Tinkering with the axial skeleton: vertebral number variation in ecologically divergent threespine stickleback populations

Abstract

Understanding how the vertebral column is impacted as populations adapt to different habitats favouring distinct body forms can provide insight into the origin of evolutionary diversity in the axial skeleton. We examined variation of vertebral number and body shape in morphologically and ecologically divergent anadromous, benthic, and limnetic threespine stickleback populations from Alaska. Variation in vertebral number was substantial and was associated with body shape variation. Both vertebral homeosis and region-specific changes in vertebral number were present. Sexual dimorphism in vertebral number was significant and resulted from vertebral homeosis; females had more abdominal vertebrae and males more caudal vertebrae, although total vertebrae did not differ. Body elongation was associated with an increase in vertebral number, especially in the caudal region, although this varied among populations. Benthics tended to have less caudal vertebrae than limnetics and anadromous stickleback generally had more abdominal vertebrae than either, although this also varied among populations. Variation among individuals indicated that external similarity in body form masks significant vertebral variation in stickleback. Although more research on the underlying mechanisms and functional significance is needed, our findings highlight the potential of the threespine stickleback as a model for studying the evolution of the vertebrate axial skeleton.