Friday, 4 April 2008

Moss life-cycle

I have been trying to decipher the life-cycle of mosses, and believe I now have it worked out. For so-called 'primitive plants', mosses have a complex life-cycle.

We'll start with the sexual generation, with male antheridia and female archegonia, which are usually - but not always - borne on separate plants. The following shot shows the cone-shaped antheridia of Polytrichum commune.

These produce the male gametes which swim (through water, hence the need for humidity around mosses) towards the female archegonia, which are tiny pockets on the stem of the female plant:
Fertilisation takes place, resulting in the sporophyte, or asexual spore-bearing generation, which grows as a parasite on the female shoot. So although the sporophyte - which consists of the seta (capsule-bearing 'stem') and the capsule (spore-bearing container) - appears to be part of the female plant, it is only 50% hers.

This shot shows the immature sporophytes of Polytrichum commune.

And these are the mature sporophytes with open capsules:


Gill said...

So, are the male and female plants diploid,and the meiosis to produce the haploids occurs in the archegonia/antheridia?
Or are the spores somehow male and female? This is where I lose the plot....

Stuart said...

Meiosis occurs in the capsule of the sporophyte, which produces spores that go on to be sexual gametophytes. The sporophyte is diploid and the gametophytes are haploid.

Gill said...

So, if the gametophytes are haploid (and the gametophyte is the moss "plant" we see) how can you have male and female structures on the same onw? ["...with male antheridia and female archegonia, which are usually - but not always - borne on separate plants"].

I'm sure I'm missing something really obvious here, but it has baffled me for years!

Stuart said...

Ok...mosses are split into two groups...

1) dioecious, with male and female sexual parts on separate plants, and

2) monoecious, with male and female organs on the same plant.

The description I gave above clearly applies to dioecious mosses, but I can't find any info on whether it applies to monoecious mosses (and I can't see how it would I agree with your point). Text and web references get a little coy on this one, so I'll have to refer to someone who will know. I have a couple of sources who should be able to help...:)

Stuart said...

Well, one oracle has spoken thus:

If the dioicous haploid species had sex chromosomes and produced XY sporophytes, then a first-generation hybrid sporophyte that underwent chromosome doubling would have two X chromosomes from one species and two Y chromosomes from the other. If the X chromosomes paired during meiosis, as did the Y chromosomes, then segregation of an X and Y to each spore would result in diploid XY gametophytes that expressed characters of both sexes.

(My italics)

So it looks like monoecious mosses have a diploid gametophyte. That would work.

I have to say that's as far as I can go with this. Any further and I can't spell any of the words.

Leif said...

I'm not fully convinced that any of the first three pictures show Polytrichum commune. Not possible to tell from the photos. At least the second photo is clearly not P. commune. It is most probably Polytrichum juniperinum, but a bit hard to see on the not so sharp photo.