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Ms on an alien chromosome

At least one alien chromosome is known to belong to each homoeologous group (BIELIG and DRISCOLL. 1973). On the assumption that groups 4 and 5 are the ones of interest, the chromosomes involved are as follows.

Group 4:

An Agropyron elongatum (HOST) BEAU. chromosome bearing a gene for blue endosperm (LARSON, personal communication in BIELIG and DRISCOLL, 1971; LARSON and ATKINSON, 1972).

A. elongatum chromosome 4E (DVORAK, SCHELTGEN and KNOTT, 1972).

Preliminary evidence indicates that chromosome D of Secale cereale L., variety Imperial, may also be a member of this group (DRISCOLL, unpublished).

Group 5:

The S. cereale chromosome 5R, bearing the hairy-neck gene, Hp (O'MARA, 1946).

Aegilops umbellulata ZHUK. chromosome C (CHAPMAN and RILEY, 1970).

Information obtained from chromosome substitution lines allows one to conclude that effective Ms genes are located on some of these chromosomes. For example, the substitutions of rye chromosome 5R for 5A and for 5D are highly fertile. The selfed seed set of monosomic 5R substituted for 5A and 5D was 70% and 65%, respectively, of that of euploid wheat under the same glasshouse conditions (BIELIG and DRISCOLL, 1970). Compensation for a mutant locus (or short deletion) may be more successful than compensation for loss of the entire wheat chromosome.

It should also be pointed out that alien-addition lines have somewhat different behaviours when they involve homozygous ms on a wheat chromosome. Normally disomic addition lines do not breed true; thus X lines would give rise to some Y and some Z plants. In the scheme proposed such Z lines would be sterile and would not contribute to Step 1 in Table 1. The Y plants would give rise to Z plants in the male block of Step 2 and would fail to contribute to that step. Maintenance of the X line itself is much easier if the 21" decay product is male sterile; however, it may even so require some monitoring.

It is known that some 22-chromosome pollen does compete successfully with 21-chromosome pollen when both are produced from a 43-chromosome plant. This would mean that some Y seed would be produced at the end of stage 2. This would eventually result in a few sterile plants in the commercial crop. These would not be expected to affect the performance of the crop, as they would be pollinated by the abundant pollen present, and they would in any case be presumably present in low numbers.

Comparison of cytoplasm and chromosomal sterility systems:

The chromosomal system is more complicated than the cytoplasmic system, particularly as the stages before the hybrid-seed-production stage are more numerous in the chromosomal system. However, there are comparative advantages in the chromosomal system which may outweigh this disadvantage.

Any deficiency of fertility restoration in the cytoplasmic system affects the commercial crop, whereas in the chromosomal system such deficiencies affect the hybrid-seed-production stage, which is less serious. Also, hybrid-seed production can be carried out in an environment other than those under which the crop is produced.

The male parent utilized in the hybrid-seed production with the cytoplasmic system contains the nuclear, fertility-restoring genes, whereas in the chromosomal system the male parent contains no such special component. It is thus possible to select the male parent on the basis of combining ability and amount of pollen shed.

Hybrids and disease resistance:

Two less obvious advantages of hybrids are as follows. A series of male parents for the hybrid-seed-production stage could be developed which differed by different genes for resistance to a particular pathogen. This would allow for management of these resistances in accordance with their needs as determined by variations in the pathogen.

The development of hybrids would also allow ready use of translocated alien segments bearing disease resistances. If such a translocation is present in one parent only, the hybrid crop not only has a single dose of alien chromatin but it has a dose of the pertinent wheat segment that is missing in pure lines involving the translocation.


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