Academic journal article Genetics

Wnt Signaling in Sexual Dimorphism

Academic journal article Genetics

Wnt Signaling in Sexual Dimorphism

Article excerpt

THE primitive embryonic gonad in Drosophila melanogaster is composed of two distinct cell types, the germ cells and the somatic gonadal precursor cells (SGPs) (Santos and Lehmann 2004). These two cell types are formed at different locations in the embryo and are specified by distinct mechanisms. The germ cells arise as pole cells at the posterior end of the precellular blastoderm embryo, and their proper specification depends on maternal determinants that are assembled in the pole plasm during oogenesis. After cellularization of the blastoderm, the germ cells must make their way into the center of the embryo and then migrate toward the newly formed SGPs in parasegments (PS) 10-13. SGPs are derived from dorsolateral mesodermal tissue in these parasegments and are specified by the hierarchical action of zygotic patterning genes. The dorsolateral mesoderm of PS 10-13 is formed under the control of tinman and zfh-1 (Moore et al. 1998) while the eventual specification of SGPs from these cells depends upon the bifunctional transcription factor eyes absent (eya) (Boyle et al. 1997; Moore et al. 1998). Although eya is required for SGP identity, it is differentially expressed in anterior and posterior SGPs. This difference depends upon the activity of the homeotic genes abdominal-a (abd-A) and Abdominal-B (Abd-B). The specification of anterior SGPs depends upon abd-A, while the specification of posterior SGPs depends upon both abd-A and Abd-B (Boyle and DiNardo 1995; De Falco et al. 2004).

In addition to differences between anterior and posterior SGPs, the embryonic gonad is sexually dimorphic. One sexspecific difference is in the activity of signaling pathways that mediate communicationbetweentheSGPs and the primordial germ cells (PGCs). Wawersik et al. (2005) found that the ligand for the JAK-STAT pathway, unpaired (upd) (Harrison et al. 2003), is expressed in a small group of SGPs at that very anterior of the embryonic gonad in male but not female embryos. [The same sex-specific expression pattern is seen for the closely related upd-3 (Hombria et al. 2005).] The upd ligand signals to the germ cells in male embryos upregulating the level and activity of the transcription factor STAT92E (Hou et al. 1996). By contrast, there is little, if any, STAT92E in the germ cells of female embryos. The activity of the JAK-STAT pathway in males and females is dependent upon the somatic sex determination pathway. The sex determination pathway can be bypassed in females by ectopic expression of upd (or upd-2 or upd-3), which then activates STAT92E accumulation in their germ cells.

Another sex-specific difference is the presence of cell types in one sex but not the other. (De Falco et al. 2003, 2008). One example of a cell type found only in males is the pigment precursor cell. These cells arise late in embryogenesis and are distributed around the outside of the embryonic gonad. Their specification depends upon the wingless ligand wnt-2, which is activated by the dsx gene. Another sex-specific cell type is the male-specific SGP (msSGP), which is clustered at the posterior end of the coalesced gonad. msSGPs are specified by a mechanism that seems to be independent of tinman and zfh-1, and they express three different molecular markers, namely Eya, the wnt-2 ligand, and the transcription factor Sox100B. While Sox100B protein is detected only in the male gonads, wnt-2 expression is observed in gonads of both sexes around the time that the germ cells and SGPs first make contact. Subsequently, at the gonad coalescence stage, wnt-2 is greatly enriched in the male gonads in the msSGPs (De Falco et al. 2003). At this stage another SGP-specific marker, Eya, is also enriched in msSGPs.

AlthoughmsSGPsarefoundonlyin thecoalescedgonads of male embryos, their initial specification is not sex specific. Thus, Sox100B/Abd-B-positive cells are detected in PS 13 of both male and female stage 13 embryos. However, survival of msSGPs is controlled by the doublesex (dsx) gene in a sexspecific manner. …

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