Chapter 14 - Chromosome Morphology

Abstract

Chromosome markers usually do not require as much development time as molecular markers, but they have the disadvantage of requiring living tissue. Once developed for a particular group, molecular methods are more efficient because multiple loci can be scored on a single gel. In addition, although DNA variation of some type is present in virtually every species, intraspecific chromosome variation may not be present. In fact it appears that marine fishes have much more stable karyotypes than freshwater fishes, so chromosome number variation is rare in them. Although chromosome number variation has been observed in shellfish, no intraspecific variation has been documented. Usually NOR staining will reveal intra-specific variation in almost any species, but it may not be stock-specific. Most species have a unique karyotype, so cytogenetic methods can be very valuable for analysis of hybrid zones between closely related species or subspecies. For fish species with intraspecific chromosome number variation, hatchery fish may have a different chromosome number than wild fish, so in that case it would be possible to estimate the percentage of hatchery vs. wild fish and identify any hybrids between the two using cytogenetic methods. Such variation is especially common in salmonid fishes, and often involves intraspecific translocation polymorphisms. In human genetics, molecular markers diagnostic for specific translocations involved in specific tumors and birth defects have been developed. These types of assays could make identification of chromosome variation more feasible for population studies in the future.