Sex determination and sexual differentiation in the avian model

Abstract

The sex of birds is determined by the inheritance of sex chromosomes (ZZ male and ZW female). Genes carried on one or both of these sex chromosomes control sexual differentiation during embryonic life, producing testes in males (ZZ) and ovaries in females (ZW). This minireview summarizes our current understanding of avian sex determination and gonadal development. Most recently, it has been shown that sex is cell autonomous in birds. Evidence from gynandromorphic chickens (male on one side, female on the other) points to the likelihood that sex is determined directly in each cell of the body, independently of, or in addition to, hormonal signalling. Hence, sex‐determining genes may operate not only in the gonads, to produce testes or ovaries, but also throughout cells of the body. In the chicken, as in other birds, the gonads develop into ovaries or testes during embryonic life, a process that must be triggered by sex‐determining genes. This process involves the Z‐linked DMRT1 gene. If DMRT1 gene activity is experimentally reduced, the gonads of male embryos (ZZ) are feminized, with ovarian‐type structure, downregulation of male markers and activation of female markers. DMRT1 is currently the best candidate gene thought to regulate gonadal sex differentiation. However, if sex is cell autonomous, DMRT1 cannot be the master regulator, as its expression is confined to the urogenital system. Female development in the avian model appears to be shared with mammals; both the FOXL2 and RSPO1/WNT4 pathways are implicated in ovarian differentiation.

High-density linkage mapping aided by transcriptomics documents ZW sex determination system in the Chinese mitten crab Eriocheir sinensis

“…The orthologs identified from E. sinensis transcriptomes are enclosed in red frames. The pathways are modified based on Hansen and Pilgrim (1999), Chue and Smith (2011) and Kopp (2012). Sxl, sex-lethal; tra, transformer; dsx, double sex; fru, fruitless; xol, XO-lethal; sdc, sex and dosage compensation; her, hermaphroditization; fem, feminization; Foxl2, forkhead box protein L2; Rspo1, R-spondin-1; Wnt4, wingless-type MMTV integration site family, member 4.…”

De novotranscriptome sequencing to identify the sex-determination genes inHyriopsis schlegelii

“…Many sex-determination genes, including Sry (sex-determination region of chromosome Y), 1) Sox9 (SRY box-containing gene 9), 2) Dmrt1 (Doublesex and mab3-related transcription factor 1) and Dmrt2 (Doublesex and mab3-related transcription factor 2), 3) Foxl2 (forkhead box L2), 4) Rspo1 (R-spondin1), and β-catenin, 5) have been implicated in sexual determination. Dmrt genes have been identified in species such as mammals, 6) birds, 7) amphibians, 8) and teleosts. 9) However, Sry genes have not been identified in non-mammalian vertebrates.…”

Chicken hemogen homolog is involved in the chicken-specific sex-determining mechanism

“…A strong candidate for male sex determination in chicken is doublesex and mab-3 (Protein MAB-3) related transcription factor 1 (DMRT1), which is on the Z chromosome (3). DMRT1 has been suggested to activate SOX9 indirectly, because there is a time lag between the expression of DMRT1 and SOX9, which are first expressed on day 4.5 and day 6.5 of incubation, respectively (4)(5)(6). Therefore, other factors that are likely to be chicken specific must be in the molecular cascade between DMRT1 and SOX9.…”