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I was wondering whether there are any eukaryotes which never have a diploid phase. I can't think of any. Fungi have diploid stages, and I know any sexually reproducing organisms will have at least transient diploid stages (I am not referring to such transient stages- I mean extended diploid stages so I am counting as a 'haploid' organism one that only ever has two sets of chromosomes for a very short time while it undergoes meiosis.)

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    $\begingroup$ what about bees, males are haploid from the gamete stage. $\endgroup$ – John Mar 16 at 0:47
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Update: There are plenty of eukaryotes that occur in haploid stage as the dominant life cycle stage. See metagenesis in cnidarian animals and "alternation of generation" in algae, protists and fungi. ......

I am not referring to such transient stages.- I mean extended diploid stages so I am counting as a 'haploid' organism one that only ever has two sets of chromosomes for a very short time while it undergoes meiosis.

  • Yes there exist a vast lot; and such life cycle pattern is called haplontic life cycle (also called haploid lifecycle)

Outline of haploid lifecycle Outline of haploid life-cycle

Most of the Green algae like Chlamydomonas, Volvox, Spirogyra etc. are prominent example.

Among fungi; Mucor, Schizosaccharomyces are some prominent example.

Among Protozoans; an example of haploid life-cycle is seen in Wood-roach(Cryptocercus)'s some gut - microfauna such as Trichonympha

(Table from Google Books, Protozoology by Karl Gottlieb Grell) enter image description here

Among slime-molds, Dictyostelium shows haploid lifecycle.

Dictyostelium life cycle diagram *Dictyostelium* life cycle from Biology- A dynamic Science, by Russell et al (Google Books)

Book-Reference:

  1. Advanced BIOLOGY, principles and applications, C.J. Clegg and D. G. Mackean, First Edition (1994), John Murray publications.

  2. College Botany, Vol-2, By Gangulee and Kar, New Central Book Agency, Kolkata.

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Some apicomplexans such as Plasmodium spp. are haploid during their asexual stages. The organism spends more time (at least in the human host) in the asexual stage than sexual. source

Plasmodium spp. are haploid in both human and mosquito hosts except for a brief stage in the mosquito midgut where two haploid malaria gametes present in the mosquito's blood meal fuse to form a diploid zygote. This zygote immediately undergoes meiosis to generate four haploid cells that continue in the haploid form, reproducing through mitosis, until several thousand are present in a small sac, the oocyst, attached to the gut wall.

“Sexual” Population Structure and Genetics of the Malaria Agent P. falciparum

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    $\begingroup$ Welcome to Biology.SE! This looks like a good answer, but answers are much more likely to receive a favorable response if you include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion. This is a good example of how to format references. ——— You may also want to take the tour and then consult the help pages for additional advice on How to Answer effectively on this site. Thank you! 😊 $\endgroup$ – tyersome Feb 20 at 20:46

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