Mapping Chromosome Evolution between Species

Sticklebacks are small fish that inhabit fresh water and marine environments throughout the Northern Hemisphere. They are established models for the study of species evolution (speciation) and are more recent subjects for the study of chromosome rearrangements, as the chromosomes of the six species within the stickleback family differ in both number and morphology (shape/size). These differences in genome architecture among sticklebacks also include sex chromosome systems. Sex chromosomes are chromosome pairs that genetically determine the sex of an individual. The XX/XY sex chromosome is called a “male heterogametic” system, in which males are XY (one X chromosome and one Y chromosome), females are XX (two X chromosomes), and the presence of the Y chromosome is required for male development. In contrast, the ZZ/ZW sex chromosome is called a “female heterogametic” system, in which females are ZW, males are ZZ, and the W chromosome is required for female development. For both XX/XY and ZZ/ZW sex chromosome systems, the X and Y or Z and W may differ in morphology; these systems are known as heteromorphic sex chromosomes. Two different XX/XY sex chromosome systems have evolved independently in threespine and ninespine sticklebacks, and fourspine sticklebacks have evolved ZZ/ZW sex chromosomes.