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According to Wikipedia:

Meiosis is a specialized type of cell division that reduces the chromosome number by half. This process occurs in all sexually reproducing single-celled and multicellular eukaryotes, including animals, plants, and fungi Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization) to form a diploid zygote that contains two copies of each chromosome, one from each parent.

So it means that a haploid plant body will give rise to either a male sex organ or female sex organ that produces gametes to form zygote thus completing the alternation of generation. We also know that chara a monoecious plant can be dioecious, which means that haploid plant body is producing both antheridia and archegonia which produce gametes by mitosis. Coming to my question, "HOW CAN A PRODUCT OF MEIOSIS BEFORE FERTILISATION BE BISEXUAL?" Does this implies that reduction division is not responsible of sex differentiation? If yes then what is responsible for sex differentiation in plants like chara where a haploid produces both gametes ? Explanation will be appreciated

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  • $\begingroup$ I am not sure I understand. You define male and female based on the size of the gametes. If the gametophyte is multicellular it can still produce gametes that are of different size. Isn't it? $\endgroup$
    – Remi.b
    Apr 8, 2016 at 18:48
  • $\begingroup$ @Remi.b So ,there are no allosomes in plants unlike humans? $\endgroup$
    – JM97
    Apr 8, 2016 at 22:26
  • $\begingroup$ An allosome is a non-sexual chromosome. Yes plants have allosomes. Most species (of plants and animals) do not even have sex chromosomes but only have allosomes. I don't understand how it relates to the post though. Looks like something is unclear in your understanding of sex and sex determination system but I don't know what. $\endgroup$
    – Remi.b
    Apr 10, 2016 at 4:31

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Meiosis does not determine sexual form. Eukaryotes use meiosis and fertilization to recombine genes to form new combinations. Meiosis does produce haploid cells from diploid cells, but that has nothing much to do with the sexual forms involved.

In the case of the algal genus Chara, the organism's life cycle is entirely haploid except for the single-celled zygote formed during fertilization; this is called a haplontic life cycle. Being haploid does not require an organism to be of one sex (or any sex), so there is no difficulty to being monoecious and having both male and female structures on a single organism. The reason for having two kinds of gametes, sperm and ova, is simply practical: the specialized job of sperm is to move to another gamete, adn the job of the ova is to provide a maximum amount of nutrition for the future zygote (which means larger size and hence limited mobility).

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  • $\begingroup$ Even Chlamydomonas. $\endgroup$
    – WYSIWYG
    Apr 8, 2016 at 19:44

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