Menzel, Margaret Y.
The hybrid of the Pearson tomato with S. lycopersicoides (Rick, 1951, P.N.A.S. 37:741-744) showed that the parental species have partially dissimilar chromosomes: chromosome pairing in the sterile F1 was variable and incomplete at metaphase I, but when the amphidiploid was obtained, fertility improved and most of the chromosomes paired as bivalents. In the present study, pachytene chromosomes in two clones of the hybrid (kindly made available by Dr. Rick) are being compared with those of the parents to determine whether chromosome differences which cause failure of chiasmata in F1 and "preferential pairing" in the amphidiploid are reflected in visible differences or failure of synapsis at meiotic prophase.
The twelve pachytene bivalents of the tomato can all be identified (Barton, 1950, Am. J. Bot. 37: 639-643) each arm except the short, nucleolar arm of chromosome 2 can be further subdivided into proximal chromatic and distal achromatic regions. Hence, the tomato genome consists of 47 recognizable regions. Comparison of the pachytene bivalents of the hybrid with Barton's map of the tomato suggests the following tentative conclusions.
Forty-three of the 47 regions of the hybrid, including all 23 achromatic regions, consist of two regularly, completely and intimately synapsed, visually identical strands, which are similar in lenght and appearance to a corresponding segment in the tomato.
The remaining four, chromatic, segments in the hybrid are composed of partners which differ in length, hence show a "deficiency" configuration. Three of the four segments have been identified as: the short (nucleolar) arm of chromosome 2, the proximal portion of the long arm of chromosome 4, and the proximal region of the long arm of chromosome 9. The fourth differential region is probably the proximal region of the short arm of chromosome 10.
The inequalities in the chromatic regions probably account for the three or four unequal bivalents seen at metaphase I in the hybrid. Since the visible pachytene differences are located exclusively in chromatic regions, whereas chiasmata appear to be confined to achromatic regions, it seem unlikely that the visible differences affect chiasma frequency. Conversely, no evidence was found for visible differences between partners in the achromatic regions where chiamata occur, nor was synapsis less successful in those regions in the hybrid than in the parents.
Study of the pachytene chromosomes of the Pearson and S. lycopersicoides parents is incomplete, but observations suggest that morphologically both chromosome sets of the hybrid resemble those of the tomato parent more closely than they do those of S. lycopersicoides.