Many nuclear male sterile genes have been assembled over a long period of years. I am indebted to several geneticists for submitting seed sample sources of male sterile types that occurred unexpectedly in their research plantings.
Upon receipt, each accession was tested initially to determine whether the sterility was cytoplasmically transmitted. For this purpose, the standard commercial inbred line WF9 has been useful since its nuclear genotype is nonrestoring for each of the cytoplasmic male sterile types cms-C, cms-T and cms-S. The procedure followed was to pollinate male sterile plants by WF9. If the progeny were male sterile, there was presumptive evidence that the sterility was cytoplasmically transmitted. Further tests were then conducted using diagnostic lines whose genotypes of nuclear restorer genes differentiate among cms-C, cms-T and cms-S cytoplasms.
These screening tests were conducted in order to eliminate cytoplasmic male steriles from further study. Screening with additional diagnostic lines provided evidence consistent with assignment to cms-T (4 sources) or cms-S (9 sources). Further confirmations of these assignments by tests of cytoplasmic components were not made. Tested sources are listed in Table 1.
Table 1. Cytoplasmic male sterile sources.
|RJL 5399||RJL 5209|
|USSR 29-54||Golden Glow (Palmer) (PI)|
|PI 213787||Manwiller (PI)|
PI sources represent numbered Plant Introduction accessions of maize varieties, or named varieties of PI accessions. PI accessions are propagated at the Regional Plant Introduction Station, Ames, Iowa. Seed samples are available from Ames upon request. PI262500 carried both cytoplasmic and nuclear male sterility.
All instances of male sterility with an apparent nuclear mode of inheritance were further confirmed by male transmission. The procedure was as follows. Male sterile plants that occurred in plantings of accessions were pollinated by WF9. F1 plants were self-pollinated and crossed as male parents on one or more standard commercial inbred lines free of nuclear male sterile alleles and carrying normal (non-sterile) cytoplasm. From selfing of F1 plants, F2 progenies were grown to confirm the segregation of male sterility. Progenies were also grown from the pollination of standard commercial lines by the same F1 plants. Several plants in each family were self-pollinated and progenies grown the following generation; it is expected that half of these progenies will segregate for male sterility due to recessive nuclear male sterile alleles that have been pollen-transmitted.
Table 2. Nuclear male sterile gene sources.
|Previous Designation||Information from Tests||New Designation|
|ACCO 1752||= ms10||ms10-6001|
|Alex 21096||= ms2||ms2-6002|
|Alex 78193||Group 1 (7L)||ms*-6004|
|Bear 1||= ms14||ms14-6005|
|Bear 2||Group 6 (9L)||ms*-6006|
|Bear 3||= ms7||ms7-6007|
|Bear 4||Group 3 (3L)||ms3-6008|
|Bear 5||Group 3 (3L)||ms3-6009|
|Bear 6||Group 1 (7L)||ms*-6010|
|Bear 7||Group 2 (9L)||ms*-6011|
|Bear 8||= ms2||ms2-6012|
|Bear 10||Group 1 (7L)||ms*-6013|
|Bear 11||Group 1 (7L)||ms*-6014|
|Holden 4439||= Holden 4442||ms*-6015|
|Holden 4442||= Holden 4439||ms*-6016|
|Holden 7469||= si1-at||si1-at-6017|
|Hooker 3879||Group 2 (9L)||ms*-6018|
|Hooker 5345||Group 7 (2L)||ms*-6019|
|Hooker 5472||Group 3 (3L)||ms3-6020|
|Hooker 8472||Group 4 (9L)||ms*-6021|
|Hooker 8508||Group 4 (9L)||ms*-6022|
|RJL (Lambert) M1||ms*-6023|
|RJL (Lambert) M11||Group 7 (2L)||ms*-6024|
|RJL (Lambert) M19||ms*-6025|
|RJL (Lambert) M37||ms*-6026|
|RJL (Lambert) M59||Group 2 (9L)||ms*-6027|
|RJL (Lambert) M70||ms*-6028|
|RJL (Lambert) M71||Group 7 (2L)||ms*-6029|
|RJL (Lambert) M72||= si1-at||si1-at-6030|
|RJL (Lambert) B76||Group 2 (9L)||ms*-6031|
|RJL (Lambert) 1015||= ms9||ms9-6032|
|RJL (Lambert) 1871||ms*-6033|
|RJL (Lambert) 5602||Group 8 (1S)||ms*-6034|
|RJL (Lambert) 7029||= ms10||ms10-6035|
|(RJL) PI 245138||ms*-6036|
