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Seed Plants

The seed plant gametophyte life cycle is even more reduced than basil taxa (ferns and lycophytes). Seed plant gametophytes are not independent organisms and depend upon the dominate sporophyte tissue for nutrients and water. With the exception of mature pollen, if the gametophyte tissue is separated from the sporophyte tissue, it will not survive. Due to this complex relationship and the small size of the gametophyte tissue, in some situations single celled, differentiating with the human eye or even a microscope between seed plant gametophyte tissue and sporophyte tissue can be a challenge. While seed plant gametophyte tissue is typically composed of mononucleated haploid cells (1 x n), specific circumstances can occur in which the ploidy does vary widely despite still being considered part of the gametophyte.

               In gymnosperms, the male gametophyte is produced inside the microsporangia (typically located inside of male cones). Four male gametophytes are created by the division of a diploid microspore mother cell through meiosis[1] .  At this stage, each male gametophyte is single celled and may divide further by mitosis before becoming a mature pollen grain. The water and nutrients that the male gametophyte depends upon are provided by the microsporangia sporophyte tissue from this point until pollination. There is great variety in the cell number of each pollen grain when the pollen is released from the microsporangia. This number varies between the gymnosperm orders. Cycadophyta have 3 celled pollen grains while Ginkgophyta have 4 celled pollen grains[1]. Gnetophyta may have 2 or 3 celled pollen grains depending on the species, and Coniferophyta pollen grains vary greatly ranging from single celled to 40 celled[2][1]. One of these cells is typically a germ cell and other cells may consist of a single tube cell which grows to form the pollen tube, sterile cells, and/or prothallial cells which are both vegetative cells without an essential reproductive function[1]. After pollination is successful, the male gametophyte continues to develop. If a tube cell was not developed in the microstrobilus, one is created after pollination via mitosis[1]. The tube cell grows into the diploid tissue of the female cone and may branch out into the megastrobilus tissue or grow straight towards the egg cell[3]. The megastrobilus sporophytic tissue provides nutrients for the male gametophyte at this stage[3]. In some gymnosperms, the tube cell will create a direct channel from the site of pollination to the egg cell, in other gymnosperms, the tube cell will rupture in the middle of the megastrobilus sporophyte tissue[3]. This occurs because in some gymnosperm orders, the germ cell is nonmobile and a direct pathway is needed, however, in Cycadophyta and Ginkgophyta, the germ cell is mobile due to flagella being present and a direct tube cell path from the pollination site to the egg is not needed[3]. In most species the germ cell can be more specifically described as a sperm cell which mates with the egg cell during fertilization, though that is not always the case. In some Gnetophyta species, the germ cell will release two sperm nucelli that undergo a rare gymnosperm double fertilization process occurring solely with sperm nucelli and not with the fusion of developed cells[1][4].After fertilization is complete in all orders, the remaining male gametophyte tissue will deteriorate[2].

Multiple examples of the variation of cell number in mature seed plant female gametophytes prior to fertilization. Each cell contains one nucleus unless depicted otherwise. A: Typical 7 celled, 8 nucleate angiosperm female gametophyte (ex. Tilia americana). B: Typical gymnosperm female gametophyte with many haploid somatic cells illustrated with a honeycomb grid and two haploid germ cells (ex. Ginkgo biloba). C: Abnormally large 10 celled, 16 nucleate angiosperm female gametophyte (ex. Peperomia dolabriformis). D: Abnormally small 4 celled, 4 nucleate angiosperm female gametophyte (ex. Amborella trichopoda). E: Unusual gymnosperm female gametophyte that is singled celled with many free nucelli surrounding a pictured central vacuole (ex. Gnetum gnemon). Blue: egg cell. Amber: synergid cell. Yellow: accessory cell. Green: antipodal cell. Peach: central cell. Purple: individual nucelli.

