BMC genomics – 2017 – Oreochromis niloticus (Nile Tilapia) – sex determination regions

Sex determination regions

The new O_niloticus_UMD1 assembly was used to study sequence differentiation across two sex-determining regions in tilapias. The first region is an XX/XY sex-determination region on LG1 found in many strains of til-apia [9, 34, 44–47]. We previously characterized this region by whole genome Illumina re-sequencing of pooled DNA from males and females [48]. We realigned these sequences to the new O_niloticus_UMD1 assembly and searched for variants that were fixed in the XX female pool and poly-morphic in the XY male pool. Figure 4 shows the FST and the sex-patterned variant alle le frequencies for the XX/XY O. niloticus comparison across the complete Orenil1.1 and O_niloticus_UMD1 assemblies, while Fig. 5 focuses on the highly differentiated ~9Mbp region on LG1 with a substantial number of sex-patterned variants, indicative of a reduction in recombination in a sex determination region that hasexistedforsometime[48].

The second sex comparison is for an ZZ/WZ sex-determination region on LG3 in a strain of O. aureus [11,49]. This region has not previously been characterized using whole genome sequencing. For this comparison we identified variant alleles fixed in the ZZ male pool and polymorphic in the WZ female pool. Figure 6 shows the FST and the sex-patterned variant allele frequencies for this comparison across the whole O_niloticus_UMD1 assembly, while Fig. 7 focuses on the differentiated region on LG3. O. aureus LG3 contains a large ~50Mbp region of differentiated sex-patterned variants, also indicative of a reduction in recombination in the sex determination region. Figure 6 also shows this differentiation pattern on several other LGs (LG7, LG9, LG14, LG16, LG18, LG22 and LG23). It is possible that these smaller regions of sex-patterned differentiation are actually translocations in O.aureus relative to the O. niloticus genome assembly.