Primary events in C. elegans sex determination and dosage compensation

An outline of the complex regulatory gene network that controls all aspects of sexual dimorphism in the nematode C. elegans is now known in considerable details. This review describes the genes and gene interactions involved in the coordinate control of sex determination and X chromosome dosage compensation in C. elegans.     

xol-1: a gene that controls the male modes of both sex determination and X chromosome dosage compensation in C. elegans

Loss-of-function mutations in the X-linked gene xol-1 cause the feminization and death of XO animals (normally males) by shifting the sex determination and dosage compensation pathways toward their hermaphrodite modes. XO-specific lethality most likely results from the reduction in X chromosome expression caused by xol-1 mutations. Mutations in genes required for the hermaphrodite mode of dosage compensation suppress lethality but not feminization, and restore X chromosome expression to nearly wild-type levels. Mutations in genes that control the hermaphrodite modes of both sex determination and dosage compensation fully suppress both defects. These interactions suggest that xol-1 is the earliest-acting gene in the known hierarchy controlling the male/hermaphrodite decision and is perhaps the gene nearest the primary sex-determining signal. We propose that the wild-type xol-1 gene product promotes male development by ensuring that genes (or gene products) directing hermaphrodite sex determination and dosage compensation are inactive in XO animals. Interestingly, in addition to feminizing XO animals, xol-1 mutations further masculinize XX animals already partially masculinized.     

sdc-1: a link between sex determination and dosage compensation in C. elegans

Mutations in the X-linked gene sdc-1 affect both sex determination and X-chromosome dosage compensation in C. elegans, providing evidence that these two pathways share a common step. In XX animals (normally hermaphrodites), sdc-1 mutations cause partial masculinization and elevated levels of X-linked gene expression, an apparent shift of both pathways toward their XO modes of expression. The masculinization occurs through effects on the major sex determination pathway, upstream of all previously identified sex-determining genes. XO animals are apparently unaffected by the sdc-1 mutations. We propose a model in which the wild-type sdc-1 activity is either a component of the primary sex-determining signal (the X/Autosome ratio) or involved in transmitting information about this signal to both the sex determination and dosage compensation pathways.     

The Caenorhabditis elegans dosage compensation machinery is recruited to X chromosome DNA attached to an autosome

The dosage compensation machinery of Caenorhabditis elegans is targeted specifically to the X chromosomes of hermaphrodites (XX) to reduce gene expression by half. Many of the trans-acting factors that direct the dosage compensation machinery to X have been identified, but none of the proposed cis-acting X chromosome-recognition elements needed to recruit dosage compensation components have been found. To study X chromosome recognition, we explored whether portions of an X chromosome attached to an autosome are competent to bind the C. elegans dosage compensation complex (DCC). To do so, we devised a three-dimensional in situ approach that allowed us to compare the volume, position, and number of chromosomal and subchromosomal bodies bound by the dosage compensation machinery in wild-type XX nuclei and XX nuclei carrying an X duplication. The dosage compensation complex was found to associate with a duplication of the right 30% of X, but the complex did not spread onto adjacent autosomal sequences. This result indicates that all the information required to specify X chromosome identity resides on the duplication and that the dosage compensation machinery can localize to a site distinct from the full-length hermaphrodite X chromosome. In contrast, smaller duplications of other regions of X appeared to not support localization of the DCC. In a separate effort to identify cis-acting X recognition elements, we used a computational approach to analyze genomic DNA sequences for the presence of short motifs that were abundant and overrepresented on X relative to autosomes. Fourteen families of X-enriched motifs were discovered and mapped onto the X chromosome.     

A small region on the X chromosome of Drosophila regulates a key gene that controls sex determination and dosage compensation

In Drosophila, flies with two X chromosomes are females, with one X chromosome, males. We investigated the presence of sex determining factors on the X chromosome by constructing genotypes with one X and various X-chromosomal duplications. We found that female determining factors are not evenly distributed along the X chromosome as had been previously postulated. A distal duplication covering 35% of the X chromosome promotes female differentiation, a much larger proximal duplication of 60% results in male differentiation. The strong feminizing effect of distal duplications originates from a small segment that, when present in two doses, activates Sxl, a key gene for sex determination and dosage compensation. Our results suggest that Sxl can be activated to intermediate levels.     

Mechanisms of sex determination and X-chromosome dosage compensation

Abnormalities in chromosome number have the potential to disrupt the balance of gene expression and thereby decrease organismal fitness and viability. Such abnormalities occur in most solid tumors and also cause severe developmental defects and spontaneous abortions. In contrast to the imbalances in chromosome dose that cause pathologies, the difference in X-chromosome dose used to determine sexual fate across diverse species is well tolerated. Dosage compensation mechanisms have evolved in such species to balance X-chromosome gene expression between the sexes, allowing them to tolerate the difference in X-chromosome dose. This review analyzes the chromosome counting mechanism that tallies X-chromosome number to determine sex (XO male and XX hermaphrodite) in the nematode Caenorhabditis elegans and the associated dosage compensation mechanism that balances X-chromosome gene expression between the sexes. Dissecting the molecular mechanisms underlying X-chromosome counting has revealed how small quantitative differences in intracellular signals can be translated into dramatically different fates. Dissecting the process of X-chromosome dosage compensation has revealed the interplay between chromatin modification and chromosome structure in regulating gene expression over vast chromosomal territories.     

