Methods for Fish Biology

Chapter 6: Chromosome Preparation and Analysis

Gary H. Thorgaard and Jane E. Disney


The study of fish chromosomes has become an active area of research in recent years. Chromosome analysis can be useful for addressing a variety of evolutionary and genetic questions about fishes. Because closely related species often differ in chromosome number and morphology, analysis of chromosomes can be useful in species identification. The degree of similarity in chromosome number and morphology can be used to estimate evolutionary relationships among species.

Chromosome number and morphology also can vary within fish species. Variations within and between populations can be used to estimate evolutionary relationships, to identify stocks for management, and for other purposes. Induced polyploidy has been investigated in several fish species recently, and chromosome counts can be used to identify polyploid fish with extra sets of chromosomes. Analysis of meiotic chromosomes from germ cells can enhance understanding about inheritance patterns within species and about chromosomal homologies among species and species hybrids.

Several useful reviews of work on fish chromosomes have been published. Gold et al. (1980), Vasil’yev (1980), Sola et al. (1981), and Yu et al. (1987) listed chromosome numbers and morphologies for diverse fish species. Denton (1973), Blaxhall (1975), and Ojima (1982) reviewed methods for making chromosome preparations from fish tissues. Ohno (1974) and Gold (1979) discussed the evolutionary and genetic significance of fish chromosome studies.

The objective of this chapter is to outline several basic methods for making chromosome preparations from fish. Techniques for counting, photographing, and analyzing chromosomes are discussed. Studies of fish chromosomes sometimes are frustrating because of technical difficulties, but they can be very rewarding. We hope that the information and references presented here will allow newcomers to the field to participate in and enjoy fish cytogenetics as much as we have.

Fish chromosomes are most easily viewed with a light microscope at the metaphase stage of mitosis when they are most highly condensed and well defined. The basic approach in making chromosome preparations is to accumulate dividing cells at metaphase and to spread chromosomes from the metaphase cells on a microscope slide for observation. The same principles apply to studies with other animals (Macgregor and Varley 1983), including humans (Yunis 1974; Priest 1977).