Aspects of chromosomal mutation and karyotype evolution in ants are discussed with reference to recently accumulated karyological data, and to detailed karyotype analyses of several species or species complexes with low chromosome number and unusual chromosomal mutations (the complexes of Myrmecia pilosula (Smith) (n=1, 5 or 9 to 16); M. piliventris Smith (n=2, 3-4, 17 or 32), and Ponera scabra Wheeler (n=3 or 4, 2n=7 or 8)). Translocations and Robertsonian polymorphisms are confirmed to be non-randomly distributed among ants -the former are found at high frequencies in species with low chromosome numbers (n<_ 12), while the latter predominate in those with high numbers (n> 12). This situation is consistent with the minimum interaction hypothesis of , under which translocations are expected to occur most frequently in low-numbered karyotypes, and that the resulting genetic risks are minimized by increases in chromosome and/or arm numbers through centric fission and pericentric inversion. Centric fusion is considered to be a transient event in karyotype evolution, resulting from telomere instability in acrocentric chromosomes.
A new sibling species of the primitive Australian ant Myrmecia pilosula has a chromosome number of n = 1. C-banding techniques confirm that the two chromosomes of workers are homologous. Males are haploid, as in other Hymenoptera, and their somatic cells contain only a single chromosome. This new species is potentially of great importance in both laboratory and field studies on gene organization.
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