and Aps-1 was performed on 2,224 yv- plants of an F2 population from the same cross. Summing up the BC and F2, a total of 5,591 gametes were analyzed for yv and Aps-1. A summary of results is presented in the Table. In general, the relative map distances between yv, coa and c conform with previous data. The same is true for a-hl. As expected, they show independence from chromosome 6 markers. Among 513 BC plants scored for nematode resistance, all the green plants (yv/+) were free from galls except one which developed a few, tiny galls at the tip of the roots. This plant was then transplanted to a pot with additional nematode inoculum and grown to maturity, revealing no change in the severity of the symptoms. Its progeny segregated for susceptible yv (associated with Aps-1+) end resistant + (associated with Aps-11 ) thus ruling out the possibility that the original plant was a recombinant between yv and Mi. Among the 250 yv plants, 5 were free from galls. Using the same approach as above, two developed galls at maturity, one died after transplanting and the other two which remained free from symptoms proved by progeny test to be susceptible. Therefore no bona fide recombinants were recovered. Likewise, all the yv (BC's and F2's) wereAps-1+/+ while the green segregants (BC only) were Aps-1+/1. Despite the lack of recombination, since there exist susceptible yv and + genotypes, resistant Aps-1+/+, susceptible Aps-1+/+, there is little doubt that yv, Aps-1 and Mi are three distinct genes, and thus pleiotropy by a single gene is unlikely. The results of this test indicate, however, that these genes are so tightly linked they map at the same locus. Previous data by Gilbert (TGC 8:15-17) suggested a distance of 1-2 cM between yv end Mi. Two possibilities may account for this discrepancy: a) Gilbert's recombinants might have been escapes since no progeny tests were performed; b) natural heterogeneity of recombination values. That the latter does occur is a well established fact, yet the causes are poorly understood according to Butler (Can. J. Genet. Cytol. 10:866-892; 19:521-529). Since all the Hawaiian lines (HES) analyzed earlier, including a few lines from J. C. Gilbert, carry Aps- 1+ (Act. Hort. 100: in press), we could assume that the specific line used in Gilbert's linkage analysis ofyv-Mi was Aps-1+. The Mi Aps-1+ lines originated from a crossing over between Aps-11 and Mi, end the adjacent genetic material of L. peruvianum introduced with Mi and Aps-11 might be present to a less extent in those HES lines. Therefore, the chromosome homology of that specific region with the corresponding region of L. esculentum is probably greater in the Aps-1+ resistant lines than in those carrying Aps-11. It is noteworthy that studies of induced deficiencies indicate that yv is located in or very near the proximal heterochromatin on 6L (Chromosoma 23:452- 454). It is possible that Aps-11 lines also carry part of the heterochromatin of L. peruvianum resulting in reduced homology and pairing at that particular chromosome section which would be the major factor preventing the recombination. As far as is known the genomes of the Lycopersicon species are homosequential, and the only marked morphological differences are in the proximal heterochromatin of some interspecific hybrids where pairing is sometimes disturbed (Proc. 10th Int. Bot. Congr. p. 222). Thus, the absence of recombination on the proximal region due to alien heterochromatin is conceivable. Despite the inherent difficulties of detecting small differences in chromosome structure, a cytological study of meiosis in Aps-1 heterozygotes would be required to corroborate this hypothesis. If it is true that the L. peruvianum heterochromatin is repressing recombination, the most logical gene order in 6L would be yv Aps-1 Mi. This would explain the occurrence of recombinants between yv and Mi observed by Gilbert since his Mi materiel was probably Aps-1+ end the corresponding yv and heterochromatin of such lines would be that of L. esculentum. Such an arrangement would allow intimate pairing and the opportunity for crossing over. Evidence, however, to prove such a hypothesis is as yet lacking. Other possible causes for heterogeneity in recombination values referred to in the literature are differences in genetic background, environment, sex, cytoplasm end age of plant. Differences in the genetic background could be e factor since the crosses involved two genetics backgrounds. The other factors mentioned, however, are not likely causes since: a) BC and F2 progenies were produced in two different greenhouse environments, one of them under ideal growing conditions and the other having a wide range of temperature variation. In addition, F2 populations were also produced in the field; b) sex and cytoplasm are unlikely causes because BC and F2's comprised the reciprocals; and c) regarding age, F2 plants analyzed were produced from F1's up to one year old.

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