Quest for the sex chromosomes in Squalius pyrenaicus (Teleostei, Cyprinidae)

In teleosts, evolutionary proximity does not dictate similarity in the sex determining systems. Hybridization, even among closely related species, often results in a huge shock. A good example of such deviation is the cyprinid species complex of Squalius alburnoides with unique characteristics that make this a very interesting model of study regarding the differentiation of sex chromosomes in a hybrid context. This complex endemic to the Iberian Peninsula resulted from the hybridization of Squalius pyrenaicus females (P genome) with males from an unknown species closely related to Anaecypris hispanica (A genome). The complex comprises males and females with different levels of ploidy and genome combinations (genomotypes) intertwined by altered modes of reproduction, ranging from normal meiosis to meiotic hybridogenesis. Sex ratio in nature is clearly biased towards the prevalence of triploid females but the complex also comprises an allegedly all-male lineage. Overall, sex chromosomes are very similar with X and Z having euchromatic regions while Y and W are completely heterochromatic. The accumulation of heterochromatin and the potential loss of genes on Y and W chromosomes are considered convergent traits in the process of sex chromosome differentiation. Bearing in mind S. pyrenaicus as the maternal lineage of S. alburnoides and the possibility of a ZW sex determining system in this complex, GISH (genomic in situ hybridization) experiments were performed in 6 specimens (3 males, 3 females; Cheleiros tributary, Portugal) trying to identify polymorphisms that could be associated with differentiating sex chromosomes in S. pyrenaicus. No differences could be found suggesting that either sex determining genes are scattered throughout the genome of this species or that, if present, sex chromosomes remain in a relatively undifferentiated state (morphological and structural). This investigation requires higher resolution cytogenetic tools like repetitive-free chromosome-specific probes, single-copy gene mapping (sex related genes) and chromosome-specific anchoring probes/markers.