Chromosomal polymorphisms involving telomere fusion, centromeric inactivation and centromere shift in the ant Myrmecia (pilosula) n=1

Abstract

Detailed karyological surveys of the ant Myrmecia pilosula species group, which is characterized by the lowest chromosome number in higher organisms (2n=2), were attempted. We revealed that this species has developed highly complicated chromosomal polymorphisms. Their chromosome numbers are in the range 2n=2, 3, and 4, and six polymorphic chromosomes are involved, i.e., two for chromosome 1 (denoted as SM₁ and ST₁), three for chromosome 2 (A₂, A′₂, and M₂), and M₍₁₊₂₎ for the 2n=2 karyotype. We suggested that these chromosomes were induced from a pseudo-acrocentric (A ^M₁ ) and A₂ as follows: (1) A ^M₁ →SM₁ or ST₁ by two independent pericentric inversions; (2) A₂→A′₂→M₂ by chromosomal gap insertion and centromere shift; and (3) ST₁+A₂→M₍₁₊₂₎ by telomere fusion, where (3) means that the 2n=2 karyotype was derived secondarily from a 2n=4 karyotype. It is a noteworthy finding that active nucleolus organizer (NOR) sites, in terms of silver staining, are tightly linked with the centromere in this species, and that both the centromere and NOR of A₂ were inactivated after the telomere fusion.