Principles of Genetics and Breeding

1.3.4 Fish chromosomes

• The work on fish chromosomes began with the studies of Retziat (1890) and Katschenko (1890).
• A revolution in the fish chromosome cytology took place when pre-treatment with colchicine coupled with squashing (Roberts, 1964) and the flame–drying method (Denton and Howell, 1969) were made.

• From about 31,000 fish species estimated to occur, the basic karyotype characteristics, i.e., diploid chromosome number (2n) and the number of chromosome arms (NF), are known for not more than 3,000 fish species.
• The first investigations of chromosomes were made by means of histological sections in the late nineteenth century.
• Then, squash techniques were performed which permitted a more accurate count of chromosome number. Later, the air-drying technique combined with colchicines and hypotonic treatment (to accumulate cells at metaphase and to ease chromosome spreading) greatly increased our knowledge not only of chromosome number, but also of chromosome morphology.
• Karyotyping, i.e. pairing of the chromosomes according to their size and morphology, then became feasible. Recent development of new differential staining techniques has allowed the demonstration of characteristic banding of chromosomes. These techniques have led to a better understanding of the fine characterization of individual chromosomes and to a better standardization of karyotypes.

Aquatic invertebrates have been investigated mostly with squash or air-drying techniques from tissues, while in fish, since 1960 there has been a boom in cytogenetic studies mainly due to the application of human cell culture technique. Because of this, there is more information on banding techniques in fish than in invertebrate species. There has been a biased recording of chromosome numbers in aquatic organisms depending on the group studied. Looking at the aquatic invertebrates, some phyla are poorly investigated, e.g. the sponges or the cnidarians. With regard to minor pseudocoelomate or eucoelomate animals, published data are even more rare.

Fish chromosomal complements contain chromosomes of two, three or even four types. In a number of teleostean fishes additionally very small “microchromosomes” had been found along with larger ones and it is very difficult to quantify such chromosomes. The role of microchromosomes is not fully understood although some authors suggest that they contain the “redundant” genetic material necessary for the cells where there is increased protein synthesis. In salmonids, “satellite chromosomes” have been reported. In these chromosomes small region are separated from the main body by narrow strangulations.

In fishes, the chromosomal complements turn out to be quite variable, the diploid numbers varying between 12 and 250. The diploid (2n) chromosome number for some of the fishes is shown in Table 2.

Table 2.

Fish

Deploid number of chromosome

Catla, rohu, mrigal Grass carp and silver carp common carp, gold fish Tor khudree Clarias batrachus Heteropnuestes fossilis Oreochromis mossambicus O. niloticus Tilapia macrocephalus T. zilli Channa striatus C. gachua C. punctatus Medaka Oryzias latipes Atlantic salmon Brown trout Rainbow trout Etroplus suratensis E. maculates the Amazon Molly, Poecilia formosa,P.mexicana Macrobrachium rosenbegii Penaeus monodon Seabass Milk fish Mullet Cobia

50 48 104 100 50 56 44 44 32 38 40 78 32 48 60 80 60 48 46 46 46 118 88 48 32 26 48

Chromosome number is constant for each species. Number has no specific significance and does not indicate in any way the evolutionary advancement of a species.