A genome-wide association study, supported by a new chromosome-level genome assembly, suggests sox2 as a main driver of the undifferentiatiated ZZ/ZW sex determination of turbot (Scophthalmus maximus).

Background: Understanding sex determination (SD) across taxa is a major challenge for evolutionary biology. The new genomic tools are paving the way to identify genomic features underlying SD in fish, a group frequently showing limited sex chromosome differentiation and high SD evolutionary turnover. Turbot (Scophthalmus maximus) is a commercially important flatfish with an undifferentiated ZW/ZZ SD system and remarkable sexual dimorphism.

Cytogenomic analysis of several repetitive DNA elements in turbot (Scophthalmus maximus).

Repetitive DNA plays a fundamental role in the organization, size and evolution of eukaryotic genomes. The sequencing of the turbot revealed a small and compact genome, as in all flatfish studied to date. The assembly of repetitive regions is still incomplete because it is difficult to correctly identify their position, number and array.

Comparative expression patterns of Sox2 and Sox19 genes in the forebrain of developing and adult turbot (Scophthalmus maximus).

The turbot, Scophthalmus maximus, belongs to the flatfishes (order Pleuronectiformes), which display substantial asymmetry of the olfactory organs and forebrain. Sox genes code for SRY-related HMG domain-bearing transcription factors involved in various developmental processes. Group B1 Sox genes as Sox2 and Sox19 appear to play major roles in neural development.

Fine mapping and evolution of the major sex determining region in turbot (Scophthalmus maximus).

Fish sex determination (SD) systems are varied, suggesting evolutionary changes including either multiple evolution origins of genetic SD from nongenetic systems (such as environmental SD) and/or turnover events replacing one genetic system by another. When genetic SD is found, cytological differentiation between the two members of the sex chromosome pair is often minor or undetectable. The turbot (Scophthalmus maximus), a valuable commercial flatfish, has a ZZ/ZW system and a major SD region on linkage group 5 (LG5), but there are also other minor genetic and environmental influences.

Consolidation of the genetic and cytogenetic maps of turbot (Scophthalmus maximus) using FISH with BAC clones.

Bacterial artificial chromosomes (BAC) have been widely used for fluorescence in situ hybridization (FISH) mapping of chromosome landmarks in different organisms, including a few in teleosts. In this study, we used BAC-FISH to consolidate the previous genetic and cytogenetic maps of the turbot (Scophthalmus maximus), a commercially important pleuronectiform. The maps consisted of 24 linkage groups (LGs) but only 22 chromosomes.

Mapping of DNA sex-specific markers and genes related to sex differentiation in turbot (Scophthalmus maximus).

Production of all-female populations in turbot can increase farmer's benefits since sexual dimorphism in growth in this species is among the highest within marine fish, turbot females reaching commercial size 3-6 months earlier than males. Puberty in males occurs earlier than in females, which additionally slows their growth. Thus, elucidating the mechanisms of sex determination and gonad differentiation is a relevant goal for turbot production.

Comparative expression analysis in mature gonads, liver and brain of turbot (Scophthalmus maximus) by cDNA-AFLPS.

Turbot is one of the most important farmed fish in Europe. This species exhibits a considerable sexual dimorphism in growth and sexual maturity that makes the all-female production recommended for turbot farming. Our knowledge about the genetic basis of sex determination and the molecular regulation of gonad differentiation in this species is still limited.