In many of the lower plants the mechanism of sex determination is restricted to single gene and it does not bring about any morphological variation between the mating types.
In individuals like Mucor, Yeast, Aspergillus etc., sex is governed by the two alleles of a single gene A and a. The zygote will have both these genes. After meiosis when segregation takes place two/races are produced A and a.
Sexual reproduction is possible only between the races carrying the opposite genes. In most of the flowering plants, which are monoecious, it is more a question of sex differentiation than of sex determination.
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The cells posses both the potentialities (male and female) and possibly hormonal influence will differentiate the male and female sex organs. (A detailed account of sex determination in plants is available in Discussions in Cytogenetics by C.R. Bumham.)
In most of the dioecious plants, sex determination is of the XX, XY type. In some bryophytes like Sphaerocarpus, Marchantia etc, the diploid spore mother cells have X and Y chromosomes.
During reduction division and spore formation these two segregate and consequently two types of spores are formed. 50% of the spores will have X chromosome and they develop into female gametophytes, while the other 50% carry Y chromosomes and develop into male gametophytes.
A similar mechanism of sex determination (XX, XY) has been-reported for the gymnosperm Cycas (Abraham and Mathew 1962). In the pteridophytic member Equisetum, there is a controversy as to whether the gametophytes are monoecious or dioecious.
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While Walker (1921) believes the prothalli to be monoecious, Kashyap (1917) opines that crowded prothalli (growing in a limited area) tend to be dioecious. Joyet Lavergne (1931) has proposed a theory of cytoplasmic sexualization with reference to the sexual behaviour of the spores in Equisetum.
According to him, the spores of Equisetum are morphologically alike, while physiologically different. Spores which have lower oxidizing power, and possessing a fat which reduces osmic acid develop into gsmetophytes which form only archegonia (female), while spores with high oxidizing power and lacking the fat will tend to develop into gametophytes that from only antheridia (male).
1. Sex determination in maize:
The maize plant is monoecious with both staminate inflorescence (tassel) and pistillate (ear) inflorescence located on the same plant.
Many genes which are sex promoting are distributed throughout the ten chromosomes maintaining a balance, so that the tassel produces the Pollen and the ear produces the seed.
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Many mutants in the sex genes in maize have been identified (Singleton and Jones, 1930). A male sterile gene has been identified on chromosome 6 with the result; the tassel does not produce pollen. Another male sterile gene induces (pistil) formation in tassel. These genes render the plants effectively female.
Jones (1930) by genetic analysis came to the conclusion that the sex determination in mutants of maize is due to a single gene. The fertile silk gene (+) mutates and forms a barren gene, sk. The gene in the homozygous condition renders the plant male, by eliminating the ears.
On the other hand other gene (recessive) ts, in the homozygous condition transform the tassel into a functional pistillate structure. The genotype ++, ts ts, would be pistillate, while, sk sk.ts ts would be staminate.
2. Monogenic sex determination in Asparagus:
Asparagus is a dioecious plant, with staminate and pistillate plants cocooning in equal numbers. In some rare instances, female flowers bear rudimentary anthers and staminate flowers bear rudimentary pistils.
Very rarely the pistils of staminate flowersjyiay produce seeds (Rick and Hanna). When these seeds are raised into plants there was a ratio of 3:1 male and female plants respectively.
When the male plants raised thus are used to pollinate pistillate flowers on female plants, one third produced only male plants and two thirds showed seggregation into male and female indicating the control of a single gene.
The genetic explanation is as follows. Sex is controlled by a pair of alleles S and s. S gene produces maleness and apparently all male plants are heterozygous (S/s), while female plants are homozygous recessive (ss).
3. Sex determination in Coccinea indica:
Coccinia indica is a member of the family Cucurbitaceae and is dioecious. Roy and his co-workers at Patna University have worked out the genetic basis of sex determination in diploid, triploid and tetraploid strains. Sex is determined by the XX, XY mechanism.
Roy found out that, the Y chromosome is essential for maleness and a single Y chromosome results in a male individual irrespective of the number of X chromosomes or autosomes. In the diploid plants, females have 2A+XX, and males have 2A+XY.The following table gives a summary of relationship of autosomes and sex chromosomes in sex determination inCoccinea indica.
4. Sex determination in Melandrium:
The mechanism – of Sex determination in Melandrium album has been studied by Warmke (1946). Here also irrespective of the number Of X chromosomes and autosomes, one or more Y chromosomes would render the individual male, and in the absence of Y chromosome, the individual becomes female.
Only when the ratio between X and Y reaches 4:7, some effects of femaleness could be discemed’inspite of the presence of Y. The number of autosomes does not seem to have an effect on the expression of sex.
Like coccinea, in Melandrium also, individuals are diploid, triploid or tetraploid. The following table gives the details of sex expression with different numbers of X chromosomes autosome sets.
The details of generic control in Melandrium as shown by Warmke indicate trait the Y chromosome is longer than the X chromosome. Plants in which the X and Y chromosomes were fragmented were studied and as a result it was found but that both the chromosomes had five different segments.
These segments are known to control the different stages of development of sex organs. Segment is common between X and Y chromosomes and it is this that helps in pairing regular disjunction during meiosis.
In Drosophila, the Y chromosome is inert as far as sexual expression is concerned since the genes for maleness are on the autosomes, while in Coccinea ‘and Melandrium, the genes for maleness are definitely on the Y chromosome.
5. Sex determination in Bacteria:
During conjugation one of the bacterial cells acts as male and the other female. A portion of the chromosome from male is passed on to the female.
In all the male bacteria, there is a structure called episome, which is abseat in females. Male cells arise by division of the pre existing ones, or when the episome gets into a female cell.