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Maize Genetics Cooperation � Stock Center

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University of California at Berkeley

 The reverse germ orientation2-VI mutation maps to chromosome 1L near Ts6 and shows allelism to ids1 --Jackson, JD, Kaplinsky, N As previously reported (MNL 70:66), rgo*-VI arose in a Rf*-VI strain of John Laughnan. This mutant was shown not to be allelic to rgo1 and was therefore called rgo2-VI. The rgo2 mutant sometimes produces extra florets within the tassel spikelets and in the female spikelets as well, giving a reverse germ orientation phenotype. Floret number seems to vary between two and three in the rgo2-V/rgo2-V tassels, but kernels on rgo2-V/rgo2-V ears are all rgo in straight rows. Another rgo* from Frances Burr was shown to be allelic to rgo2-VI and was therefore referred to as rgo2-Burr. Mutations with similar phenotypes have been reported by Brieger (MNL 22:55,1948), Joachim (MNL 29:53,1955; MNL 30:84-85,1956; Proc. Minn. Acad. Sci. 24:37-43,1956) and Sachan and Sarkar MNL 52:119-120, 1978).

One of these is reversed germ orientation1 (rgo1). This is a recessive mutation, which causes the spikelet meristem to produce three instead of the normal two floret meristems. Development of the third floret causes reversed kernels due to the distichous pattern of floret initiation in the spikelet. rgo1 also prevents the abortion of lower florets in some ear spikelets. rgo1 maps to chromosome 9 near wx1 (Nick Kaplinsky et al., Maize Genetics Conference Poster Abstracts 41: 1999).

The rgo2 phenotype is similar to the rgo1 phenotype in the tassel and the ear. Ears from plants heterozygous for rgo1 and rgo2 have a reversed germ phenotype, suggesting that rgo1 and rgo2 are in the same developmental pathway and are sensitive to each other�s dosage.

Crosses were undertaken with the comprehensive set of B-A translocations (TB�s) to determine the chromosomal location of the rgo2-VI mutation. Plants homozygous for rgo2 were crossed by the respective TB�s and seeds from the resulting F1 ears showing the rgo phenotype were planted in the field and scored for hypoploid status by plant height and pollen sterility. The tassels on hypoploid plants were checked for floret number and ears were self-pollinated and at harvest scored for rgo phenotype. The results of crosses with TB-1Sb, TB-1La, TB-7Sc and TB-9Lc are summarized in Table 1. Results placed rgo2-VI to chromosome 1.

Table 1. Summary of TB crosses with rgo2.
 
Mutant Arm-locating TB Cross Summary of hypoploid ears floret # / spikelet in hypoploid tassels Mutants on Same Chromosome with Similar Phenotype
rgo2-VI TB-1Sb 3 ears all rgo:

2 ears some rgo:

7 ears some rgo

 3

3

2

  
rgo2-VI TB-1La 10 ears all rgo:

11 ears all rgo:

1 ear some rgo:

5 ears some rgo 

 3

2

3

2

 Ts6, ids1
rgo2-VI TB-7Sc 16 ears all +rgo:

5 ears some rgo

 2

2

  
rgo2-VI TB-9Lc 26 ears all +rgo:

8 ears some rgo

 -

-

 rgo1 maps to 9L

Crosses were then made of rgo2-VI with Vg1, Kn1 and Ts6 on chromosome 1 to confirm chromosomal arm placement. These results are summarized in Tables 2-4. The linkage test was set up as a backcross as indicated. Kernels from the backcross ears were planted in the field and the resulting plants were scored for the chromosome 1 markers and self-pollinated. The resulting ears were scored for reversed germ orientation. Tight linkage was obtained with Ts6 placing rgo2-VI on chromosome 1L very close to Ts6.

Table 2:

2 point linkage data for rgo2-Ts6

Testcross: [Rgo2 Ts6 x rgo2 ts6 ] x rgo2
 
Region Phenotype No. Totals
0 Ts + 84  
  + rgo 97 181
1 Ts rgo 5  
  ++ 2 7

% recombination rgo2-Ts6 =3.7+1.4

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Table 3:

2 point linkage data for rgo2-Kn1

Testcross: [Rgo2 Kn1 x rgo2 kn1] x rgo2
 
Region Phenotype No. Totals
0 Kn + 60  
  + rgo 62 122
1 Kn rgo 23  
  ++ 28 51

% recombination rgo2-Kn1=29.5+3.5

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Table 4:

2 point linkage data for rgo2-Vg1

Testcross: [Rgo2 Vg1 x rgo2 vg1] x rgo2
 
Region Phenotype No. Totals
0 Vg + 25  
  + rgo 46 71
1 Vg rgo 22  
  ++ 13 35

% recombination rgo2-Vg1 =33.0+4.6

Another mutation with a similar phenotype (indeterminate spikelet1=ids1) has been mapped to 1Lwith recombinant inbred lines (George Chuck et al., Genes & Development 12:1145-1154, 1998). According to Ben Burr, ids1 maps to the long arm of chromosome one and showed zero recombination with RFLP marker chi1, which is out on 1L, 15 cM distal to Ts6 (George Chuck, personal communication).

The rgo2 phenotype is similar to the ids1 phenotype in the tassel and they both give reversed kernels. Southern blots using ids1 as a probe suggested that rgo2 is an allele of ids1 (Nick Kaplinsky et al., Maize Genetics Conference Poster Abstracts 41: 1999).

Crosses were then undertaken to test for allelism between ids1 and the two rgo2 alleles. Results are presented in Table 5. The allelism crosses were between plants homozygous for rgo2 and heterozygous for ids1. In both cases crosses gave a good 1:1 segregation indicating a positive allelism test. In further crosses the mutant segregated out in good 3:1 ratios as would be expected. We suggest the proposed new designations for these two new alleles of ids1.

Table 5:

Allelism tests of ids1 and rgo2
 
previous designation allelism test with +/ids1 self of + ear from Allelism Test self of rgo ear from Allelism Test proposed new designation
rgo2-VI 38 + : 32 rgo (1:1) 5 + : 2 rgo (3:1)   ids1-rgo2-VI
rgo2-Burr 32 + : 36 rgo (1:1) 11 + : 4 rgo (3:1) 0 + : 27 rgo (all mut) ids1-rgo2-Burr

 

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