Experimental bases for the minimum interaction theory. I. Chromosome evolution in ants of the Myrmecia pilosula species complex (Hymenoptera: Formicidae: Myrmeciinae)

Chromosome evolution in primitive Australian ants of the Myrmecia pilosula species complex is investigated in the context of the minimum interaction theory. Under the minimum interaction theory, selection favors rearrangements tending to reduce the occurrence of deleterious chromosomal mutations, and hence chromosome numbers are expected to increase. The complex is chromosomally highly heterogeneous (2n = 2-32), and 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 indicate that chromosome evolution of the complex proceeds as a whole towards increase in chromosome number by centric fission and, inversions converting chromosomes from aero- to metacentrics (A-M-inversion). These conclusions are consistent with the predictions of the minimum interaction theory. Both centric fusion and A-M-inversion serve to eliminate constitutive heterochromatin (visible as C-bands), which appears to increase in a saltatory fashion after centric fission, probably due to telomere instability. Newly observed phenomena which we term "fusion burst" and "fission burst" suggest that rates of chromosome evolution in M. pilosula have fluctuated with time.