Algae, Tree, Herbs, Bush, Shrub, Grasses, Vines, Fern, Moss, Spermatophyta, Bryophyta, Fern Ally, Flower, Photosynthesis, Eukaryote, Prokaryote, carbohydrate, vitamins, amino acids, botany, lipids, proteins, cell, cell wall, biotechnology, metabolities, enzymes, agriculture, horticulture, agronomy, bryology, plaleobotany, phytochemistry, enthnobotany, anatomy, ecology, plant breeding, ecology, genetics, chlorophyll, chloroplast, gymnosperms, sporophytes, spores, seed, pollination, pollen, agriculture, horticulture, taxanomy, fungi, molecular biology, biochemistry, bioinfomatics, microbiology, fertilizers, insecticides, pesticides, herbicides, plant growth regulators, medicinal plants, herbal medicines, chemistry, cytogenetics, bryology, ethnobotany, plant pathology, methodolgy, research institutes, scientific journals, companies, farmer, scientists, plant nutrition
Select Language:
 
 
 
 
Main Menu
Please click the main subject to get the list of sub-categories
 
Services offered
 
 
 
 
  Section: Genetics » Sex Determination, Sex Differentiation, Dosage Compensation and Genetic Imprinting
 
 
Please share with your friends:  
 
 

X/A ratio and multiple numerator elements (Drosophila and Coenorhabditis)

 
     
 
Content
Sex Determination, Sex Differentiation, Dosage Compensation and Genetic Imprinting
Chromosome Theory of Sex Determination 
Balance Theory of Sex Determination X/A ratio in Drosophila
Triploid intersexes in Drosophila and genie balance theory
X/A ratio and gynandromorphs in Drosophila
X/A ratio in Coenorhabditis elegans (a free living nematode)
Balance Between Male and Female Factors
- Diploid intersexes in gypsy moth (Lymantria)
- X/A ratio and multiple numerator elements (Drosophila and Coenorhabditis)
Sex Determination in Plants
Methods for determining heterogametic sex in plants
Sex determination in Coccinia and Melandrium
Sex determination in other dioecious plants
Sex Chromosomes in Mammals Including Humans (Homo sapiens)
TDF, ZFY and SRY genes in humans
H-Y antigen and male development in mammals
Single gene control of sex
Sex determination in Asparagus
Tassel seed (ts) and silkless (sk) genes in maize
Transformer gene (tra)in Drosophila
Haploid males in Hymenoptera
Hormonal control of sex
Environmental Sex Determination in Reptiles
Dosage Compensation in Organisms with Heterogametic Males
X-chromosome inactivation in mammals
Position effect variegation
Hyperactivity of X-chromosome in male Drosophila
Lack of Dosage Compensation in Organisms with Heterogametic Females
Genetic imprinting
It has been shown in recent years, that the X/A ratio only works as a primary sex determining signal, which acts to set several major regulatory genes in a functional state, thus governing three independent processes : (i) dosage compensation, (ii) somatic sexual development and (iii) germ line sexual development (Fig. 17.10). Dosage compensation is the process, where the total gene expression from single X in one sex is made equal to total gene expression from the two X-chromosomes in the other sex (for details see later in this section). Failure to achieve this compensation is often lethal. Somatic sexual development refers to sex specific differentiation in all tissues except the germ line. Germ line sexual development refers to the type of gametes made by the germ line, e.g. oocytes in female and sperms in male, or both oocytes and sperms in the hermaphrodite (e.g. in ♀+♂ C. elegans).

General organization of sex determination and sex differentiation genes in Drosophila melanogaster and C. elegans.
Fig. 17.10. General organization of sex determination and sex differentiation genes in Drosophila melanogaster and C. elegans.


In the X/A ratio, X is the numerator and A is the denominator and there are genetic elements on both, described as 'numerator elements' and 'denominator elements' that contribute to this ratio. The elements interact with the target gene of X/A ratio signal. For instance, in Drosophila, the target gene of X/A ratio is the gene Sxl (sex lethal)and the two numerator elements which interact with this gene Sxl, are sis-a and sis-b (sisterless). In C. elegans, direct microinjection of cloned mnDp8 (a small X chromosome duplication) was able to shift the sexual phenotype of 2X : 3A male animals towards intersexes or hermaphrodites. Although,' all duplications for X-chromosome segments tested lead to increase in X/A ratio in 2X : 3A individuals from 0.67 to 0.70, but all these DNA clones were not effective. There is also some evidence that there may be abundant non-coding sequences on the nematode X chromosome that may correspond to numerator elements, which in Drosophila have been found to be involved in the synthesis of sex linked transcription factors.

