Chromosomal Variation in Cutthroat Trout

Chromosome numbers and morphologies (karyotypes) are relatively slowly-evolving but both show informative variation among cutthroat trout subspecies.  This information is laborious to collect but does not require expensive reagents. Westslope and coastal cutthroat have higher chromosome numbers (66 and 68-70, respectively) than other subspecies and a high frequency of short second arms on many chromosomes.  Samples of several inland subspecies (Utah, Colorado River and Yellowstone) have 64 chromosomes and few chromosomes with short second arms. Presumptive evolutionary relationships of subspecies based on analysis of karyotypes gave some contrasting results to those with other types of genetic markers.  Surprisingly, although aligned to westslope and coastal based on other types of markers, the Lahontan cutthroat have a karyotype like the inland subspecies.   Some gaps in knowledge of cutthroat trout karyotypes include coastal cutthroat north of California and the Rio Grande cutthroat.   The relatively slow rate of evolution of chromosomes makes them useful for tracing relationships among subspecies.  Results to date have not shown variations useful for identifying distinctive populations within subspecies but this might change with broader sampling.