1. The chromosome number in the males of
Dicranocephalus agilis is 2n=14. The complement includes a pair of m-chromosomes and a pair of sub-equal sex chromosomes.
2. The first division of meiosis is reductional for the differential regions of the autosomes and for the m-chromosomes which are completely differential. It is equational with respect of the sex chromosomes.
3. The sex chromosomes are positively heteropycnotic at the prophase of meiosis I and are associated non-homologously. They are double when the autosomes pair at zygotene.
4. At metaphase of first and second division the autosomes congress to form a ring on the periphery of the spindle. This is in keeping with their size, valency and the space available.
5. At equilibrium, the sex chromosomes auto-orientate side by side in the ring with the autosomes at first metaphase. The sex pseudo-bivalent congresses in the centre of the spindle at second metaphase where less space is available.
6. The sex chromosomes become negatively heteropycnotic at first metaphase and the sex chromatids form a pseudo-bivalent during their anaphase separation. This is facilitated by their side by side orientation at metaphase (a consequence of their earlier association), their convergence as they approach the poles and their reduced repulsion.
7. The early formation of the sex pseudo-bivalednt in
D. agilis and
Oncopeltus fasciatus is considered adaptive in the absence of an interphase which suggests a more rapid division than in
Cimex and
Oncopeltus nigriceps where the pseudo-obivalent forms later and the second division is of longer duration.
8. The m-chromosomes are unpaired at prophase of the first division but associate in the centre of the spindle at first metaphase when they are understained. They congress to the centre of the spindle at second division also. They, and the sex chromosomes remain understained until the completion of meiosis.
9. The differential staining of the sex and in-chromosomes compared with the autosomes is important only in that it provides a visible demonstration of allocycly which can be inferred on other grounds.
10. The centromere in
D. agilis is diffuse. It is suggested that the time and place at which the chromosomal spindle fibres are organised are partly responsible for determining whether the first division of meiosis is reductional for the differential segments (as is general in the Heteroptera) or equational (as is generally the case in the Homoptera).
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