A high incidence of the sources of cytoplasmic male sterility (cms) in the Maize Research Institute (MRI) gene bank

--Vancetovic, J, Vidakovic, M, Vidakovic, M, Rosulj, M

Finding a restorer cytoplasm, which exists only theoretically, as proposed by Hermsen (Euphytica 14:221–224, 1965; Euphytica Supp. 1:63–67, 1968), would allow easier and genetically cleaner hybrid seed production based on male sterility in a plant species than any other system imposed so far. Only a limited amount of research has been done in this area, and all of the results were negative (Kohel and Richmond, Crop Sci. 3:361–362, 1963, on cotton; Rutger and Jensen, Euphytica 16:350–353, 1967, on barley; and Washnok, MNL46:25–27, 1972, on maize). This encouraged us to start a huge experiment, and search all of our gene bank for the presence of the restorer cytoplasm for the gene ms10. Unfortunately, we have not found it. So the objective of the original study was quite different from the results reported herein, not to find new sources of cms, but to find the restoring cytoplasm for the gene ms10 in maize.

In a search for the restorer cytoplasm (Vidakovic et al., J. Hered. 93:444–447, 2002) for the gene ms10, each accession from the MRI gene bank was crossed with a heterozygous Ms10/ms10 tester. The plants from the two crossed ears of each heterogeneous accession (OP varieties, races, synthetics), and from one ear of the crossed lines, were subsequently backcrossed with the Ms10/ms10 tester. Apart from the Ms10/ms10 genetic constitution, the heterogeneous tester consisted of various genotypes from different maturity groups, in order to cover almost all of the vegetation span of the gene bank accessions. Crossing and backcrossing was done in isolation, with detasselled gene bank accessions used as females. Twenty plants of the first cross per each ear were used for backcrossing within each accession. Approximately the same number of kernels from the backcrossed ears were mixed within an accession, and subsequently planted in a population of about 60 plants for testing for the presence of the restoring cytoplasm for the ms10 gene. Indication of the presence of the restoring cytoplasm would be a 100% fertile test, and average expected frequency of the sterile ms10/ms10 plants was 1/8 for each accession.

Restorer cytoplasm wasn’t found, but a high incidence of the sources of cytoplasmic male sterility was obvious within our gene bank. Since plants within each test were descendants from a single ear, they all shared the same cytoplasm. As an indication that an accession contains a sterile cytoplasm we took the presence of more than 30% sterile plants in the test, which is significantly higher than the theoretical 1/8 which would arise from ms10/ms10 plants only. In Table 1 there is a list of accessions where sterile cytoplasm was indicated.

In the year 2000, spare seed from the crosses and/or backcrosses of these accessions was used for multiplication by open-pollination (cytoplasm is only transmitted through female plants). Genotypes with more than 40-50% sterile plants in the phase of multiplication (from the original seed lots) were chosen for testing of the type of cytoplasm involved (± in Table 1).

Some more sources of sterile cytoplasms were indicated from an unexpected source — from so-called test-candidates for the restoring cytoplasm, which were 100% fertile in the test. Within those tests, selfing of individual plants and outcrossing to the ms10/ms10 tester was performed. A 100% fertile self plus 100% sterile or 1:1 (Ft:St) segregating test would indicate the presence of the restorer cytoplasm. Such pairs of selfs-tests we did not find, but in some of the selfed progenies an excess of sterile plants was observed, far greater than 0.25, which would be expected if the original plant was heterozygous for the ms10 gene. These progenies were also included in the multiplication field in 2000, and few were (Table 2) chosen for the test of the type of cytoplasmic sterility (based, again, on more than 40–50% sterile plants in the phase of multiplication).

In the multiplication field, progenies were also included from the ears of the sterile plants found in the accessions of the Yugoslav Collection, taken from the open-pollination. Again, for further classification, only progenies with more than 40% sterile plants were chosen, indicating the presence of the sterile cytoplasm, and not solely some of the ms genes. These accessions are listed in Table 3.

Kol 146 will be included twice in the classification test (Table 1 and 3), chosen by the 2 criteria (descendants are from the 2 different ears from this variety).

The high incidence of sterile cytoplasms in our gene bank is somewhat surprising. We have chosen 50 sources for a test of the type of cytoplasmic male sterility, but it seems that the number of the cms sources is at least twice that high, giving a number of about 100 independent sources of cms. In comparison with the total active number (about 5000) of accessions in our gene bank (accessions that are actually present), this gives a total of 2%. It raises a question of whether there may be any evolutionary significance of a high number of, mostly, populations with cytoplasmic male sterility, possibly in making some self-compatibility barrier protecting such a population from foreign pollen not carrying the appropriate restorer (Rf) gene.

Sterile plants from the accessions chosen are crossed for further classification in 2002 with 3 heterozygous testers: RfT/rfT, RfC/rfC and RfS/rfS. Field classification for these 3 types of cms will be done in 2004, since our breeding nursery in 2003 was destroyed by a storm.

We would like to thank our technical staff, Stanija Mladenovic and Snezana Veselinovic, and our workers Milijana Kalisanin and Milena Petrovic, for performing a huge amount of work in the laboratory and in the field.

† Kol=Collection of OP varieties from the territory of the former Yugoslavia
INT=Heterozygous foreign material
INT-L=Foreign and domestic inbred lines
‡ LB=Sterile plants in the test (some or all) exhibited the late break of sterility
§ +=Chosen for the sterility type classification
¶ = 1 of 2 progenies gave 100% St test
# = The test was almost 100% sterile
††= Originally received as segregating for St plants

† Kol=Collection of OP varieties from the territory of the former Yugoslavia
INT=Heterozygous foreign material
INT-L=Foreign and domestic inbred lines
‡ +=Chosen for the sterility type classification

† +=Chosen for the sterility type classification