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.

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The mammalian amelogenin (AMEL) genes are found on both the X and Y chromosomes (gametologous). Comparison of the genomic AMEL sequences in five primates and three other mammals reveals that the 5 portion of the gametologous AMEL loci began to differentiate in the common ancestor of extant mammals, whereas the 3 portion differentiated independently within species of different mammals. The boundary is marked by a transposon insertion in intron 2 and is shared by all species examined. In addition, 540-kb DNA sequences from the short arm of the human X chromosome are aligned with their Y gametologous sequences. The pattern and extent of sequence differences in the 5 portion of the AMEL loci extend to a proximal region that contains the ZFX locus, and those in the 3 portion extend all the way down to the pseudoautosomal boundary (PAB)1. We concluded that the AMEL locus spans an ancient PAB, and that both the ancient and present PABs were determined by chance events during the evolution of mammals and primates. Sex chromosome differentiation likely took place in a region that contains the male-determining loci by suppressing homologous recombination.chromosomal rearrangement ͉ evolutionary strata ͉ recombination suppression L ahn and Page (1) have proposed that there are four distinct evolutionary strata on the human X chromosome, and that differentiation of the X from the Y chromosome was initiated one stratum at a time. This hypothesis is based on the observation that the average extent of the sequence divergences at synonymous sites between X and Y homologous, or more precisely gametologous, loci is Ϸ10% in stratum 4 in contrast to 30% in stratum 3, 50% in stratum 2, and 100% in stratum 1. Stratum 4 spans Ϸ20 megabases on the short arm region of the X chromosome and is bounded by the amelogenin (AMEL) locus and pseudoautosomal boundary (PAB)1. Among seven loci examined in stratum 4 (1), AMEL has been more extensively studied in animals other than humans (2-7). Notably, primate intron 3 sequences suggest that AMEL on the X chromosome (AMELX) began to differentiate from that on the Y chromosome (AMELY) before the split of Old World and New World monkeys (4). On the other hand, cDNA or amino acid sequences analysis of gametologous AMELs shows greater relatedness within a species than among different mammalian species (5).Recently, Iwase et al. (8) compared human BAC clones that encompass the AMELX and AMELY loci. They found that, although the region downstream from intron 2 exhibits Ϸ10% sequence differences per site, the upstream region exhibits a high level that is similar to stratum 3 (Ͼ 20%; Ϸ30% if multiple-hit substitutions are taken into account). This finding does not contradict previous results (4, 5), because AMEL exons 1 and 2 almost exclusively encode the 5Ј untranslated region and are excluded from comparisons of intron 3 or amino acid sequences. Therefore, Iwase et al. (8) pointed out that the boundary between strata 3 and 4 on the human X chromosome lies in AMEL intron 2. Their preliminary study of ge...

A quantitative analysis of the alterations of constitutive heterochromatin in eukaryotic chromosomal evolution was attempted using the accumulated C-banding data available for mammals, amphibians, fish, ants, grasshoppers, and plants. It was found that these eukaryotes could be classified into two types by their C-banding patterns: 1) Type I included mammals, fish, and ants, and 2) Type II included amphibians, grasshoppers, and plants. C-bands were rather scarce in Type I eukaryote chromosomes and were found around the pericentromeric region when present at all, whereas the predominance of interstitial or terminal C-bands was found in Type II eukaryote chromosomes. The Type I and II C-banding patterns can best be interpreted by assuming that in the former group of eukaryotes the saltatory increase in constitutive heterochromatin occurs preferentially at the pericentromeric regions of telocentric chromosomes induced by centric fission, with C-bands being eliminated almost completely by centric fusion and/or pericentric inversion. On the other hand, C-bands appear in the Type II eukaryotes both interstitially and in the telomeric regions of chromosomes, and there may be no effective mechanism to eliminate these bands once they are integrated.

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