EXPERIMENTAL BASES FOR THE MINIMUM INTERACTION THEORY .1. CHROMOSOME EVOLUTION IN ANTS OF THE MYRMECIA-PILOSULA SPECIES COMPLEX (HYMENOPTERA, FORMICIDAE, MYRMECIINAE)

Chromosome evolution in primitive Australian ants of the Myrmecia pilo sula species complex is investigated in the context of the minimum int eraction theory. Under the minimum interaction theory, selection favor s rearrangements tending to reduce the occurrence of deleterious chrom osomal mutations, and hence chromosome numbers are expected to increas e. The complex is chromosomally highly heterogeneous (2n = 2 - 32), an d comprises at least 5 karyotypically distinct species: M. croslandi ( 2n=2-4), M. imaii (2n=6-8), M. banksi (2n=9-10), M. haskinsorum (2n=12 -24), and M. pilosula (2n=18-32). Statistical considerations using the karyograph method and chromosomal alteration network analysis indicat e that chromosome evolution of the complex proceeds as a whole towards increase in chromosome number by centric fission and inversions conve rting chromosomes from acro- to metacentrics (AM-inversion). These con clusions are consistent with the predictions of the minimum interactio n theory. Both centric fusion and AMBAR-inversion serve to eliminate c onstitutive heterochromatin (visible as C-bands), which appears to inc rease in a saltatory fashion after centric fission, probably due to te lomere instability. Newly observed phenomena which we term ''fusion bu rst'' and ''fission burst'' suggest that rates of chromosome evolution in M. pilosala have fluctuated with time.