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From wikipedia > Biological Life Cycle:

  • haplontic life cycle - the haploid stage is multicellular and the diploid stage is a single cell, meiosis is "zygotic".
  • diplontic life cycle - the diploid stage is multicellular and haploid gametes are formed, meiosis is "gametic".
  • haplodiplontic life cycle [..] - multicellular diploid and haploid stages occur, meiosis is "sporic".

To my knowledge, all mammal species are diplontic. It might even be true for all vertebrates (but I am not sure). In plants and in fungi, there is a much greater diversity of life cycle however. In many (all?) bryophytes for example, the sporophyte (the diploid phase) is multicellular and its life is completely dependent on the gametophyte. This diversity of life cycle lead to the following question:

What drives the evolution of the relative time an organism spend in the haploid vs in the diploid phase of the life cycle?


This post comes as a follow up to @JAD's comment here

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  • $\begingroup$ I appreciate that English is not your first language, but at 60k you can surely come up with a clearer title and question. How can “relative importance” evolve (either in English or French)? Are you sure your question can’t be rephrased to something like “What determines the relative…”? $\endgroup$ – David Jun 19 at 18:09
  • $\begingroup$ By "relative importance", I was referring to the ratio of time spent in the diploid pahse over the time spent in the haploid phase (or the same ratio but in terms of number of mitosis). I chose "What drive the evolution" to clarify my question was about the evolutionary pressures and not about the physiology. I made a few edits but please feel free to edit it yourself if you can improve language and clarity. $\endgroup$ – Remi.b Jun 19 at 18:31
  • $\begingroup$ I’d rather you do it yourself. I’m sure you’re able to. The title can be simplified because it’s just a title. Think about the question. Two sentences is often the solution. $\endgroup$ – David Jun 19 at 18:35
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I just want to report the finding of one paper here. I don't know much about the rest of the literature and can't comment on it.

Otto et al. (2015) investigate this question in sexually reproducing organisms. As I understand it, they assume that the amount of time spent in on phase of the life cycle is proportional to the amount of selection happening in this phase. With a theoretical model Otto et al. (2015) showed that there is a sexual conflict over the length of the haploid phase. Mothers favour a longer haploid phase while fathers favour a shorter haploid phase. The outcome of the model depends upon who is in control of the length of this phase.

For mothers, producing a baby is often costly (see Bateman's principle). As such, the mother might want to ensure that its baby has high fitness. In the haploid phase, deleterious recessive alleles are no masked by their dominant counterpart and hence can be purged out of the population. A long haploid phase, therefore allows purging of deleterious alleles and make the diploid phase more resistant which is in the mothers interest.

Among fathers, the evolutionary pressures are different. Fathers evolve to mask mutations carried by their haploid gametes (e.g., by provisioning with diploid gene products), causing their sperm to be more competitive. As such, a father has interest that the haploid phase is short.

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