As mentioned above, when 108-2 or 108-3 was used in the crosses, the
frequency of multivalents was relatively high in the hybrids. On the other
hand, when 108-5 was crossed to the common female parent as 108-2 or 108-3,
the frequency of multivalents was low or they were not observed at all (Table
2). Among these three strains, no multivalent was observed in a hybrid
108-2 x 108-3 but in 108-5 x 108-3, a quadrivalent was observed at a very
low frequency (1.6%). These observations would indicate that there is a
minor translocation between 108-2 or 108-3 and several other strains including
108-5. Type E1 was thus considered to be heterogeneous for minor
translocations. An attempt was made to identify chromosome types differing
with minor translocations. As shown in Table 2,
no multivalent was found in the hybrids between 108-5 and 8541, 8736B, 8935,
8937B or 8943, but the frequency of multivalents varied greatly (0.0-36.4%)
in the hybrids between 108-3 and these strains. That is, the frequency of
quadrivalents produced by the estimated minor translocation varied greatly
in different hybrid combinations. Therefore, for the sake of accuracy, each
strain was classified into a subtype only when two or more hybrids were
available.
First, 108-5 and 108-3 were chosen as the standard E1a and E1b
strain, respectively, based on the observations described above. Strains
8541, 8736B, 8935 and 8943 were classified into type E1a because
no multivalent was observed in hybrids with 108-5 but multivalents were
observed at a low frequency in hybrids with 108-3. Also, 8536, 8736A, 8817
and 8821C were included into the E1a type since no multivalent
was observed in two or more hybrids with 108-5 or other E1a strains
mentioned above. No multivalent was formed in hybrids between the three
strains, 1959B, 1976B and 1978B. This indicates the absence of a translocation
between them. In the hybrid 1959B x 8536, no multivalent was formed but
a quadrivalent was observed at a low frequency in hybrids between 1976B
or 1978B and 108-3. Therefore, these three strains were also classified
into type E1a. Based on a similar rationale, 108-2 was included
into E1b. The average frequency of cells with multivalents in
11 hybrids between E1a and E1b was 16.3% (1.6-36.4%).
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