Taxonomy of Finfish (1+2)

6.1.7 Trends in Karyotypes Evolution

Chromosome evolution in fishes can be traced back to the protochordates. The tunicates, considered to be a primitive group among protochordates, contain microchromosomes only. A microchromosome is generally less than 0.5 micrometer (µm) in size, whereas average size of fish chromosomes range between 2 to 5 µm. It is believed that large chromosomes have arisen by increase of DNA content through polyploidy and fusion of microchromosomes. It is interesting to note that the lungfish, Lepidosiren paradoxa possess very large chromosomes of 15 to 30µm in length.

In the cartilaginous fishes – sharks, skates and rays, the diploid number of chromosomes varies between 28 to 106, with most species having chromosomes between 52 to 100. The chromosomes are often present.

In the chondrostein fishes of the families Polyodonidae and Acipenseridae, the chromosome numbers and genome sizes are very large. Microchromosomes are present in these species. It is believed that fishes of these families are polyploid of various degrees i.e tetraploid to octaploidy. The karyotypes of the tetraploid species (Acipenser huso, 2n=118+2 and Polyodon spathula , 2n=120) could be divided into groups of four chromosomes of similar morphology. The holosteian fishes, Lepisosteous oculatus) has chromosome number 2n=68 and Amia calva has 2n=46, which includes some microchromosomes. Holostei are believed to be the closest relatives of the Acipenseriform fishes but the chromosome number in the former is comparatively low. This point also suggests that Acipenseriform fishes originated by polyploidy.

The teleostean fishes show a wide variation in chromosome number but they lack microchromosomes. In the order Salmoniformes, the chromosome numbers of the family Salmonidae is comparatively higher than the members of the related families. The members of this family are believed to have originated by polyploidy. Among Cyprinformes, all species of the family Catostomidae and some members of the families Cyprinidae and Cobitidae are polyploids. Of about 450 cyprinidae species studied, it is remarkable to find majority of them having 50 chromosomal complements in their karyotypes. The variations of chromosome number is more pronounced in the order Siluriformes. Polyploid is found in the family Callichthyidae of this order. On the other hand, the chromosome number is less variable in the order Perciformes, where most species possess 2n=48 chromosomes.

Many fish cytogeneticists believe that 48 acrocentric chromosomes constitute the basic diploid karyotypes in teleosts. The karyotypic diversity in fishes arose by polyploid, Robertsonian fission/fusion and other chromosomal rearrangements like pericntric inversion. Chromosomal non-disjunction (i.e. unequal separation of two chromosome sets into two daughter cells) during meiosis produces of chromosomal polymorphism within a species. Robertsonian fission/fusion takes place when the chromosome number changes without any corresponding change in the fundamental arm number (FN). For example, if two acrocentric chromosomes get fused to form a metacentric chromosome or a metacentric chromosome gets split into two acrocentric chromosomes, occurs no change in the FN of chromosome arms. There are three interesting points to be noted in the study of fish karyotypic evolution. i) The presence of polyploid forms in some orders. ii) The chromosomes are distinguishable in only a few species. iii) Intra-specific chromosomal polymorphism occurs in some species.