Caldwell, in Herpetology Fourth EditionSex Determination Whether an individual is a male or female has cascading effects on its life history, behavior, physiology, often morphology, and a suite of other functions throughout its life. Although it might seem intuitive that sex would be determined in amphibians and reptiles by differences in sex chromosomes, referred to as genetic sex determination GSD as in humans and all mammalsthis is not entirely true.
Received Jan 5; Accepted Apr 6. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly credited.
This article has been cited by other articles in PMC. Associated Data Figure S1: In the mouse, Fgf9 and Wnt4 are expressed in gonads of both sexes prior to sex determination. However, the relationship between these signals and the male sex-determining gene, Sry, was unknown.
We show through gain- and loss-of-function experiments that fibroblast growth factor 9 FGF9 and WNT4 act as opposing signals to regulate sex determination. In the mouse XY gonad, Sry normally initiates a feed-forward loop between Sox9 and Fgf9, which up-regulates Fgf9 and represses Wnt4 to establish the testis pathway.
These data suggest that the fate of the gonad is controlled by antagonism between Fgf9 and Wnt4. The role of the male sex-determining switch— Sry in the case of mammals—is to tip the balance between these underlying patterning signals.
In principle, sex determination in other vertebrates may operate through any switch that introduces an imbalance between these two signaling pathways. Introduction The development of sexually dimorphic reproductive organs is a common feature among animal species.
The testis and ovary represent two divergent pathways of development from the bipotential embryonic gonad. The switch that initiates divergent development of the gonad is highly diverse among species; however, the underlying mechanisms that lead to the establishment of ovary or testis pathways are likely to be conserved.
In all species, the embryonic gonad is made up of a mixed population of germ cells and somatic cells. This tissue is remarkable in that all of its cells are believed to be bipotential, and can differentiate into ovarian or testicular lineages [ 12 ].
Consistent with the idea that cells in this primordium are poised between two developmental pathways, some of the genes that are involved in establishing sexual dimorphism, including Dax1 dosage-sensitive sex reversal-congenital adrenal hypoplasia critical region on the X chromosome protein 1Sox9 Sry-like HMG box 9Fgf9 fibroblast growth factor 9and Wnt4 wingless-related MMTV integration site 4are initially expressed in similar patterns in XX and XY gonads [ 3 — 8 ].
The conventional view of mammalian sex determination is that the basic pathway of organ development is ovarian, and that the testis-determining gene operates by diverting this program toward testis development by simultaneously influencing the fate of the key supporting cell lineage and initiating a male-specific morphogenetic program.
All of the experimental evidence suggests that these two processes are closely interwoven. For example, both proliferation [ 9 ] and migration of cells to trigger testis cord formation [ 1011 ] appear to be closely integrated with Sertoli cell differentiation.
Sry, a Y chromosome-linked gene, is the primary sex-determining gene in mammals [ 12 — 14 ].
In the absence of Sry expression—in XX embryos, or in XY embryos carrying a deletion of Sry—cells in the gonad follow an ovarian differentiation pathway. Genetic evidence from chimeric mice [ 15 ], and expression studies using reporter transgenes [ 216 ], indicate that Sry expression is required only in precursors of the somatic supporting cell lineage.
Expression of Sry in these bipotential cells leads to their differentiation as testis-specific Sertoli cells rather than as follicle cells, the parallel cell type of the ovary [ 2 ]. It is believed that the Sertoli cell is the first cell type to differentiate in the gonad [ 17 ].
There is substantial evidence that a critical threshold number of Sertoli cells is required to establish testis differentiation [ 91518 — 20 ].
In cases where this threshold is not reached, ovary differentiation ensues.Gonads are the male and female primary reproductive organs.
The male gonads are the testes and the female gonads are the ovaries. These reproductive system organs are necessary for sexual reproduction as they are responsible for the production of male and female gametes.
Introduction to the germ line * Cartoon representation of post-embryonic hermaphrodite gonad development. Comparative size of gonads at different stages is not to scale. and germline sex determination go hand in hand and several key genes function in multiple processes (see Sex-determination in the germ line; Germline.
Gonads were isolated from all-female and all-male tilapia before (5–15 days after hatching [dah]) and after (25–70 dah) morphological sex differentiation. Introduction. Sex determination, unlike many developmental processes, is characterized by a lack of conservation throughout the vertebrates.
May 23, · Introduction. The development of sexually dimorphic reproductive organs is a common feature among animal species.
The testis and ovary represent two divergent pathways of development from the bipotential embryonic gonad. It is defined as the development of phenotypic structures consequent to the action of hormones produced following gonadal determination. Sexual differentiation includes development of different genitalia and the internal genital tracts, breasts, body hair, and plays a role in gender identification.
Sex determination in mammals: coordinated control by the endocrine system An analysis of sex- chromosome aneuploids demonstrates that mammalian sex determination and differentiation are quite different from those of Drosophila (see Table ).