X/A ratio, sex linked master control genes and autosomal regulatory genes Drosophila melanogaster
(a) Sex linked master control
genes. As discussed above, the X/A ratio works as a signal and sets the master control genes into functional state. These master genes include Sxl (sex lethal)gene in Drosophila and xol-1, sdc-1 and sdc-2genes in C. elegans. In Drosophila, Sxl responds to the dosage of sis-a and sis-b genes and also to other genes like da (daughterless) and snf (sans-fille). It (Sxl)acts in early embryogenesis and becomes set in one of the two modes, either active (in females) or inactive (in males). Whenever Sxl mutates, losing function (lf)in female or gaining function (gf)in male, the mutants are lethal and hence the name sex lethal. Once set in female mode, Sxl acts as a regulator of four processes : (i) dosage compensation, (ii) somatic sex, (iii) germ line sex and (iv) regulation of its own activity. In male it has no function (Fig. 17.11).

Probable role of Sxl gene (and regulation of its activity) in sex determination and sex differentiation in Drosophila melanogasler.
Fig. 17.11. Probable role of Sxl gene (and regulation of its activity) in sex determination and sex differentiation in Drosophila melanogasler.

The function of Sxl is achieved by differential control on splicing of its transcript (HnRNA), which has a stop codon (in frame) undergoing one of the following two fates : (i) in males, due to stop codon, full length protein is not obtained, and the truncated proteins produced are non-functional; (ii) in females during early phase, stop codon is removed by splicing (due to the activation by sis elements), so that a functional protein is obtained, which during later phase not only further promotes this productive splicing of transcripts leading to positive autoregulation, but also controls the splicing of target genes like tra. This autoregulation is necessary, so that stable response to X/A ratio is achieved, because the apparent X/A ratio may change due to dosage compensation (increased activity of X in XY males). The Sxl active protein also causes productive splicing of tra (transformer) transcript leading to an active feminizing product, so that in the absence of active Sxl protein, default splicing of tra transcript results in male soma. It is not clearly understood, how Sxl achieves gametogenesis in female and male fruit flies.

(b) Autosomal regulatory genes for somatic and germ line sexual phenotypes. There are also four major autosomal regulatory genes (tra, tra-2, dsx = double sex, ix = intersex), which interact in an orderly cascade to determine somatic sexual phenotype (this set of genes in Drosophila has no effect in germ lines, but in C. elegans corresponding genes determine both soma and the germ line). We already mentioned that active Sxl protein causes productive splicing of tra (Fig. 17.12). Following this, tra and tra-2 gene products jointly act on the target gene dsx, which, due to female splicing in XX flies represses male development, thus permitting female development, and in XY flies, due to default splicing, represses female development, permitting male development. The gene ix is needed only for the female development and not for the male development.


Somatic cascade in Drosophila melanogaster; in XX flies, Sxl promotes productive splicing of tra transcript to produce tra protein which in association with tra-2 promotes female specific splicing of dsx transcripts and finally dsx product in association with ix product leads to female somatic development; in XY flies both tra and dsx adopt a default mode of splicing.
Fig. 17.12. Somatic cascade in Drosophila melanogaster; in XX flies, Sxl promotes productive splicing of tra transcript to produce tra protein which in association with tra-2 promotes female specific splicing of dsx transcripts and finally dsx product in association with ix product leads to female somatic development; in XY flies both tra and dsx adopt a default mode of splicing.


For gametogenesis, many genes have been studied, but it is not known if any of these genes can exercise a choice between tra-2 and ovo, which in mutant state leads to abortive oogenesis, but not to spermatogenesis. The gene tra-2 has a male specific role in the germ line, in addition to female specific role in the soma, but the details are not yet known with certainty.

X/A ratio, sex linked master control genes and autosomal regulatory genes Coenorhabditis elegans.
(a) Sex linked master control
genes. In C. elegans, X/A ratio works as a signal for a master control gene having functional similarity to Sxl. This gene is sdc-2 (sex and dosage compensation) which is sex linked (located on X) and activates femaleness. This gene works with another gene sdc-1, so that both sdc-1and sdc-2are under negative control of xol-1(XO lethal) in XO animals (although xol-1mutations affect both XX and XO animals). This gene xol-1is compared with da (daughterless) in fruitfly, and its activity is negatively correlated with high or low X/A ratio (Fig. 17.13). The interactions in C. elegans are thus more complex than in fruitfly.
 
