Gilbert, J. C.
Evidence that the gene (Mi) for resistance to the common race of root knot nematodes was linked with c in linkage group four was offered in the TGC 8. Trisomic tests,described by Rick at the same time placed this group in chromosome 6. When the Mi gene (obtained originally from L. peruvianiim P. I. 128657) was used in a backcrossing program with the determinate "island line" tomatoes with uniform unripe fruit color, considerable difficulty was had with increased susceptibility to concentric fruit cracking in MiMi segregates. Genes modifying the size of the stem scar, time of maturity and thickness of the ovary wall were also suspected as present in this linkage group from the breeding work in Hawaii. Enlarged stem scars and increased concentric cracking were the most difficult problems in MiMi lines obtained here but recombinations were eventually obtained which yielded MiMi plants with normal size stem scars and fruits with no greater susceptibility to concentric cracking tlmn their Mi^+ parent lines. Late maturity also proved a persistent problem in the MiMi lines but yielded to continued selection and the use of distinctly early varieties in new crosses.
Concentric cracking scores were obtained on MiMi, Mi^+ and ++ sister plants by use of a ten fruit sample per plant and a 1 to 5 cracking scale with 1 representing no cracks and 5 extremely severe concentric cracking. The sum of the products of these class values times the number of fruits in each class was used as the cracking score for each plant. The low scores thus represent less cracking than higher scores. The mean score of a ten plant sample was used as the cracking score for each line in each replicate.
In the first cracking trial using the above scoring system, eight unselected MiMi progenies and four unselected ++ progenies from HES 5149 (IL+) yielded the following scores. The varieties Molokai and Lanai are also included because they were the commercial lines most recently involved in the derivation of HES 5149 (an F3 selection of HES 4679 x Lanai, a 5th backcross to L. esculentum from L. esculentum x L. peruvianum).
Concentric cracking indices of some
unselected progenies of HES 5149
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MiMi lines Rep. I Rep. II Rep. III
5149-E 35.7 31.2 31.0
5149-G 30.3 31.5 31.0
5149-H 33.2 30.7 30.7
5149-K 28.8 28.3 28.5
5149-M 34.6 30.4 31.5
5149-P 30.6 30.8 27.7
5149-T 31.4 28.8 29.9
5149-W 33.1 29.0 27.8
++ lines
5149-I 18.4 16.9 17.3
5149-L 16.3 16.9 16.8
5149-R 18.9 19.1 17.7
5149-S 21.2 14.7 13.0
Molokai 21.1 17.3 13.0
Lanai 15.1 16.8 13.2
The variance analysis showed all the MiMi progenies to be significantly more
susceptible to concentric cracking according to this scoring method than
either their ++ parent lines or ++ sister lines even though there had been two
generations of selection for horticultural worth (including reduced cracking)
since the 4679 x Lanai cross. Both 5149 and iTs parent, 5038, were Mi^+
plants and more closely approached their commercial type predecessors in
concentric crack resistance than any of their MiMi progenies. Crack indices
of other Mi^+ lines in this family were also somewhat intermediate between the
MiMi and ++ lines. These cracking notes were made in April 1958 at the
Poamoho Farm, Oahu, T. H. with weather conditions favoring light concentric
cracking on standard Hawaiian varieties.Although our experience with the Mi locus in the breeding program has led us to believe that at least one gene modifying the susceptibility of tomatoes to concentric cracking lies fairly close to Mi on chromosome 6, it has not suggested that a simple type of inheritance is involved in concentric cracking susceptibility. At least four different levels of resistance to concentric cracking have already been established in the efforts to improve the crack resistance of the MiMi lines. When the more highly susceptible lines, like HES 5229, are used in crosses with less crack susceptible ++ lines the F1 hybrids have been somewhat intermediate in their cracking tendencies. Very definite improvement in the cracking behavior of these hybrids has been effected by substituting certain new crack resistant MiMi lines for the older crack susceptible sources of the Mi gene. Thus STEP 234 (5229 x Maui) and STEP 275 (5229 x STEP 174) are both more crack susceptible than 6048 x Maui or 6048 x STEP 174. The MiMi line, HES 6048, is less susceptible to concentric cracking under average conditions in Hawaii than the standard "island lines" (like Maui), and its hybrids with such lines are less crack susceptible than the Mi+ parents. This suggests that the association between the Mi locus and concentric crack susceptibility noted above was not due to any pleiotropic effect of the Mi gene but rather to linkage, possibly with something carried over from the original L. peruvianum source of this gene.