Speaker: Dr. Asano Ishikawa
Institute: Ecological Genetics Laboratory, National Institute of Genetics, Mishima, Japan
Talk title: Genetic molecular mechanisms underlying life-history evolution in sticklebacks
The annual timing of reproduction is a key life history-trait having a large effect on fitness. Populations often show convergent evolution of the reproductive period to adapt to different seasonality between habitats. However, little is known about the molecular genetic mechanisms. To address this question, we use the three-spined stickleback as a model. Stickleback are primarily marine, but colonized and radiated into postglacial freshwater habitats. First, we found that the multiple freshwater populations exhibited an extended breeding season. We showed that gonad development is suppressed by short photoperiod in marine ecotypes, but not in freshwater ecotypes. Transcriptome analysis demonstrated that thyroid stimulating hormone beta 2 (TSHb2) exhibits photoperiodic changes in marine ecotypes, but not in multiple freshwater populations. Knockout marine fish of TSHb2 demonstrated that fish showed signs of maturation even under short photoperiod. They were also similar to freshwater individuals in overall brain transcriptome and body size, suggesting pleiotropic effects. Linkage mapping and allele-specific expression analysis revealed that the parallel loss of TSHb2 response occurred via different mechanisms; cis-regulatory mutations in Canada and trans-regulatory mutations in Japan. Therefore, the convergent evolution of TSHb2 pathway underlies variation in reproductive seasonality in the three-spined sticklebacks. We also found the intraspecies variation in photoperiod-dependent TSHb2 responses in the nine-spined sticklebacks and medaka fishes, suggesting the convergent evolution of TSH pathway underlying variation in life-history traits in further diverse taxa.