The roles of XO lethal gene (xol-1)and sex and dosage compensation genes (sdc-1, sdc-2)in C. elegant; mutations in these two classes of genes have essentially opposite effects; sdc mutations are epistatic to xol-1 mutations (barred arrows i.e. —| indicate negative interaction or down regulation).
Fig. 17.13. The roles of XO lethal gene (xol-1)and sex and dosage compensation genes (sdc-1, sdc-2)in C. elegant; mutations in these two classes of genes have essentially opposite effects; sdc mutations are epistatic to xol-1 mutations (barred arrows i.e. —| indicate negative interaction or down regulation).


(b) Autosomal regulatory genes for somatic and germ line sexual phenotypes. A cascade of regulatory genes located on autosomes are found in the nematode C. elegans. Relative to corresponding genes in fruitfly these genes in the nematode, are many more in number and involve many more steps. They also have a dual effect in soma and germ line, unlike in Drosophila, where separate genes control the soma and the germ line. The activity of the last gene of the cascade i.e. tra-1 (transformer-1) is essential for female somatic development, so that dominant gain of function (gf)mutation leads to XO female, absence or low activity of tra-1 leads to male, and nul1 mutation (loss of function = lf) leads t0 masculinization of XX individuals (Table 17.4).

genteic botany Biocyclopedia.com

There are seven genes in the cascade (her-1, tra-2, tra-3, fem-1, fem-2, fem-3and tra-1)and five of these have been cloned. The transcription level of her-1is high in males and low in females, so that the next genes tra-2, tra-3have low activity in males and high in females and so on (Fig. 17.14). But the activity of genes does not involve splicing of the transcript as in fruitfly, and is controlled in a much more complex manner.
 
Somatic cascade inferred for C. elegans; her (hermaphroditization gene), fern (feminization gene); tra wild allele feminizes; the seven genes involved adopt opposite activity states in the two sexes; in XX animals, her-1activity is low and in XO it is high, which ultimately ends up with high activity of tra-genes in XX leading to femaleness and low activity of tra genes in XO leading to maleness.
Fig. 17.14. Somatic cascade inferred for C. elegans; her (hermaphroditization gene), fern (feminization gene); tra wild allele feminizes; the seven genes involved adopt opposite activity states in the two sexes; in XX animals, her-1activity is low and in XO it is high, which ultimately ends up with high activity of tra-genes in XX leading to femaleness and low activity of tra genes in XO leading to maleness.

The same seven genes that control somatic sex in C. elegans, also control germ line sex, but following differences are noteworthy : (i) fern genes have dual role, masculinizing directly by inducing spermatogenesis and by down regulating tra-1(in the absence of fern gene activity, only oocytes are produced, irrespective of the state of tra-1); (ii) the role of tra-1is complicated in gametogenesis, but some level of tra-1activity is necessary for gametogenesis in both sexes; (iii) fog-2 (feminization of germ line)down regulates tra-2 transiently, making fern genes active, thus promoting spermatogenesis. The tra-2 activity is under general negative control in XO animals (presumably by her-1)and under germ line specific negative control in XX animals (presumably by fog-2). A model showing interactions involved in the germ line development are shown in Figure 17.15.
 
Germ-line cascade in C. elegans; low her-1activity in XX leads to hermaphrodite, while high her-1 activity in XO leads to male; high fern activity leads to spermatogenesis and low fern activity leads to oogenesis; mutants at fog-2and some tra-2mutants (gf = gain of function) eliminate spermatogenesis from XX rendering them female, but they do not affect XO individuals, so that XX female and XO male system develops.
Fig. 17.15. Germ-line cascade in C. elegans; low her-1activity in XX leads to hermaphrodite, while high her-1 activity in XO leads to male; high fern activity leads to spermatogenesis and low fern activity leads to oogenesis; mutants at fog-2and some tra-2mutants (gf = gain of function) eliminate spermatogenesis from XX rendering them female, but they do not affect XO individuals, so that XX female and XO male system develops.

 
     
 
 
     




     
 
Copyrights 2012 © Biocyclopedia.com | Disclaimer