Complementary genes Rf4, Rf5 and Rf6 are not the unique genetic system for fertility restoration in cmsC of maize (Zea mays L.)
--Vidakovic, M; Vancetovic, J; Vidakovic, M

Using a group of five inbred lines in C-cytoplasm, converted to full restorers as female testers for the group of 24 inbreds known as restorers for cmsC, we found cases of segregation in F2 generations in the ratio of 15:1 male fertile to male sterile. Male sterile segregants did not appear in any of the two backcrosses, what was actually a crucial criterion for "absolute restorerness" of the crossed parents.

The above mentioned situation is possible only if fertility restoration of the two parents used in crosses was based on different genetic systems. It is obvious that the dihybrid segregation is in question. Therefore we assume that complementary genes Rf4, Rf5 and Rf6 represent the first system (System I), and as for the second one, it could be either a duplication, partial or complete of the first system or another parallel and independent genetic system with the same phenotypic effect.

The results of our screenings during two consecutive seasons (1995 and 1996) of all generations of the B73 cmsC RfC x V-312A RfC cross (RfC - a completely male fertile inbred containing cmsC cytoplasm) are presented in Tables 1 and 2.

The seeds for the 1996 screening were newly produced for all generations on the basis of consecutively selfed and pedigreed inbred plants in order to provide the maximal reliability of experimental results.

Table 1. Segregation of the B73 cmsC RfC x V312 RfC cross in Zemun Polje in 1995.
 

	Plants per fertility class
Genotype	Fertile	Sterile
B73 cmsC RfC x V312A RfC (F1)	40	0
(B73 cmsC RfC x V312A RfC) (F2)	272	13	1.39
(B73 cmsC RfC) (BC1)	80	0
(V312A RfC) (BC2)	78	0

Table 2. Segregation of the B73 cmsC RfC x V312A RfC cross in Zemun Polje in 1996.
 

	Plants per fertility class
Genotype	Fertile	Sterile
B73 cmsC RfC (P1)	27	0
V312A cmsC RfC (P2)	23	0
B73 cmsC RfC x V312A RfC (F1)	260	0
(B73 cmsC RfC x V312A RfC) (F2)	2227	127	2.936
(B73 cmsC RfC) (BC1)	303	0
(V312A RfC) (BC2)	1599	3

As can be seen there are no discrepancies between the results obtained in 1995 and 1996 (Tables 1 and 2) and therefore they represent a direct proof of our previous report (Vidakovic, Maydica 33:51-64, 1988) that a system of three complementary genes might not be a unique genetic device for fertility restoration in cmsC. Otherwise, if it was unique, the F2 segregation would be impossible.

As for the dilemma duplications vs. parallel genetic system, it may be difficult to solve by classical genetic analysis of this type, and molecular geneticists might contribute much to it.

What is evident is the fact that both of the crossed parents studied here possess one recessive gene each, but in opposite genetic systems for fertility restoration in cmsC. 

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