which a gametic combination could be made homozygous immediately should be of practical use to the plant breeder. One method, the utilization of haploids by doubling their chromosome number, has been suggested by many workers. It seems to be a feasible method in crops in which pollinations can be made on a large scale and genetic markers are available to aid in their recognition.
A second method for obtaining such homozygous lines is one I am calling an "Oenothera" or multiple translocation method. In this method, all the chromosomes of the haploid set are to be involved in translocations in such a way that the F1 of crosses with normal stocks will have at meiosis a ring containing the entire diploid number of chromosomes. Such a plant should produce two kinds of functional spores corresponding to the two parental gametic combinations of chromosomes. Among the offspring from selfing such a plant there would be the heterozygotes with the chromosome ring recognizable by high spore abortion; and in addition two types of normals, each homozygous for one of the two parental gametic combinations. These two types of normals would have normal pollen, the normal number of chromosome pairs, and could be distinguished by crossing them with standard normal stocks. The normal type not carrying the translocations would constitute the homozygous line.
The degree of homozygosity in these lines thus isolated depends on the amount of crossing over which has occurred at meiosis in the formation of the functional spores. Crossovers in the differential segments result for the most part in spores carrying interchanges and would be eliminated. Crossovers in the outer or interchanged arms of the chromosomes would be the ones most likely to result in recombinations of characters between the two parental gametes. The amount of recombination may not be very large, since crossing over is usually greatly reduced in regions near the translocation points and reduced to a lesser degree in regions farther away. It might be necessary, however, to establish several normal sub-lines from each F1 plant to eliminate, or at least to measure, heterozygosity from that source.
For practical use, the multiple translocation stock would be crossed with the heterozygous source being used for new gene combinations (e.g. a variety, or a single- or double-cross hybrid). Each F1 plant then represents a different gametic combination from that source combined with the multiple translocation gamete, and is the starting point of a different homozygous line to be established in F2. Selected lines thus isolated could be utilized in breeding tests similar to those used with lines heretofore established by continued inbreeding. The frequency of "superior" lines should correspond to the frequency of "superior" gametes in the heterozygous population being sampled. In this "Oenothera" method the gametic combination is established in homozygous condition immediately. In Stadler's "gamete selection" method, the selected gametic combination is combined with a gamete from an inbred line. Further breeding, selection and testing are necessary to isolate lines which carry at least part of the new germ plasm.
The "Oenothera" method has not been tried but crosses are under way by which it is hoped to eventually produce such a multiple translocation stock in corn. The plan of procedure is to choose for crossing only those translocations involving one chromosome in common in which the breaks in this common chromosome are far enough apart to furnish a "differential segment." A crossover in this segment will combine the two translocations in the same gamete.
Spore abortion will undoubtedly increase as more translocations are added, but it is hoped that it will not preclude dehiscence of the anthers or the production of sufficient seeds to utilize the method. It is possible that in the larger rings more of the disjunctions will fall into the zigzag type and thus reduce the degree of spore abortion.
C. R. Burnham