Gilbert, J. C.
Backcross tests with several marker genes have shown independent segregation of Mi and a, d, l, al, y, H and u. The gene tf segregated independently of Mi in an F2 test. Backcross seedling tests with Mi and c showed linkage with recombination values of 35.2%, 41.2% 38.O% and 40.3% in four trials involving 227, 188, 137 and 171 plants respectively. Four mature plant backcross tests totaling 952 plants produced recombination figures of 39.1%, 41.2%, 44.2% and 44.0% between Mi and c. The plantings which appeared most satisfactory from the standpoint of uniform survival of all classes were those which yielded recombination values between 39 and 41%. The low value of 35.2% came from a test in which the la gene was derived from HES 4245 whereas all the others used HES 5295 as the resistant parent. These two plants were quite different in that 4245 was an MiMi line established at the second backcross level from the original esculentum x peruvianum cross while 5295 was an MiMi line established at the fifth backcross level.
Mi-yv linkage (HES 5295 x LA 304) F2 The results of the above tests would seem to place this gene for root knot resistance in linkage group IV. If so, Rick's yv mutant also falls in this group since it appears to be very closely linked with Mi. It was found that the yv^* and yv seedlings could not be grown together very long over strong root knot inoculum as the yvyv class simply disappeared in a few weeks. An early separation of the yv^* and yv seedlings produced satisfactory tests, however. In one trial, where only the yv yv segregates were retained, 51 out of 54 yv seedlings were heavily galled after a six-week exposure to root knot. Eventually only one of the 54 proved to be resistant. A second F2 coupling test yielded the following:
Root Knot Root Knot
Resistant Susceptible
yv^+ 587 6
yv yv 2 158
Because of the viability problem in tests involving exposure of seedlings to
root knot (which greatly increases the susceptibility of the Mi^+Mi^+ plants
to damping off under some conditions) the writer would prefer recombination
figures from backcross rather than F2 tests. This will be attempted when seed
is available.3-point linkage tests with c, sp, Mi. A total of 952 plants of (5295 x LA 13) x LA 336 were exposed to root knot in four plantings and examined as mature or nearly mature plants. LA 336, received from C. M. Rick is spsp cc Mi^+^Mi^+ while the F1 was Mi sp c^+/Mi^+ sp^+^c. HES 5295 is a highly determinate, root knot resistant line while its F1 with Rick's LA 13 was a vigorous, indeterminate plant. The characters were not difficult to read, with very few exceptions, since several branches per plant were available for study. The writer is quite familiar with the parental material and has more confidence in the accuracy of the sp-c data than anything else in this series of tests. Only 17 crossovers between c and sp were found in the 952 plants. Three plants were questionable and appeared to be quite intermediate in the sp sp^+ character. They are not included in the data. The four tests gave recombination values between c and sp of 2,3%, 2.6%, 1.3% and 1.7%. The last two tests were of 310 and 354 plants each, grown in the field to full maturity. Of the 17 sp-c crossovers found, there were eleven which retained the parental combinations at the sp-Mi loci and six at the c-Mi loci. Although these numbers are too small to throw much light on the relative position of c, sp and Mi, they suggest the possibility of close linkage between Mi and Cf\2 if the gene order is c-sp-Mi. Since we don't have Cladosporium fulvum or leaf mold disease, in Hawaii, the Mi-Cf\2 situation should be tested elsewhere. Although two additional genes have apparently been placed in linkage group IV from these tests in Hawaii, the gap between the c-sp group and the yv-Mi group is still to large and additional genes are needed to map this area.