Principles of Genetics and Breeding

1.3.6 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 30 µm 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.