I've been reading on Wikipedia about how plants alternate generations between a diploid sporophyte (usually the dominant part) and haploid gametophyte (in flowering plants, the pollen and ovule sacs).

Mammals also produce haploid gametes. But in mammals we don't describe the gametes as a separate generation that alternates with the dominant "sporoid." The gametes are just the organism's means of reproduction.

What is it about plant reproduction that warrants describing their gametes as a separate "generation" or "phase"?

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    $\begingroup$ Well, I do. But my biologist friends don't find it funny. $\endgroup$ – lvella Jun 19 '19 at 12:40
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    $\begingroup$ There's an old joke that a zygote is just a gamete's way of producing more gametes. $\endgroup$ – StackOverthrow Jun 19 '19 at 18:07

The big difference is that in humans, there is no mitosis in the haploid phase.

There are three terms that are important here:

  • Haplontic: Most of the life is spent in the haploid phase
  • Diplontic: Most of the life is spent in the diploid phase
  • Haplodiplontic (aka. diplohaplontic): About as much time is spent in the haploid phase than in the diploid phase

At the extreme, when there is absolutely no mitosis in one of the phase, then the organism is clearly not haplodiplontic. Haplodiplontic organisms have an alternation of generations.

Note that the term alternation of generations is also used for systems that are much more complex than haplodiplontie. Examples of such complex alternation of generation include many parasites such as the giant liver fluke

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    $\begingroup$ Hmm, I guess the follow-up question would be why plants benefit from haploid mitosis, while mammals do not? Is it merely divergent evolution, or is there some conditions that make it more useful for plants or v.v.? $\endgroup$ – JAD Jun 19 '19 at 13:03
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    $\begingroup$ This is a good question but one that is difficult to answer. I don't think a clear consensus has been reached on this question but there certainly are some research (there certainly are some theoretical models at least). I opened a new post (here) to ask this question and we will see if someone can come up with a nice answer $\endgroup$ – Remi.b Jun 19 '19 at 14:35
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    $\begingroup$ @JAD In addition to the hypothesis in Remi.b's linked question, you can check out one of our canonical answers here: biology.stackexchange.com/questions/35532/… which might also apply to some extent. $\endgroup$ – Bryan Krause Jun 19 '19 at 18:18

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