Three point linkage tests with c, md and Mi.<p>
    
Gilbert, J. C.<p>

The mottled leaf mutant, md, described by Burdick (TGC 8:9) was placed in a
three point test in Hawaii after Burdick found it to be linked with c on
chromosome 6.  Backcross seed was made in Hawaii with a triple recessive
parent recovered from F2 seed sent to us from Burdick's tests.  Two backcross
trials were carried out with the same seed in root knot inoculated soil, but
all seedlings were started in clean soil to give the weaker plants a chance to
survive.  The viability of the mdmd classes in these plantings was
approximately 90%.<p>

The root knot tests with seed obtained from the F1, +/c - +/md - Mi/+
backcrossed to the triple recessive c/c - md/md - Mi^+/Mi^+ produced the
following results.<p>

<pre>    
            First backcross test with c, md, Mi

                    No. of plants

+/c +/md Mi/+              89            Parental class
c/c md/md Mi^+/Mi^+        94            Parental class
 
c/c md/md Mi/+             56            Single crossovers
+/c +/md Mi^+/Mi^+         52               Region 2

c/c +/md Mi/+              50            Single crossovers
+/c md/md Mi^+/Mi^+        32               Region 1

c/c +/md Mi^+/Mi^+         20            Double crossovers
+/c md/md Mi^+              8            Double crossovers
                          ___
                          401  Total plants

            Second backcross test with c, md, Mi

+/c +/md Mi/+             113            Parental class
c/c md/md Mi^+/Mi^+        97            Parental class

c/c md/md Mi/+             49            Single crossovers
+/c md/md Mi^+/Mi^+        67               Region 2

c/c +/md Mi^+/Mi^+         57            Single crossovers
+/c md/md Mi/+             39               Region 1

c/c +/md Mi^+/Mi^+         21            Double crossovers
+/c md/md Mi/+             21            Double crossovers
                          ___      
                          464  Total plants

</pre>
In reading the above table it should be remembered that Mi represents a
dominant mutant for resistance to the common races of root knot in Hawaii as
well as most of the warmer regions.  Its "normal" allele, Mi^+, is the
recessive gene for susceptibility to root knot galling from the nematode
species, Meloidogyne incognita as well as several other (but not all) of the
species or races of root knot nematodes.<p>

In the first of these two tests there were 203 resistant plants and 198 root
knot susceptible seedlings.  In the second test there were 240 resistant and
224 well galled susceptible plants.  The plants were exposed to the nematodes
for five weeks before making the readings.  The Mi gene was used in these
tests rather than the closely linked yv gene because md shows up best against
a green background color in the leaves.<p>

The two tests reported above were in close agreement regarding the
recombination rates between c, md and Mi. In the first test the rates were c
27.4 md 33.9 Mi while in the second test they were c 29,7 md 34.1 Mi. Although
the recombination values between c and md are higher in these trials than the
22% reported by Burdick in TGC 9, the writer does not feel that this
represents a difficulty in properly classifying the plants. The mottled leaf
character is clearly expressed in Hawaii and it seems possible that the
greater recombination values obtained from this one lot of seed used in the
two trials here may represent a real difference in the behavior of Burdick's
parental material and ours.  In most of our earlier backcross tests with c and
Mi, recombination rates ranged from 39% to 42% but in these two trials they
are 47.3% and 45.7% when only the c - Mi data are considered, as in the
earlier tests (and therefore not including the double crossovers in the
recombination classes).<p>

The map distances for c, md and Mi suggested for the revised chromosome map of
the tomato are, of course, tentative and represent a compromise between
Burdick's results and these tests in Hawaii.  The fortunate discovery of
Burdick's md marker for a locus between the c - ap group and the Mi - yv group
on chromosome 6 has greatly increased the total map distance between these two
groups since a considerable number of double crossovers were found. A
comparison of the expected and the observed number of double crossovers
produced coincidence values of .75 for the first trial and .93 for the second.<p>

From the rather weak linkage found between c and Mi it would now seem that the
use of c as the only marker for chromosome 6 in testing new mutants will allow
us to miss some of the genes on this chromosome and it is suggested that Mi or
yv be added to c for this purpose.<p>

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