Figures

- Molecular patterns of sex determination in the animal kingdom: a comparative study of the biology of reproduction

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Figure 1   Haplodiploid reproduction. In Hymenoptera, unfertilized eggs develop into uniparental haploid males whereas fertilized eggs into biparental diploid females.

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Figure 2   Single-locus complementary sex determination (sl-CSD). In single-locus complementary sex determination (sl-CSD), heterozygotes at a single sex locus develop as females whereas hemizygotes and homozygous diploids develop as males. However, homozygous diploid males are generally sterile, unable to mate or not viable.

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Figure 3   Matched matings in sl-CSD. In matched matings, half the diploid offspring are homozygous at the sex locus and turn into diploid males, which are unable to contribute to reproduction.

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Figure 4   The X:A ratio determines sex in Drosophila melanogaster. In Drosophila melanogaster sex is determined by the X:A ratio, which is communicated through the balance between the X numerator elements and the autosomal denominators in the presence of several maternally derived proteins. An X:A ratio of 0.5 leads to a non-functional SXL and male development, whereas an X:A ratio of 1 maintains SXL in its active state and is conducive to female development.

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Figure 5   Sex-specific splicing of the sxl mRNA. Early activation of the sxl gene through a different primer (PE) in females allows the appearance of an early SXL protein that guides the splicing of the mRNA originating from the 'standard' primer (PM). This alternative splicing leads to a functional 'mature' SXL protein that then takes up the role of retaining its active state. In males, where no early transcripts can be found, a male-specific exon is included which contains many early stop codons thus leading to the creation of a truncated and non functional protein.

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Figure 6   Genes involved in sex determination in Drosophila melanogaster. The SXL protein regulates the female-specific splicing of the tra mRNA. The TRA protein then forms dimers with TRA-2 which regulate the sex-specific splicing of dsx mRNA. DSXF is the result of said sex-specific splicing in females, whereas DSXM is present in males. All of the above also interact with other genes in turn, in order to mediate sexual development.

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Figure 7   The currently known X-signal elements in C.elegans. In C.elegans, the X-signal elements, such as the SEX-1 and FOX-1 proteins, control the levels of XOL-1 and help determine sex.

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Figure 8   Sex gene pathway in C.elegans. A simple depiction of the sex determination gene pathway as it is known today in the soma of C.elegans. The interactions between several groups of gene products that have been observed to have an inhibitory effect on each other follow the switch of xol-1. The result is that several of these proteins remain active only in males only and others only in hermaphrodites.

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Figure 9   Sex determining interactions on a cellular level. Suggested protein interactions in the later stages of the sex determination pathway in C.elegans. While the SDC proteins have also been known to serve as part of the dosage compensation mechanism in C.elegans, HER-1 has been pictured as capable of binding to the TRA-2 receptor, which then releases the FEM molecules in males. Those in turn bind to the TRA-1 transcription factors rendering them inactive. In hermaphrodites, the TRA-2 receptors retain their hold on FEM, and TRA-1 is free to act as a transcription factor on the genome.

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Figure 10   Gene interactions that allow spermatogenesis and oogenesis in the hermaphrodite C.elegans, as opposed to the gene pathway in the soma. The top half of each frame displays the gene pathway for sex determination in the C.elegans soma and the bottom half the changes that concern the hermaphrodite germline. During the fourth larval stage (L4), a special set of genes expressed in the germline (fog-2, gld-1, laf-1) allows spermatogenesis to occur in hermaphrodites by interfering with the original sex determination pathway (inhibition of tra-2 that leads to the activation of the fem gene products and others such as fog-1 and fog-3). Once spermatogenesis is over and the hermaphrodite enters its mature stage (M), the original sex determination pathway is re-established (tra-2 becomes active again) in the germline of adult hermaphrodites and makes the switch to oogenesis (by inactivating the genes fem, fog-1 and fog-3 gene products that allowed spermatogenesis).

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Figure 11   Aromatase. Aromatase is a cytP450 enzyme that allows the conversion of androgens into estrogens.

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Figure 12   Temperature-dependent sex determination. Aromatase activity levels during the thermosensitive period (TSP) are regulated by the temperature of the environment and control gonadal differentiation. Changes in the environment temperature before and after TSP do not seem to affect sex.

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Figure 13   Sex determination in the medaka. Although many details for the molecular model of sex determination in the medaka are still missing, the mechanism is known to be based on the presence of XX/XY sex chromosomes. Following a stage of undifferentiated gonads, males exclusively express DMY, a gene bearing a DM domain, which is a genetic feature that considered central in sex determination pathways of various species. Among the genes induced downstream is DMRT1, which participates in gonadal development and differentiation in fish, birds and mammals. In XX females, the exact genetic cascade triggered in the absence of DMY is unclear, but it supposed to involve sex-specific gene expression, such as FIGa and sex steroid/aromatase regulation.

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Figure 14   The role of ZPKCI and ASW (WPKCI) in ZW sex determination. According to one theory, the ZPKCI proteins form homodimers in ZZ males that stimulate a factor required for the differentiation of the testes. Whereas in ZW females, the ASW (also known as WPKCI) proteins form heterodimers with ZPKCI that may prevent the activation of that factor or stimulate directly the differentiation of ovaries.

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Figure 15   The multistage model of sex chromosome evolution. The mammalian X and Y chromosomes are thought to derive from a common initial autosomal pair. By a gradual process of genetic instability, which may have been related to failure in the recombination process, the chromosomes have begun to differ from each other. The first area to acquire a sex-specific role is considered to be the locus around the major sex determinant gene, i.e. SRY. Thus, in evolutionary lower mammals with a more conserved chromosomal content, such as monotremes, X and Y retain homology in all their length but for the SRY region. Subsequent stages of X-Y recombination failure have led to other, transient forms of X-Y structure, such as those observed in marsupials and primates. The greatest level of heterogeny is considered to be that found in modern humans.