Tetraploidy in paddlefish, Polyodon spathula? - 34 years later

Symonová R. Flajšhans M. Gela D. Pelikánová Š. Rábová M. Ráb P

The hypothesis on tetraploid origin of North American paddlefish, Polyodon spathula based on simple karyotyping of 31 cells from 2 males by Dingerkus & Howell (1976) was recently confirmed by molecular phylogeny but chromosomes of this highly endangered, ancient fish has never been analysed using both conventional and molecular cytogenetic protocols. We examined chromosomes of paddlefish derived from leukocyte cultures using sequentially Giemsa- and Ag-staining, CMA3- and DAPI-fluorescence and FISH with 28 S, 5 S rDNA and telomeric (TTAGGG)n probes. Diploid chromosome number 2n=120 (48 macro-, 72 microchromosomes) and macrochromosomes apparently grouped in quadruplets, including acrocentric “acipenseriform” chromosome marker, were reconfirmed. The major (28S) and minor (5S) rDNA sites co–localize in p arms of four (likely) smallest metacentric chromosomes; while all four 28 S sites displayed signal with the same intensity, 5S sites displayed one pair with remarkable stronger signal intensity. Telomeric signals were situated in telomeres, no interstitial telomeric site was observed. CMA3-fluorescence revealed only four signals onto p arms of four (likely) smallest metacentric chromosomes corresponding to rDNA sites and Ag-staining and four signals in pericentromeric regions of acrocentric “acipenseriform” marker. The DAPI-fluorescence revealed only weak signals in centromeric regions of four the second largest and four smaller metacentric chromosomes and also in pericentromeric region of a one submetacentric pair of chromosomes (i.e., not in quadruplet) in karyotype. Such evident fourfold chromosomal characteristics support the hypothesis about tetraploid origin of paddlefish but presence of a single submetacentric pair with AT-rich heterochromatin indicates some remodelling of its karyotype after tetraploidization.