GrainGenes

II. 9. Defective endosperm xenia (dex) mutants.

R. T. Ramage and Chere L. Crandall. Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721, U.S.A. "R"

We have suggested that the symbol dex be used to symbolize those mutants expressing xenia that have endosperm so defective as to prevent stand establishment under normal field conditions (1). These mutants may be considered as zygotic lethals that express xenia. The first of this type to be described was given the symbol sex2b (2). We have proposed changing the symbol sex2b to dex1a (1).

This communication reports our collection of defective endosperm mutants and our progress in establishing allelism among them (Table l). All of the mutants reported in Table l are monofactorial recessives and all are of spontaneous origin with the exception of dex4d. The mutant dex4d was selected from a M2 population of the cultivar 'Betzes' that had been treated with the chemical mutagen, 'diethyl sulfate'. All of the defective endosperm mutants are zygotic lethals that express xenia and are maintained in heterozygous condition.

Table 1. Defective endosperm xenia (dex) mutants.

Defective endosperm mutants dex2b, dex3c and dex4d are phenotypically indistinguishable from dexla, which has been previously described as sex2b (3). Seed that are homozygous recessive for the mutations grow normally for a few days after fertilization. Then, growth stops and the seed begin to shrivel. At maturity, shrunken endosperm seed are so reduced in size that they are barely visible within the lemma and palea. Defective endosperm prevents stand establishment under normal field conditions. The mutants express xenia and behave as zygotic lethals. Seed produced on heterozygous plants are found in a ratio of about 3 normal, viable seed : 1 defective, inviable seed.

Five of the defective endosperm mutants (dex2b, dex4d, dex,,e, dex3f and dex,,h) have been allele-tested with the shrunken endosperm mutant, sex1a, and found to be non-allelic to it. The defective endosperm mutant, dex1a, is considered to be non-allelic to sex1a due to the fact that dex1a is located on chromosome 7 and sex1a is located on chromosome 6 (2).

Since the defective endosperm mutants are zygotic lethals, allele tests among them are accomplished by crossing heterozygous stocks. Allelic mutants, when crossed, will produce two types of F1 plants: those with all normal, plump seed and those with spikes segregating in a ratio of about 3 normal, plump : 1 defective endosperm, shriveled seed. The two types of F1 plants (normal and segregating 3 : 1) should be found in about a 1 : 2 ratio. Seed planted from Fl spikes segregating 3 normal : 1 defective endosperm seed are expected to produce two types of F2 plants (all normal and segregating  3 : l) in a ratio of 1 : 2.

If the mutants are non-allelic, three types of F1 plants are expected from crossing two heterozygous plants: those with all normal seed, those with spikes segregating in a ratio of about 3 normal : 1 defective endosperm seed and those with spikes segregating in a ratio of about 9 normal : 7 defective endosperm seed. The three types of F1 plants (normal, segregating 3 : 1 and segregating 9 : 7) should be found in a ratio of about 1 : 2 : 1. Seed planted from F1 spikes segregating 9 normal : 7 defective endosperm seed are expected to produce the same three types of plants (all normal, segregating 3 : 1 and segregating 9 : 7) in the F2 as expected in the F1, but, in a ratio of 1 : 4 : 4. The F2 allows us to confirm nonallelic crosses.

The results of allele crosses tested to date are shown in Table 1. Based on these results, dexla, dex2b, dex3c, and dex4d are all non-allelic and have been assigned to loci. The mutants dex3c and dex3f are allelic. Allele tests are being continued.

References:

1. Ramage, R. T. and C. L. Crandall. 1981. A proposed gene symbol for defective endosperm mutants that express xenia. Barley Genetics Newsletter 11: 30-31.

2. Scheuring, J. F. and R. T. Ramage. 1976. A tertiary trisomic balanced for both msg2 and sex2. Barley Genetics Newsletter 6: 60-62.

3. Scheuring, J. F. and R. T. Ramage. 1976. Description of Genetic Stocks - sex2h. Barley Genetics Newsletter 6: 134.

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