The primary trisomics of the tomato are being studied in several aspects They are being classified according to morphological type, in which they differ extensively. They are being identified with their corresponding linkage group appropriate breeding tests. Dr. D. W. Barton is collaborating in the program by identifying in pachytene the extra chromosome of each Primary trisomic type. After these three phases are completed, an attempt will be made to evaluate individual chromosomes for their contribution to quantitative economic characters. Up to the present time, the genetic analysis has been completed with eight trisomic types and nine linkage groups, and seven trisomics have been identified cytologically.
Until the trisomic types have been described, a complete progress report of the other work here would not be very meaningful. It is appropriate, however, to call attention to data that seem to indicate a condensation of linkage groups VI and VIII into a single group. Lutescent (l) was used as a marker for group VI and the anthocyanin difficiency a\2\ for VIII, The trisomic concearned here carries the morphological type number "3".
In the F2 of type 3 homozygous for L x diploid l, the segregation was 92+: 10 l, lutescent comprising 9.9 +\- 3.1% of the family. In other families grown under the same conditions segregation for l was 479+ : 135 l (22.0 +\- 1.7% lutescent). In the same F2 none of the 38 plants that were trisomic type 3 were lutescent, whereas for other genes segregating in the same family, 11/38 trisomics were d; 10/38 were c; 8/38 were r; and 7/38 were y. Type 3 is therefore trisomic for the chromosome that bears linkage group VI.
The evidence for group X is as follows. Since a\2\ cannot be scored in the field at Davis, the analysis was limited to seedling segregations. The F2 of these 3 homozygous for A\2\ x diploid a\2\ included 153+ and 34 a\2\, the latter comprising 18.2 +\- 2.8% of the family. In families not descending from type 3 the segregation was 390+ : 154 a\2\ (283 +\- 1.9% a\2\). A X^2 test of all families indicates the following: the heterogeneity x^2 for all families taken together is significant at the 0.02 level, whereas for all families excepting the one involving type 3 it corresponds to a P of 0.2. The F2 for type 3 disturbs the homogeneity of all other families segregating for a\2\ and differs significantly from them. The deviation, furthermore, is in the direction of a trisomic segregation, but is not so marked as might be expected. The evidence is not so conclusive as for l, yet it strongly suggests that trisomic type 3 corresponds also to linkage group X. Further tests are in progress.
Trisomic analysis can further tomato genetics in several ways, but especially by providing an absolute seperation of the linkage groups. It is worthwhile to determine as soon as possible whether any additional gene groups established by absence of linkage with other groups need to be consolidated.