X chromosome sites autonomously recruit the dosage compensation complex in Drosophila males

It has been proposed that dosage compensation in Drosophila males occurs by binding of two core proteins, MSL-1 and MSL-2, to a set of 35–40 X chromosome “entry sites” that serve to nucleate mature complexes, termed compensasomes, which then spread to neighboring sequences to double expression of most X-linked genes. Here we show that any piece of the X chromosome with which compensasomes are associated in wild-type displays a normal pattern of compensasome binding when inserted into an autosome, independently of the presence of an entry site. Furthermore, in chromosomal rearrangements in which a piece of X chromosome is inserted into an autosome, or a piece of autosome is translocated to the X chromosome, we do not observe spreading of compensasomes to regions of autosomes that have been juxtaposed to X chromosomal material. Taken together these results suggest that spreading is not involved in dosage compensation and that nothing distinguishes an entry site from the other X chromosome sites occupied by compensasomes beyond their relative affinities for compensasomes. We propose a new model in which the distribution of compensasomes along the X chromosome is achieved according to the hierarchical affinities of individual binding sites.     

Rapid de novo evolution of X chromosome dosage compensation in Silene latifolia, a plant with young sex chromosomes

Silene latifolia is a dioecious plant with heteromorphic sex chromosomes that have originated only ～10 million years ago and is a promising model organism to study sex chromosome evolution in plants. Previous work suggests that S. latifolia XY chromosomes have gradually stopped recombining and the Y chromosome is undergoing degeneration as in animal sex chromosomes. However, this work has been limited by the paucity of sex-linked genes available. Here, we used 35 Gb of RNA-seq data from multiple males (XY) and females (XX) of an S. latifolia inbred line to detect sex-linked SNPs and identified more than 1,700 sex-linked contigs (with X-linked and Y-linked alleles). Analyses using known sex-linked and autosomal genes, together with simulations indicate that these newly identified sex-linked contigs are reliable. Using read numbers, we then estimated expression levels of X-linked and Y-linked alleles in males and found an overall trend of reduced expression of Y-linked alleles, consistent with a widespread ongoing degeneration of the S. latifolia Y chromosome. By comparing expression intensities of X-linked alleles in males and females, we found that X-linked allele expression increases as Y-linked allele expression decreases in males, which makes expression of sex-linked contigs similar in both sexes. This phenomenon is known as dosage compensation and has so far only been observed in evolutionary old animal sex chromosome systems. Our results suggest that dosage compensation has evolved in plants and that it can quickly evolve de novo after the origin of sex chromosomes.     

Fertility,sex determination,and the X chromosome

Because of its function, the X chromosome has a special status in mammalian genomes, with the specific occurrence of genes that influence both female and male fertility. Long ago, the XO karyotype (Turner syndrome) was associated with infertility, proving the correlation between normal X chromosome dosage and normal female fertility. Nevertheless, the search for specific X-borne fertility genes was not completely successful and suggested, instead, that female X-linked fertility, for example, depends upon groups of X-linked genes. Conversely, X-linked hyperfertility has been observed in sheep, where a mutation in BMP15 leads to a hyperfertile phenotype, but only in the heterozygous state. Many male fertility genes map to the X chromosome, consistent with a genetic model developed in the early 1990s. Ironically, NR0B1 (formerly DAX1), once presented as the paradigm of genes responsible for ovarian development and function, is probably one of these male fertility factors and is active in the maintenance of spermatogenesis. Indeed, duplications of this gene on the human X chromosome lead to XY sex reversal, as NR0B1 is able to counterbalance the effect in humans. Nevertheless, invalidation experiments in mice demonstrate the effect of this factor on male germ-cell production.     

Caenorhabditis elegans dosage compensation regulates histone H4 chromatin state on X chromosomes

Dosage compensation equalizes X-linked gene expression between the sexes. This process is achieved in Caenorhabditis elegans by hermaphrodite-specific, dosage compensation complex (DCC)-mediated, 2-fold X chromosome downregulation. How the DCC downregulates gene expression is not known. By analyzing the distribution of histone modifications in nuclei using quantitative fluorescence microscopy, we found that H4K16 acetylation (H4K16ac) is underrepresented and H4K20 monomethylation (H4K20me1) is enriched on hermaphrodite X chromosomes in a DCC-dependent manner. Depletion of H4K16ac also requires the conserved histone deacetylase SIR-2.1, while enrichment of H4K20me1 requires the activities of the histone methyltransferases SET-1 and SET-4. Our data suggest that the mechanism of dosage compensation in C. elegans involves redistribution of chromatin-modifying activities, leading to a depletion of H4K16ac and an enrichment of H4K20me1 on the X chromosomes. These results support conserved roles for histone H4 chromatin modification in worm dosage compensation analogous to those seen in flies, using similar elements and opposing strategies to achieve differential 2-fold changes in X-linked gene expression.