II.8. Allelic diversity in the U.S.D.A. world barley collection.
A. L. Kahler and R. W. Allard. Department of Genetics, University of California, Davis, California 95616, U.S.A.
A survey of 1,500 entries in the U.S.D.A. world collection of cultivated (Hordeum vulgare) and wild (H. spontaneum) barleys has shown that wide differences exist in the number of alleles which occur at different enzymatic loci. The numbers of alleles found to date at seven esterase loci (designated EA, EB, ....EG), one peroxidase locus (PxA) and two acid phosphatase loci (PaA, PaB) are: EA, 7: EB, 12; EC and ED; 6 each; EE, EF, and EG, 2 each; PxA, 2; PaA, 7 and PaB, 2.
The electrophoretic techniques used in this survey were those described by Kahler and Allard (1970) for esterases; the techniques for peroxidases and acid phosphatases were modified from those of Shaw and Koen (1968). At least 4 seedlings were assayed for each accession. A standard stock of known banding pattern was included in each gel to aid in establishing migrational distances. However, despite careful standardization, it was necessary to consider bands with closely similar migrational distances (<0.1 to 0.2 centimeters) to be the same. For accessions which gave a band or bands at a position or positions distinctly different (>0.2 cms.) from the positions of known alleles (Kahler and Allard, 1970; and unpublished) mixed extracts were prepared from a composite tissue sample of such accessions and a set of appropriate standard stocks. When band positions were determined to be different by this method formal genetic studies (analyses of F2 and/or test cross segregations) were initiated to verify the allelism of the new variant.
The Esterase A locus is characterized by single-banded homozygotes and double-banded heterozygotes. Six band positions (0.2, 1.0, 1.8, 2.0, 2.3 and 2.6 cms. from the origin when the frontal zone has migrated 8.0 cms.) and a null expression (no band) have been observed at this locus. Five of the banded allozymes have been verified by formal genetic studies to be governed by codominant alleles. These alleles are designated EA 0.2, EA 1.0, EA 1.8, EA 2.0, and EA 2.6, the numbers corresponding to migrational distances. The allelism of the null expression has also been verified by formal genetic studies, the null allele (EAN) being consistently recessive to banded. Formal genetic studies of the band at 2.3 cms. have not been completed but its allelism can be inferred from the appearance of bands at 2.0, 2.3 and 2.6 cms. in mixed extracts from individuals with single bands at these positions.
Bands representing the EB locus overlap those of the EA locus in migrational distance but are distinguishable because the EB locus bands stain more darkly. Eleven band positions and a null expression have been observed for the EB locus. The allelism of 5 of the banded codominant allozymes (EB 1.6, EB 2.0, EB 2.7, EB 3.0, EB 3.9), and the recessive null allele (EBN) has been verified by F2 and/or test-cross segregation ratios. The remaining banded allozymes (1.4, 1.8, 2.3, 3.4, 3.6, 4.0) have been distinguished by composite sample tests.
At the EC locus, five double-banded expressions (homozygotes are double banded and heterozygotes quadruple banded at this locus) and a recessive null expression have been found. The allelism of three of the banded allozymes (designated EC 4.4, EC 4.9, EC 5.4 from the position of the darker staining lead band) and the recessive null expression has been verified by formal genetic studies. Allozymes at positions 3.9 and 5.8 were identified by the composite sample test.
Six alleles have been established by formal genetic studies for the ED locus, a recessive null allele (EDN) and 5 codominant banded alleles (ED 6.2, ED 6.4, ED 6.5, ED 6.6, ED 6.8). Homozygotes at this locus are double-banded, and, as for locus EC, alleles are designated by the position of the faster migrating band.
The EE, EF and EG loci each have a single allele producing a banded expression (EE 6.9, ER 7.3, EG 8.0) and a recessive null allele (EEN, EFN, EGN).
The peroxidase A locus (PxA) has two alleles, one giving a band at 1.2 cms. (PxA 1.2) and the other giving a recessive null expression.
Six quadruple-banded expressions at 1.3, 1.5, 1.7, 2 3, 2.5 and 2.8 cms., and a null expression, have been found for the acid phosphatase A locus (distances refer to the second and darkest staining band from the origin). The results of inheritance studies and mixed extract experiments are consistent with single-locus inheritance in which the six quadruple-banded expressions represent homozygotes of six different codominant alleles, and the null represents the homozygous expression of a recessive allele. However, the data available are still inadequate to eliminate the possibility that the observed banding patterns are governed by two or more closely linked loci, or that the multiple banded expressions represent a conformer series (e.g. Cann, 1966; Shaw, 1968).
The acid phosphatase B locus was until recently believed to be invariant in the barley species on the basis that a band had appeared uniformly at 5.5 cms. in more than 500,000 individual plants examined in this laboratory. However, a single accession in the U.S.D.A. collection with null expression at this locus has now been found, establishing that mutation occurs at this locus and that certain mutants are viable and can be incorporated into populations.
Since 20 or more accessions from each of the barley growing regions of the world were represented among the 1,500 entries assayed it seems likely that this study has identified a high proportion of the allozymes in the barley species which have distinctly different migrational mobilities (> 0.1 to 0.2 cms.) in starch gels. Detailed descriptions of the geographical distribution of these allozymes, and factors responsible for the observed distributions are given in several forthcoming publications (e.g. Allard, Kahler, and Weir, in press; Weir, Allard and Kahler, in press; Kahler and Allard, in preparation).
This research was supported in part by grants from the National Science Foundation (GB-13213) and the National Institutes of Health (GM-10476).
Allard, R. W., A. L. Kahler and B. S. Weir. The effect of selection on esterase allozymes in a barley population Genetics (in press).
Cann, J. R. 1966. Multiple electrophoretic zones arising from protein buffer interaction. Biochemistry 5:1108-1112.
Kahler, A. L. and R. W. Allard. 1970. Genetics of isozyme variants in barley I. Esterases. Crop Sci. 10:444-448.
Kahler, A. L. and R. W. Allard. Worldwide patterns of variation of esterase allozymes in barley (Hordeum vulgare and H. spontaneum). (In preparation).
Shaw, C. R. 1968. Molecular basis of isozymes. Proc. 12th Int. Cong. Genet. p. 31-35.
Shaw, C. R. and A. L. Koen. 1968. Starch gel zone electrophoresis of enzymes, in Chromatographic and Electrophoretic Techniques, Edited by I. Smith, Vol. 2:325-364.
Weir, Bo S., R. W. Allard, and A. L. Kahler. Analysis of complex allozyme polymorphisms in a barley population. Genetics (in press)
BGN 2 toc
BGN Main Index