|(RJL) PI 267218||= ms9||ms9-6037|
|(RJL) PI 262500||Group 7 (2L)||ms*-6038|
|JRL (Laughnan) 303||ms*-6039|
|JRL (Laughnan) 305||ms*-6040|
|JRL (Laughnan) 308||Group 7 (2L)||ms*-6041|
|JRL (Laughnan) 1623||= ms9||ms9-6042|
|JRL (Laughnan) 1638||Group 3 (3L)||ms3-6043|
|Sprague 75-6||Group 8 (1S)||ms*-6044|
|Sprague 111||Group 4 (9L)||ms*-6046|
|Sprague 345||Group 4 (9L)||ms*-6047|
|COOP 56-112||= ms1||ms1-6050|
|COOP r1-g wx1||= ms11||ms11-6051|
|COOP adh-ms W23||ms*-6052|
The information included in Table 2 is intended to serve several purposes. In the first column are indicated the previous temporary laboratory designations of male sterile gene stocks. In the last column the proposed new designations are indicated, carrying a 4-digit laboratory number in order to conform to maize terminology rules. Since limited distribution of some of these stocks has been made to a few geneticists under the old designations, the listing of old and new designations will permit them to relate their stocks to the new symbols.
The new temporary symbols consist initially of a generic symbol for the category of the trait; here the symbol ms has been chosen. An asterisk denotes a temporary gene designation and the 4-digit laboratory number differentiates a specific male sterile gene source from others. There is no assurance even that the ms symbol will be valid in future since the mutant allele may prove to be allelic to a locus named by some other symbol (e.g., as1 or si1). Once a particular gene in this collection has been shown to be allelic to a symbolized locus, then that permanent symbol is used, followed by a dash and retention of the 4-digit laboratory designation to distinguish the allele itself. In the event the new gene source is not allelic to any currently symbolized gene, a new gene locus symbol may be assigned. The first gene symbol in the listing (ms10-6001) indicates that the male sterile gene originally carried as ACCO 1752 proved to be allelic to ms10 and is now designated as allele 6001 at that locus. The new symbol si1-at-6017 is an example of an instance in which the generic symbol (ms) was replaced when allelism with si1 was found.
The middle column in Table 2 indicates positive outcomes of allelism tests with symbolized loci, or assignments to groups based on allelism with each other and common location within identified chromosome segments.
All the mutant male sterile alleles reported here have shown a recessive expression. As indicated in Table 2, some of them have been assigned to one of eight groups, mostly on evidence that the male sterile alleles have been "uncovered" (are present in hemizygous condition, ms/-) in plants hypoploid for a B-A translocation. Members of the same group are allelic with each other.
Evidence for the different groups is as follows:
Group 1 (7L)--members of this group are uncovered in plants hypoploid for TB-7Lb. They apparently are not allelic to ms7 and there is no clear indication of allelism with va1. Included in this group are ms*-6004, ms*-6010, ms*-6013 and ms*-6014.
Group 2 (proximal 9L)--Uncovered by hypoploid TB-9Lc and linked to hypoploid TB-9Sb. Not allelic to ms2 nor to male steriles in Groups 4 or 6. Included in this group are ms*-6011, ms*-6018, ms*-6027 and ms*-6031.
Group 3 (3L)--Uncovered by hypoploids of the compound TB-3La-2S6270. Allelic to ms3, which is the basis for assignment to the long arm of chromosome 3. The locus of ms3 is thus within the segment bounded by 3L.1 (the interchange point of TB-3La) and 3L6, the chromosome 3 interchange point of T2-3 6270. Included in this group are ms3-6008, ms3-6009, ms3-6020 and ms3-6043.
Group 4 (distal 9L)--Uncovered by hypoploid TB-9Lc and hypoploid TB-9La. Not allelic to male steriles in Groups 2 or 6. Alleles at this locus show about 10 percent recombination with Bf1. Included in this group are ms*-6021, ms*-6022, ms*-6046 and ms*-6047.
