CHROMOSOME BEHAVIOUR IN DIPLOID AND TETRAPLOID CELLS OF SPHODROMANTIS GASTRICA AND ITS BEARING ON CHROMOSOME EVOLUTION IN THE MANTIDS.

Abstract

1. In common with many other mantids, Sphodromantis gastrica possesses a multiple sex chromosome mechanism of the X1X2Y ♂: X1X1X2X2♀-type. At meiosis in spermatocytes and at mitosis in embryo neuroblast cells it can be seen that S. gastrica differs from other species so far described in that the X2 element is acrocentric. Chiasma formation always takes place in the short arms of the chromosome. 2. 24 spontaneously autotetraploid cells at metaphase I were analysed with respect to the types and frequencies of multivalent associations, both for the sex chromosomes and for the autosomes. The chiasma frequencies in the autotetraploid cells were also recorded for comparison with those of diploid cells. The mean 4n cell total was found to be 26.7/cell, fully twice the 2n mean of 13.3/cell. 3. In autotetraploid cells, two aberrant types of allosomal pairing associations suggest that the Y was, ancestrally, an isochromosome. 4. It is suggested that, in addition to X-autosomal translocation, both polyploidy and centromeric misdivision have been involved in the evolution of the chromosome complement of S. gastrica and, perhaps, of all other X1X2Y mantids.