The genetic basis of Sn instability --G. Gavazzi, M. Mereghetti, G. Consonni and C. Tonelli We had previously shown that Sn:bol3 differs from the other Sn accessions so far tested for its higher pigment potential in mesocotyl tissues and its instability. Instability relates to its frequent changes from an original condition, indicated as Sn-s, to Sn-w, where -s and -w stand for strong and weak and refer to the two levels of mesocotyl pigmentation. In heterozygous Sn-bol3 genotypes (R sn/r Sn) weak derivatives are also recovered on the chromosome originally devoid of Sn as if the heterozygous association had promoted "contamination" of one chromosome (recipient) with Sn coming from the other (donor). Recombination between R and Sn can be ruled out since this event is not associated with outside marker recombination.

Of the R alleles tested in R sn/r Sn heterozygous genotypes, R-r appears the most efficient in eliciting "contamination" of the recipient chromosome. The effect of R constitution in trans on Sn instability is detected by germinating colored kernels derived from testcrossing the heterozygotes to a homozygous r-g sn line: the yield of nonparental seedlings with red mesocotyl in fact amounts to 50% if the R allele on the receiver chromosome is R-r and about 10% if it is R-g.

The levels of mesocotyl pigmentation ranging from a strong red to a very weak red are inherited and their transmission appears associated to R in its inheritance.

One way to demonstrate the trans effect induced by R in the heterozygotes and to prove its inheritance consists of planting colored kernels produced in the testcrosses of the heterozygotes, growing their plants, crossing them to r-g sn males and analyzing their progeny to see if those receiving the R marked chromosome acquired the potential for mesocotyl pigmentation. Results pertaining to this point using either R-r or R-g as a contrasting R allele in the recipient chromosome are given in Table 1.

Table 1. Mesocotyl pigment score of the progeny ears derived by growing colored kernels from the mating of R sn/r Sn-s females to r-g sn males. A sample of 50 seeds from each ear was germinated to ascertain the mesocotyl pigmentation conditioned by the R marked chromosome originally present in the parental R sn/r Sn-s genotype.

The results indicate that about 50% of the progeny plants from the heterozygotes receiving the R marked chromosome show the potential for mesocotyl pigmentation. However the level of pigmentation differs according to the R constitution of the recipient chromosome, being predominantly strong if the resident R allele is R-r and only weak if it is R-g.

To find out if the Sn derivatives isolated in the donor and recipient chromosome reflect identical or different genetic events we tested their response to Pl. Presence of Pl and Sn in the same genome in fact leads to constitutive pigment production in pericarp tissues while Sn pl genotypes are strictly light dependent for pigment accumulation in these tissues.

Starting material for this experiment was a sample of seeds obtained by pollinating a heterozygous R-r sn/r Sn-s female with a homozygous r-g sn male. Seeds were germinated and their seedlings classified for mesocotyl pigment as follows:

mesocotyl pigment score among:

colored kernels
colorless kernels
S
I
W
NR
S
I
W
NR
0
8
40
18
38
0
24
0

Seedling selections were grown in a greenhouse in winter 88 and the plants used as female parents in a cross with a homozygous Pl stock. At least 20 seeds from each resulting sib ear were grown in the field in summer 89 to ascertain their pericarp pigmentation. The results (Table 2) indicate that Sn derivatives can be grouped into two classes: (i) the parental Sn-s and its weak derivatives on the donor chromosome that maintain their capacity to interact with Pl unaltered and (ii) those recovered on the R marked homologue that lost their capacity to give a constitutive pericarp pigmentation in presence of Pl.

Table 2. Pericarp color in the progeny of different selections isolated from the mating R-r sn/r Sn-s x r-g sn and outcrossed to Pl/Pl stocks.

This conclusion is in accord with another set of data obtained in the progeny from the cross R sn/r Sn-s, Pl/Pl X r-g sn, pl. The scoring of pericarp pigmentation in the progeny grown from cross (a) gave in fact the following results:

pericarp color in progeny ears from

colored kernels
colorless kernels
n
red
nonred
red
nonred
 
7
333
273
11
634

The results, consistent with the previous conclusion, allow one to map Sn in respect to R since the few nonparentals observed in the progeny of colored and colorless kernels are expected as a result of crossing over between R and Sn.

The recombination value so obtained, 2.88%, does not differ from the value obtained by the mesocotyl scoring in the progeny of testcrosses of R-st sn/r Sn-s, the only difference being that using Pl as a probe of the Sn constitution, recombination appears reciprocal while classification for Sn on the basis of mesocotyl score in testcrossed progeny leads to an apparently nonreciprocal recombination.

In conclusion it seems that the Sn-w derivatives arising in the progeny of the parental Sn-s allele represent changes of expression of the resident Sn allele while the Sn phenotype associated with R on the recipient chromosome as observed in the colored progeny of heterozygous R sn/r Sn-s plants is more likely interpretable as a change of expression of R rather than "contamination" or activation of a silent Sn residing on the recipient chromosome.


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