The maize inbred line Va20 carries a new restoring gene for S-type
cytoplasmic male sterility (CMS)
--Kamps, T and Chase, C
CMS is the maternally inherited inability to shed viable pollen, and a CMS plant is male sterile unless it carries the appropriate gene that restores fertility. These restorer genes are generally referred to as restorers of fertility (Rf). In maize, three major groups of male sterile inducing cytoplasms occur and these groups are, in part, defined by their nuclear restorer genes. Our investigations have focused on fertility restoration of the S-type cytoplasm.
CMS-S maize plants are characteristicaly restored to fertility by the gametophytically expressed nuclear gene, Rf3. Additional CMS-S Rf genes were uncovered among unexpected male fertile progeny by Laughnan and Gabay (Maize Breeding and Genetics, pp. 427-447, 1978). These new restorers are distinct from Rf3 by map positions and, with the exception of RfIV, by their deleterious pleiotropic effects. Laughnan and Gabay localized the position of Rf3 to the long arm of chromosome 2 (2L). We have since identified a more precise location of the Rf3 gene from the inbred Ky21(S) (Rf3-Ky21) to be in the interval between whp and bnl17.14 (MNL 66:45, 1992).
Linkage analyses for fertility restoration by the inbred lines CE1(Vg) and Va20(CA) were also performed. An analysis of 45 testcross progeny revealed the estimated map position of CE1 restorer (Rf3-CE1) to be 8.9 cM distal to whp and 8.9 cM proximal to bnl17.14. This is similar to the results reported for the Rf3-Ky21 gene. Conversely, analysis of Va20(CA) testcross progeny showed no linkage of fertility restoration with either the whp or the bnl17.14 marker. Neither was linkage detected with other 2L RFLPs including npi291, npi297, npi122, npi456 and npi298, indicating that the Va20(CA) restorer was not allelic to either Rf3-CE1 or Rf3-Ky21.
The population from the three-way-cross W182BN(CA) X [Va20(CA) X Ky21(S) was generated to conduct a direct linkage analysis between the Va20(CA) restorer and Rf3-Ky21. All but one of 76 progeny examined were semi-fertile, i.e. shed pollen was composed of approximately 50% normal, starch-filled grains and 50% aborted, empty grains as expected for gametophytic restorer genes. Southern analysis with the Rf3-Ky21 linked markers, whp and bnl17.14, showed a segregation ratio of 2 Ky21 : 1 Va20 alleles. This segregation pattern is indicative of two major unlinked gametophytically expressed restorer genes and is consistent with our earlier data.
Additional studies compared fertility restoration in F1 and BC1 populations generated by crossing the Va20(CA), Ky21(S) and CE1(Vg) with four different male sterile inbreds. Male fertility was assessed by examining pollen shed from individual progeny. Va20 progeny were more variable in male fertility than either CE1(Vg) or Ky21(S) and exhibited the most frequent occurrence of unexpected male steriles. Some Va20 F1 hybrid combinations produced male steriles whereas all hybrid combinations using either Ky21(S) or CE1(Vg) parents were semi-fertile. Furthermore, we have observed that populations generated by recurrent crossing of Va20 restored progeny to the male sterile inbred W182BN(CA) tend to show an increase in male sterility. These sterility data combined with the linkage analysis suggest that the Va20 CMS-S restorer system is unique and is likely more complicated than the classic CMS-S restorer, Rf3. The Va20 inbred does not exhibit any of the deleterious effects characteristic of the 9 "new" CMS-S restorers reported by Laughnan and Gabay. The possibility that the Va20 restorer and RfIV are different genes has yet to be examined. This can initially be achieved by conducting a direct linkage experiment, like that described above, between these two restorers.
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