Menzel, MargaretY. (Florida State U., Tallahassee.)Pachytene chromosomes of the intergeneric hybrid Lycopersicon esculentum X Solanum lycopersicoides.Amer. Jour. Bot. 49(6): 605–615. Illus. 1962.—Pachytene bivalents of the hybrid were compared with those of the parents to determine whether the chromosome differentiation assumed to underlie chiasma failure and sterility of the F1and preferential pairing in the corresponding amphidiploid was reflected in visible differences or failure of synapsis between homeologues. The total pachytene complement length ofS. lycopersicoidesDun. was nearly 1.5 times that of the ‘Pearson’ tomato parent; the complement of the hybrid was of intermediate length. Pairing in the hybrid showed that the chromosomes of the parents were visually similar and completely synapsed in 43 of the 47 identifiable regions into which the pachytene complement may be divided. The similar regions included all 23 distal, achromatic segments in which chiasmata were localized, and 20 of the proximal, chromatic segments. The other 4 chromatic segments differed in length and hence exhibited a “deficiency” configuration. According to Barton's system of numbering, these were the short (nucleolar) arm of chromosome 2, the proximal segments of the long arms of chromosomes 4 and 9, and the proximal segments of the short arm of chromosome 10. Sporadic inversion, deletion and translocation configurations found in about 1–5% of pachytene nuclei of the hybrid probably represented newly‐arisen spontaneous aberrations rather than constant differences between the parental genomes. No evidence was found that the chromosomes of the parents differed by small structural rearrangements. Either the rearrangements were below the resolving power of the methods employed, or differentiation has affected some property or properties of the chromosomes other than linear order. Evidence from metaphase I suggested that inequalities in chromatic regions of hybrid bivalents inhibited chiasma formation in adjacent achromatic segments but enhanced it in achromatic segments of the opposite arm. The mean number of bivalent arms having chiasmata at metaphase I was nearly as high in the hybrid as in the ‘Pearson’ parent Univalents at metaphase I in the hybrid result not from failure of synapsis or lowered chiasma frequency but from an unbalanced distribution of chiasmata among the chromosome arms.