Molecular structures of centromeric heterochromatin and karyotypic evolution in the Siamese crocodile ( Crocodylus siamensis ) (Crocodylidae, Crocodylia)

Crocodilians have several unique karyotypic features, such as small diploid chromosome numbers (30–42) and the absence of dot-shaped microchromosomes. Of the extant crocodilian species, the Siamese crocodile ( Crocodylus siamensis ) has no more than 2n = 30, comprising mostly bi-armed chromosomes with large centromeric heterochromatin blocks. To investigate the molecular structures of C-heterochromatin and genomic compartmentalization in the karyotype, characterized by the disappearance of tiny microchromosomes and reduced chromosome number, we performed molecular cloning of centromeric repetitive sequences and chromosome mapping of the 18S-28S rDNA and telomeric (TTAGGG) n sequences. The centromeric heterochromatin was composed mainly of two repetitive sequence families whose characteristics were quite different. Two types of GC-rich CSI- Hin dIII family sequences, the 305 bp CSI- Hin dIII-S (G+C content, 61.3%) and 424 bp CSI- Hin dIII-M (63.1%), were localized to the intensely PI-stained centric regions of all chromosomes, except for chromosome 2 with PI-negative heterochromatin. The 94 bp CSI- Dra I (G+C content, 48.9%) was tandem-arrayed satellite DNA and localized to chromosome 2 and four pairs of small-sized chromosomes. The chromosomal size-dependent genomic compartmentalization that is supposedly unique to the Archosauromorpha was probably lost in the crocodilian lineage with the disappearance of microchromosomes followed by the homogenization of centromeric repetitive sequences between chromosomes, except for chromosome 2.