The establishment and application of a genotyping assay to study the sex ratios of pike populations from Rügen

The Northern Pike (Esox lucius) has been at the forefront of research focused on sex determination systems in recent years. The characterization of a new master sex determining (MSD) gene, and the proceeding breakdown of genetic variations among pike populations, resulted in a by this time impressive state of knowledge in regard to this species’ genome. This study aimed for the establishment of a genotyping assay to verify the sex determination system in Northern Pike populations from Rügen, based on the existing knowledge of the involved genes. A straightforward PCR assay was established based on a pool of confirmed male and female individuals, and successfully verified the presence of the presumed sex determination system in these populations. The gained knowledge was further used to establish a marker for Next Generation Sequencing (NGS), enabling an in-silico sexing approach. External sex determination in Northern Pike, primarily based on morphological differences and the examination of the urogenital opening, has been found to be unreliable. This methods are particularly inconsistent during the spawning season and in earlier life stages. The implementation of a genotype-based sex determination assay, once established, offers numerous advantages, including its consistency, reliability, and applicability across various developmental stages and seasonal variations. The established marker allowed for the comparison of genotyped and externally sexed individuals and revealed a discrepancy in the external sex determination in approximately 9,2 % of the individuals. The majority of the incorrectly sexed individuals were sampled during spawning season, underlining the limitations of external sexing during that period. On one side the findings show the potential of straight forward non-invasive genotyping assays for sex determination, on the other side they revealed the challenges in obtaining unbiased estimates of sex ratios due to selective sampling methods and sex specific growth.