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

Abstract

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-HindIII family sequences, the 305 bp CSI-HindIII-S (G+C content, 61.3%) and 424 bp CSI-HindIII-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-DraI (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.

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Abbreviations

AMI:

Alligator mississippiesis

ASI:

Alligator sinensis

BCO:

Boa constrictor

BrdU:

5-bromodeoxyuridine

BSG:

barium hydroxide/saline/Giemsa

CCD:

charge-coupled device

CCR:

Caiman crocodilus

CLA:

Caiman latirostris

CNI:

Crocodylus niloticus

CSI:

Crocodylus siamensis

Cy3:

orange-fluorescing cyanine

DDBJ:

DNA Data Bank of Japan

Dig:

digoxigenin

EQU:

Elaphe quadrivirgata

FISH:

fluorescence in-situ hybridization

FITC:

fluorescein isothiocyanate

GGA:

Gavialis gangeticus

GHO:

Gekko hokouensis

Mya:

million years ago

NF:

fundamental number

PCR:

polymerase chain reaction

PI:

propidium iodide

PFL:

Protobothrops flavoviridis

PMO:

Python molurus bivittatus

PSI:

Pelodiscus sinensis

rDNA (RNA):

ribosomal DNA (RNA)

SDS:

sodium dodecyl sulfate

SSC:

saline sodium citrate

TSC:

Tomistoma schlegelii

UV:

ultraviolet

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Acknowledgements

This work was supported by Grants-in-Aid for Scientific Research (nos. 15370001 and 16086201) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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Correspondence to Yoichi Matsuda.

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Kawagoshi, T., Nishida, C., Ota, H. et al. Molecular structures of centromeric heterochromatin and karyotypic evolution in the Siamese crocodile (Crocodylus siamensis) (Crocodylidae, Crocodylia). Chromosome Res 16, 1119–1132 (2008). https://doi.org/10.1007/s10577-008-1263-1

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Key words

  • Archosauromorpha
  • crocodilian
  • genomic compartmentalization
  • heterochromatin
  • karyotype evolution
  • microchromosome
  • repetitive sequence