1. The chromosome complements of
O. viridulus and
Ch. brunneus agree in number, size range and centromere position. They differ however in size, those of
O. viridulus especially the L-bivalents being longer.
2. This is paralleled by a marked difference in the mean and variance of the chiasma frequency and suggests that the increased chiasma frequency of
viridulus may depend, at least in part, on structural change.
3. The differences between the two species depend upon changed chiasma properties in the three long and the three longest of the medium bivalents. In the L-bivalents the pattern is consistent, all three showing a significant increase in the mean value in
O. viridulus. In the M-bivalents the pattern is inconsistent. The M
4 of
O. viridulus has a higher mean than that of
Ch. brunneus with no difference in the variance while the M
5 of
O. viridulus shows an increased mean and variance over the M
5 of
Ch. brunneus. By contrast the M
6 of
O. viridulus has a lower mean and a lower variance than that of
Ch. brunneus.
4. The increased chiasma values of
O. viridulus are shared by the five longest bivalents (L
1-M
5 inclusive).
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