A variant of C-type cytoplasmic male sterility with altered fertility restoration patterns
-- M. E. Smith and E. D. Earle
Among plants regenerated from callus cultures of the inbred W182BN, a male sterile individual was identified. This plant was pollinated by W182BN in the normal cytoplasm background, and the resulting progeny were all fully male sterile, indicating that the plant was a cytoplasmic male sterile (cms). Subsequent crosses with known differential restorers suggested that the cytoplasm was of the C type, since it was restored to fertility when pollinated with Pa884P (a known cms-C restorer), but not when pollinated with A636, A664 or A665 (known cms-S restorers) or with NYD410 (a known cms-T restorer). In addition, progeny of this plant showed no sensitivity to Bipolaris maydis race T toxin, which is diagnostic for the presence of cms-T. Mitochondrial DNA analysis (carried out by D. Pring, D's lab at the University of Florida) showed no detectable differences between the new cytoplasm and standard C cytoplasm. The new cytoplasm has been designated CR.
Additional crosses of numerous inbred lines onto CR and two other cms-C types of cytoplasm (C and PR) were made, and fertility restoration evaluated on the F1 progeny of these crosses in either Aurora, NY or Homestead, FL. Results of the fertility restoration data (Table 1) suggest that CR varies somewhat from both C and PR cytoplasms in its fertility restoration behavior. Progeny of crosses with A632Ht, A634Ht, A635, B89, CB59G, LH145Ht, LH148, Oh51A and Va20 were more sterile in the CR cytoplasm background than in the C cytoplasm background, while the reverse was true for progeny of crosses with Ay499, CrS4HLa, FR22 and Mo17. Fewer combinations with PR cytoplasm were available, but differences in fertility restoration between CR and PR were detected. Progeny of crosses with A634Ht were more sterile in CR than PR cytoplasm, while those from CrS4HLa, Mo17 and Oh51A were more fertile in CR than PR cytoplasm.
Table 1. Male fertility ratings* of F1 crosses between different inbreds
and W182BN-CR, W182BN-C and W182BN-PR. Ratings made in Aurora, NY or Homestead,
FL (indicated by +).
|Fertility ratings when crossed with:|
|A634Ht||1||5||1 + 1-3|
|Ay499||5||1 + 1-3||5|
|CB59G+||1 + 4||4||-|
|CrS4HLa||5||1||1-3 + 5|
|LH148+||1 + 1-3||3||-|
|Mo17||1 + 1-3||1||1|
|Oh51A||1 + 1-3||1-3 + 3||1|
The plant initially identified in the field as carrying the CR cytoplasm was recorded as being derived from a normal cytoplasm culture of W182BN. Recent evidence that mitochondrial populations within a cell may be heterogeneous suggests a possible mechanism by which cms-C plants might be derived from a normal cytoplasm culture. At the time the plant carrying CR cytoplasm was regenerated, there were both normal and cms-C cultures of W182BN in the laboratory. Thus it is also possible that a labelling error occurred, and that this plant actually was derived from a culture of cms-C W182BN.
Regardless of the exact origin of this plant, it appears to differ in
fertility restoration behavior from cms-C plants of W182BN from the C and
PR types. Although male fertility, particularly for cms-C, varies with
environment and hence might be different in Aurora and Homestead, each
set of F1s with the same male parent was evaluated in one or the other
environment. Thus differences in ratings between CR and the other cms-C
types for a given male parent cannot be explained by environmental influence
alone. Genetic differences and/or differences in genotype-by-environment
interaction among the cytoplasms are indicated. Given these differences,
CR may prove to be a useful source of additional cytoplasmic diversity
within the cms-C group.
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