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  Section: Genetics » Sex Determination, Sex Differentiation, Dosage Compensation and Genetic Imprinting
 
 
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Sex determination in Coccinia and Melandrium

 
     
 
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
Although a number of dioecious plant species have so far been examined for the presence of sex chromosomes, a detailed study was undertaken only in Coccinia and Melandrium. The mechanism of sex determination in Coccinia indica, a member of family Cucurbitaceae was studied in some detail by Prof. R.P. Roy and his co-workers at Patna University. They studied the sex in diploid, triploid and tetraploid plants with and without Y chromosome and observed that irrespective of the number of X-chromosomes and/or autosomes, presence of a single Y chromosome gave a male individual (Table 17.7).

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A similar case earlier worked out by H.E. Warmke in 1946 refers to Melandrium album. In case of Melandrium, diploids, triploids and tetraploids having different doses of X and chromosomes were studied with respect to their sex. Results of such studies are summarised in Table 17.8. Thus, in Coccinia as well as in Melandrium, a plant is male when one or more chromosomes are present and is female when chromosome is absent. Female potential in the presence of Y chromosome showed expression only when the ratio of Y : X reached 1 : 4 (Table 17.8). The number of autosomes did not visibly affect the sex expression.

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Chromosomes at meiotic metaphase I in a male Melandrium album plant, showing eleven (11) normal autosomal bivalents and one heteromorphic bivalent involving X and Y chromosomes (drawn from a photograph by Warmke, 1946).
Fig. 17.16. Chromosomes at meiotic metaphase I in a male Melandrium album plant, showing eleven (11) normal autosomal bivalents and one heteromorphic bivalent involving X and Y chromosomes (drawn from a photograph by Warmke, 1946).


In Melandrium, Y chromosome is longer than X-chromosome, and they form a heteromorphic bivalent at meiosis (Fig. 17.16). Plants having different individual fragments of Y chromosome were also studied and as a result, it was possible to divide Y and X-chromosomes into five different segments. These segments are known to control different stages of development of sex organs (Fig. 17.17). The X and the Y chromosomes have a common segment IV, which helps in pairing and regular disjunction of X and Y chromosomes during meiosis. The remainder of Y chromosome has three segments, namely I, suppressing femaleness; II, initiating anther development and III, controlling late stage of anther development. The X-chromosome also has a differential segment V, which should promote femaleness in the absence of female suppressing segment I on Y chromosome.

In view of the above, while comparing Drosophila with Coccinia and Melandrium, we find that male determining genes are present on autosomes in Drosophila, but on Y-chromosome in these plants.
 
X and Y chromosomes in Melandrium album showing diffe­rent segments (I, II, III, IV, V) control­ling different stages of sex determina­tion and sex differentiation (see text for details).
Fig. 17.17. X and Y chromosomes in Melandrium album showing diffe­rent segments (I, II, III, IV, V) control­ling different stages of sex determina­tion and sex differentiation (see text for details).

 
     
 
 
     




     
 
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