Group 5 (1S)--Allelic to ms9. Uncovered by hypoploid TB-1Sb and hypoploid TB-1Sb-2L4464. The locus of ms9 is thus within the segment bounded by 1S.05 and 1S.53. Included in this group are ms9-6032, ms9-6037 and ms9-6042.
Group 6 (distal 9L)--Uncovered by hypoploid TB-9Lc and hypoploid TB-9La. Not allelic to male steriles in Groups 2 or 4. Included as an allele at this locus is ms*-6006.
Group 7 (distal 2L)--Uncovered by hypoploid TB-1Sb-2L4464, but not by hypoploid TB-1Sb. Assignment to 2L also confirmed by the fact that alleles at this locus show about 15 percent recombination with Ch1. They are also uncovered in duplicate-deficient plants derived from T2-5f and T2-8(8376), that are deficient for terminal segments of the long arm of chromosome 2. Included in this group are ms*-6019, ms*-6024, ms*-6029, ms*-6038 and ms*-6041.
Group 8 (1S)--Uncovered by TB-1Sb and hypoploid TB-1Sb-2L4464. The male sterile locus is thus within the segment bounded by 1S.05 and 1S.53. Tests for allelism with as1, ms9, ms12, ms14 and ms17 all have appeared to be negative. Included in this group are ms*-6034 and ms*-6044.
Included in this report is a listing of 79 male sterile sources. Of these, 13 have shown a cytoplasmic mode of inheritance. Of the nuclear gene sources assembled more than ten years ago, 17 appear to be alleles of male sterile genes with permanent symbols, 20 have been located to chromosome, but not shown to be alleles of male sterile loci with permanent gene symbols and 15 are unlocated to chromosome and lacking unambiguous indication of allelism with male sterile gene loci with permanent gene symbols. An additional 14 sources are recent acquisitions that are being perpetuated and evaluated, but which have not been subjected to a significant amount of testing. In general, allelism testing of various sources has not included tests with male sterile genes assigned permanent symbols during the past ten years.
In the course of allelism testing, repeats of test plantings in different seasons have frequently exhibited erratic and differing results. This result has occurred even when the same seed source has been planted in different seasons. The ambiguous results have been mainly of two types: (1) different results observed from allelism test crossing made in reciprocal directions, and (2) in families containing male sterile plants, the occurrence of such plants in frequencies often lower than expected from a cross yielding a positive allelism result; it is as if many plants homozygous for a male sterile allele had expressed male fertility. The same result could be produced by the action of gametophyte factors operating in conjunction with linked male sterile gene loci. This possibility does not seem at all likely, however, since the number of male sterile gene loci in the genome showing aberrant ratios would require postulation of numerous instances of linkage with gametophyte factors showing fortuitous linkage. The widespread distortion of ratios might also occur if alleles at male sterile gene loci might themselves act as gametophyte factors under some conditions. This might particularly be true of mutant alleles arising from insertion or removal of transposable elements. Whatever the actual reason, there is a strong suggestion that the expression of male sterility may be strongly modified by background genotypes and/or environmental influences. It is noteworthy that Marc Albertsen, in his plantings of some of these male sterile sources, has experienced the same kinds of anomalous and ambiguous results (personal communication).
The expression of male sterility appears to be more consistent when recessive male sterile alleles are present in hemizygous condition rather than as homozygotes. The preferred strategy, then, may be to locate male sterile genes to specific chromosome regions by hemizygosity in hypoploid B-A translocation plants or by linkage to the deficient segment in hypoploid plants. As a second stage, allelism would need to be tested only by direct crosses among male sterile genes located in the same general chromosome regions.
Numerous repeats of tests have been conducted in order to obtain results
deemed suitable for a convincing assessment of the question of allelism.
As a result, it is expected that virtually all conclusions of allelism
reported here will be validated by future investigations. However, if anyone
requests seed samples of male sterile sources reported to represent alleles
at a particular locus with a view to testing for allelic differences, it
would be only prudent to reconfirm that the sources are indeed allelic
before embarking on further labor-intensive or expensive investigations.
It should also be pointed out that two or more allelic male sterile sources
submitted by the same person have a higher probability of representing
identical alleles than if submitted by different people, since alleles
unknowingly transmitted through different lineages or plantings at one
laboratory may frequently be detected separately, yet trace back to a common
pedigree source in an earlier generation.
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