Cytological studies.

Barton, D. W.

Mapping of the pachytene chromosomes of the tomato (Barton, Amer. J. Bot. 37 (7): in press 1950) has made possible cytological studies not previously adaptable to this plant. Several lines of cytogenetic research are now in progress.

1. Translocations. A series of translocations have been selected from F1 progeny from X-ray treated pollen. All translocations identifeLed thus far have caused about 30% pollen abortion in the heterozygote. Pachytene configurations are as expected and synapsis is quite good in most trans locations. Rings, chains, pairs and univalents are found at diakinesis, and the proportion of these configurations can be used to identify the translocation at diakinesis. Table 1 indicates the percentage of the associations at diakinesis for some translocations.

                  Table 1
               Ring  Chain  12\II\  10\II\  1\III\  11\II\
Translocation   IV     IV            plus 1\I\      plus 2\I\
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  T  2-12*      --     63    33          3.3           0.7
  T  9-12       36     58     4            2            --
  T  3-8        21     75     3            1            --
  T  5-7        30     54    10            6            --
  T  7-9         4     84    --           12            --
  T  1-9         2     68    30           --            --
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     *Prometaphase data

2. Deficiencies. Plants deficient for chromosome segments in the heterozygous condition have (thus far) had accompanying changes evident in plant morphology. Physiological upset ranges from extreme to slight. Work is in progress to determine the degree of transmission of these deficiencies through the pollen and ovules. Pollen abortion is over 50%.

3. Species. The pachytene morphology of several Lycopersicon species has been mapped. Despite previous evidence that chiasma frequency at diakinesis (and presumably pairing at pachytene) are only slightly reduced in species hybrids, the pachytene morpholoy of other species deviates markedly from that of esculentum. L. pimpinellifolium and L. cerasiforme, which cross so easily with esculentum, have chromosome morphology similar to esculentum. On the other hand, L. peruvianum var. dentatum, L. peruvianum var. humifusum, and L. hirsutum chromosomes are much less similar, although certain chromosomes may be recognized as being homologous those of esculentum.