Elasmobranch Cytogenetics: Methods and Sex Chromosomes

Conventional cytogenetic investigations of elasmobranchs (sharks, skates, and rays) have lagged behind that of other vertebrate groups because methods effective for other vertebrates are ineffective for elasmobranchs. Short-term culture and hypotonic swelling of blood cells methods have yielded good metaphase preparations of a wide range of elasmobranch species. A flow cytometric procedure was also developed to measure the DNA contents of elasmobranch erythrocytes. These methods, in a survey of elasmobranch karyotypes, revealed evidence of sex chromosome differentiation in a number of species. Two guitarfish (Rhinobatoidea) exhibited unequivocal male heterogamety (XY). The Y chromosome in Rhinobatos productus (shovelnose guitarfish) was the smallest and the Y chromosome in Platyrhinoidis triseriata (thornback) was the largest chromosome in respective male karyotypes. Possible male heterogamety was observed in Carcharodon carcharias (white shark), Rhizoprionodon terraenovae (Atlantic sharpnose shark), Carcharhinus acronotus (blacknose shark), and the Carcharhinus limbatus (blacktip shark). Possible female heterogamety was observed in Dasyatis americana (southern stingray). Heteromorphic chromosomes were not observed in either male or female Raja eglanteria (clearnose skate) or Galeocerdo cuvier (tiger shark) karyotypes. Male heterogamety appears to predominate in elasmobranchs. Development of methods to use blood samples for conventional cytogenetic studies of elasmobranchs permitted, for the first time, a broad survey of elasmobranch karyotypes. Observations indicated that arm number has been the most conservative genomic parameter in elasmobranch evolution. This process has occurred gradually since the radiation of modern elasmobranchs (neoselachians) began before or during the Jurassic.