Abstract
Colcemid-treated blood sampling methods permitted conventional cytogenetic studies of elasmobranch karyotypes. Representatives were karyotyped from the superorders: Galeomorphii (4 orders), Squalomorphii (3 orders), Squatinomorphii (1 genus), and Batoidea (4 or 5 orders). The 36 elasmobranch species karyotyped by this method, together with 20 species using colchine, represent ~4.3% of living chondrichthyans. DNA content exhibited the greatest variability. Chromosome arm numbers, centromere numbers and DNA content data for 47 species indicated the direction of karyotypic change during evolution within the elasmobranchs. Thus arm number has been the most conservative genomic parameter in elasmobranch evolution. A fusion model (rather than fission or modal models) best explained the data obtained for the galeomorphs and batoids studied and explains karyotypic change in other superorders.
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