WOOSTER, OHIO
USDA-ARS and Ohio Agric. Res. Dev. Ctr.

Conversions of the interchanges in reciprocal chromosomal translocations in maize to homozygosity for linkages to genes

--Findley, WR, Jr., Jones, M

Reciprocal chromosomal translocations have been successfully used to establish linkage relationships between translocation breakpoints and the genes that control important traits such as blight and virus resistance (Findley et al., Crop Sci. 18:608–611, 1973; Jenkins et al., Agron. J. 49:197–201, 1957). In 1946 the Agricultural Research Service, U. S. Department of Agriculture and the California Institute of Technology established a cooperative project to irradiate maize seeds during the atomic bomb explosion on the Bikini atoll. From this project, a large number of irradiated maize materials containing chromosomal translocations and chromosomal rearrangements were released and are available from the California Institute of Technology, Pasadena, California 91109 and the Maize Genetics Stock Center, S-123 Turner Hall, 1102 South Goodwin Avenue, Urbana, IL 61801-4798 (Longley, ARS 34-16, 1961; Freeling and Walbot, The Maize Handbook, Springer-Verlag, NY, p. 364, 1994). However, use of these stocks in linkage studies has been tedious and slow because microscopic examinations of pollen grains were needed to determine which plants carried the translocations.

In a heterozygous plant at the pachytene stage of microsporogenesis, chromatids with the reciprocally translocated chromosome segements pair with the homologous chromosome segements of their normal counterparts. This pairing of homologous segements leads to a configuration of a cross in the chromatids with the translocation breakpoints located at the center. Disjunction of the chromatids may occur along alternate or adjacent planes resulting in spores where one fourth of them are homozygous translocations, one fourth are homozygous normals, and one half contain unbalanced chromosome complements. Pollen grains with these unbalanced chromosomes do not accumulate starch and appear clear when viewed with a 40× microscope. This phenomenon was used to identify plants heterozygous for the translocation. These plants were self pollinated to obtain progeny plants that segregated as homozygous translocations, homozygous normals, and heterozygous translocations. When these progeny plants were testcrossed to a normal line, the homozygous translocations were differentiated from the homozygous normals by the 50% seed set of the semi-sterile progeny versus the completely fertile progeny. To avoid microscopic examination of pollen grains and simplify the selection process for useful genotypes, all translocations were converted to homozygosity.

Most of the translocation stocks listed in Table 1 were obtained from Dr. W. A. Russell, Iowa State University in the early 1960’s. Additional stocks were later obtained from the Maize Genetics Stock Center, Urbana, Illinois. Conversion to homozygosity of the translocation stocks was initiated during the season of 1985. During the conversion each stock was backcrossed to inbred M14 a sufficient number of times to obtain at least 10 doses of the recurrent parent. Marker genes su1, y1 o2, and wx were incorporated into the stock translocations and inbred M14 to mark the short arm of chromosome 4 (near centromere), long arm of chromosome 6 at .10, short arm of chromosome 7 at .16 and the short arm of chromosome 9 at .56. Some stocks were developed by repeated backcrosses to semi-sterile types; others by sib-mating semi-sterile types to plants classified as homozygous normal. In the development of these latter stock translocations, a close linkage with the translocation or marker gene was assumed. However, no amount of backcrossing to the recurrent parent would overcome linkages between blocks of genes that are close to points of chromosomal interchange (Jenkins et al., 1957).

The use of these converted translocations for linkage studies can be enhanced by the following procedures. Cross lines to a series of translocation stocks to mark each chromosome arm at least twice. Use F1’s or backcrosses to inbred M14 with an appropriate marker gene to express the trait and show linkage to a chromosome arm. Use translocations with breakpoints near the middle of the arm to test the entire chromosome arm. Determine gene placement to a chromosome arm by the presence or absence of the trait and the presence or absence of the translocation. Confirm an association of a gene to a chromosome arm by comparing its reaction to other chromosome stocks that have at least one arm in common with each other. Incorporate marker genes to identify the chromosome arm involved in expression of the trait. Lastly, use additional translocation stocks with breakpoints near the calculated gene site to produce more precise determinations.

In evaluation of the data, the frequency of plants showing reaction to the studied trait will indicate the degree of linkage to a chromosomal breakpoint. The degree of linkage also may be estimated by the frequency of plants that show the expected reaction to a trait (Findley et al., 1973). When appropriate marker stocks are used in gene linkage studies, the significance of the results may be determined by analyses of mean ratings for the observed traits. Presently, tests for the whole chromosome arm are possible with the forty-five converted translocation stocks, except for chromosome 6 and perhaps 10. However, to convincingly mark the short arm of chromosomes 3, 5, 8 and 9 two other stocks such as 3S.44 and 3S.56 and 8S.37 and 8S.67 may be required. Thirty-five homozygous translocation stocks were submitted to the Maize Genetics Stock Center in 1994, and the remaining 10 in 2002. In 2003, all 45 homozygous stocks should be available from the Maize Genetics Stock Center.

*Doses of M14
**Breakage points reported in Freeling and Walbot (1994). Other breakage points in A. E. Longley (1961).