1. Spermatogonial chromosomes of
Grylloblatta sp. are found to be 30 in number, including an XY pair of sex-chromosomes. X is a large metacentric chromosome, its shorter arm being as long as the largest of the acrocentric autosomes. Y is not identified from the evidences at hand, but if a certain one of the largest acrocentric chromosomes is assumed to be Y, the situation would be analogous to what should be expected from the centric fusion of X with a member of a large acrocentric autosomal pair.
2. In
Oecanthus indicus and in an unidentified species the chromosomes are 19 in the male and 20 in the female. Accordingly, the condition of the sex-chromosomes is XO-XX, as usually found in the Orthopteroid insects. The chromosomes-complexes of both species are similar and very much alike to that of
Oe. longicauda except for the presence of a small Y-chromosome in the latter. From the similarity of autosomal assortments and size variation of the Y found by Makino, it may probably be assumed that the XO-condition of the
indicus-type has been derived from the apparatus of
longicauda by gradual diminution of the Y-element until its complete disappearance. The evolutional process of the XO-condition found in
Oe. nigricornis cannot be accounted for in the same way, since X is metacentric having two arms of almost equal length.
3. The primitive condition of the sex-choromosomes in the
Oecanthus group is assumed to be of XO-XX type, X being acrocentric. The evolutional process of these different types of sex-chromosomes in this group may be explained as follows: if breakage happened to occur in an X and translocation of its distal part to the centric end of the other had taken place in the female, metacentric neo-X and an acrocentric neo-Y would have been formed. The XO-XX condition of
nigricornis may be of the metacentric neo-X, while the XY pair of
longicauda would be consisted of an acrocentric original X and a neo-Y. The XO-XX condition of
indicus-type, of which X being acrocentric, would have been brought about from XY-XX of
longicauda-type by gradual diminution to complete disappearance of Y.
4. The relation between acrocentric and metacentric types of the XO-XX conditon which are reported in closely related species of Orthopteroid insects would be interpreted in the same way.
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