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. Here we describe a new long-read turbot genome assembly used to disentangle the genetic architecture of turbot SD by combining genomics and classical genetics approaches. RESULTS: The new turbot genome assembly consists of 145 contigs (N50 = 22.9 Mb), 27 of them representing >95% of its estimated genome size. A genome wide association study (GWAS) identified a ~ 6.8 Mb region on chromosome 12 associated with sex in 69.4% of the 36 families analyzed. The highest associated markers flanked sox2, the only gene in the region showing differential expression between sexes before gonad differentiation. A single SNP showed consistent differences between Z and W chromosomes. The analysis of a broad sample of families suggested the presence of additional genetic and/or environmental factors on turbot SD. CONCLUSIONS: The new chromosome-level turbot genome assembly, one of the most contiguous fish assemblies to date, facilitated the identification of sox2 as a consistent candidate gene putatively driving SD in this species. This chromosome SD system barely showed any signs of differentiation, and other factors beyond the main QTL seem to control SD in a certain proportion of families.

Effects of carbohydrate sources on growth, body composition and tissue lipid deposition of blackspot seabream, Pagellus bogaraveo (Brunnich).

Rearing blackspot seabream has been associated with low growth rates and excessive lipid accumulation, resulting in a reduction of the edible yield. The effect of extruded diets containing different carbohydrate sources (wheat vs. wheat bran) was evaluated on 100 g blackspot seabream growth performance, feed utilization and fat deposition, taking into consideration the optimal dietary protein and lipid level described for smaller-sized fish. A fish meal-based diet was also tested as a control to assure maximal growth rates were achieved. The experiment was held in sea cages at environmental conditions. Duplicate groups of fish were distributed among six cages with a stocking density of 1.3 kg/m(3) and hand-fed each diet for 7 months. At the end of the experiment, fish in all groups doubled their body weight attaining 190-230 g. Specific growth rate (0.3-04), feed conversion ratio (1.3-1.6) and protein gain (0.5-0.6 g/kg/day) were similar among treatments. The Hepatosomatic Index, the Viscerosomatic Index and final whole body composition did not vary significantly among dietary treatments; nevertheless, the inclusion of wheat bran induced a significant increase of liver lipid content and the highest mesenteric fat index. All diets were effective in reducing whole body and mesenteric fat compared with initial values.