chapter  16
15 Pages

Genetics of Sharks, Skates, and Rays

ByEdward J. Heist

CONTENTS 16.1 Elasmobranch Cytogenetics ......................................................................................................... 471

16.1.1 Genome Sizes.................................................................................................................. 471 16.1.2 Chromosome Complements ............................................................................................ 472 16.1.3 Sex-Determination........................................................................................................... 472 16.1.4 Genetic Oddities.............................................................................................................. 473

16.2 Population Genetics, Stock Structure, and Forensics.................................................................. 473 16.2.1 Molecular Markers .......................................................................................................... 473

16.2.1.1 Isozymes and Allozymes ................................................................................. 473 16.2.1.2 Mitochondrial DNA ......................................................................................... 474 16.2.1.3 Nuclear DNA ................................................................................................... 474

16.2.2 Measuring Stock Structure with Molecules ................................................................... 476 16.2.3 Forensic Identification and Cryptic Species ................................................................... 478

16.3 Molecular Ecology ....................................................................................................................... 479 16.3.1 Philopatry and Sex-Biased Dispersal.............................................................................. 479 16.3.2 Parentage and Multiple Paternity.................................................................................... 480

16.4 Summary....................................................................................................................................... 481 References .............................................................................................................................................. 481

16.1 Elasmobranch Cytogenetics The genetic code of animals including elasmobranchs is compartmentalized into two cellular organelles: the nucleus and the mitochondrion. The vast majority of DNA is found in the nucleus where it is packaged into discrete chromosomes (Futuyma, 1998). Chromosomes segregate during meiosis in germ cells, a process that ultimately leads to the formation of haploid gametes (sperm and egg). Thus, nuclear DNA exhibits biparental inheritance; each diploid parent contributes a haploid chromosome complement to form a new diploid offspring. Mitochondrial (mt) DNA is a haploid code that typically exhibits maternal inheritance in vertebrates (Futuyma, 1998) including, presumably, elasmobranchs. Every cell contains numerous mitochondria each with multiple copies of the mitochondrial genome. When an egg is fertilized, only the mtDNA derived from the female parent is retained in the developing embryo. Thus, all mtDNA in an elasmobranch is derived from a small number of copies present in the ovum.