               The female gametophyte in gymnosperms differs from the male gametophyte as it spends its whole life cycle in one organ, the ovule located inside the megastrobilus (typically located inside female cones)[5]. Similar to the male gametophyte, the female gametophyte normally is fully dependent on the surrounding sporophytic tissue for nutrients and the two organisms cannot be separated. However, select female gametophytes do contain chlorophyll and can produce some of their own energy, though, not enough to support itself without being supplemented by the sporophyte[6]. The female gametophyte forms from a diploid megaspore that undergoes meiosis and starts being singled celled[7]. The size of the mature female gametophyte varies drastically between gymnosperm orders. In Cycadophyta, Ginkgophyta, Coniferophyta, and some Gnetophyta, the single celled female gametophyte undergoes many cycles of mitosis ending up consisting of thousands of cells once mature. At a minimum, two of these cells are egg cells and the rest are halploid somatic cells, but more egg cells may be present and their ploidy, though typically haploid, may vary[5][8]. In select Gnetophyta, the female gametophyte stays singled celled. Mitosis does occur, but no cell divisions are ever made[4]. This results in the mature female gametophyte in some Gnetophyta having many free nucelli in one cell. Once mature, this single celled gametophyte is 90% smaller than the female gametophytes in other gymnosperm orders[5]. After fertilization, the remaining female gametophyte tissue in gymnosperms serves as the nutrient source for the developing zygote (even in Gnetophyta where the diploid zygote cell is much smaller then, and for a while lives within the single celled gametophyte)[5].

              The precursor to the male angiosperm gametophyte is a diploid microspore mother cell located inside the anther. Once the microspore undergoes meiosis, 4 haploid cells are formed, each of which is a singled celled male gametophyte. The male gametophyte will develop via one or two rounds of mitosis inside the anther. This creates a 2 or 3 celled male gametophyte which becomes known as the pollen grain once dehiscing occurs[9]. One cell is the tube cell, and the remaining cell/cells are the sperm cells[10]. The development of the three celled male gametophyte prior to dehiscing has convergently evolved multiple times and is present in about a third of angiosperm species allowing for faster fertilization after pollination[11]. Once pollination occurs, the tube cell grows in size and if the male gametophyte is only 2 cells at this stage, the single sperm cell undergoes mitosis to create a second sperm cell[12]. Just like in gymnosperms, the tube cell in angiosperms obtains nutrients from the sporophytic tissue, and may branch out into the pistil tissue or grow directly towards the ovule[13][14]. Once double fertilization is completed, the tube cell and other vegetative cells, if present, are all that remains of the male gametophyte and soon degrade[14].

               The female angiosperm gametophyte develops in the ovule (located inside the female or hermaphrodite flower). Its precursor is a diploid megaspore that undergoes meiosis which produces four haploid daughter cells. Three of these independent gametophyte cells degenerate, the one that remains is the gametophyte mother cell which normally is composed of one nucelli[15]. In general, it will then divide by mitosis until it consists of 8 nuclei separated into 1 egg cell, 3 antipodal cells, 2 synergid cells, and a central cell that contains two nucelli [15][12]. In select angiosperms, special cases occur in which the female gametophyte is not 7 celled with 8 nuclei[8]. On the small end of the spectrum, some species have mature female gametophytes with only 4 cells, each with one nuclei[16]. Conversely, some species have 10 celled mature female gametophytes consisting of 16 total nuclei[17]. Once double fertilization occurs, the egg cell becomes the zygote which is then considered sporophyte tissue. Scholars still disagree on whether the fertilized central cell is considered gametophyte tissue. Some botanists consider this endospore as gametophyte tissue with typically 2/3 being female and 1/3 being male, but as the central cell before double fertilization can range from 1n to 8n in special cases, the fertilized central cells range from 2n (50% male/female) to 9n (1/9th male, 8/9th female)[12]. However, other botanists consider the fertilized endospore as sporophyte tissue. Some believe it is neither[12].

Peer Review - Kylie

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Overall, you did a great job covering a ton of information in this section. I really like that you created an introduction paragraph with the gymnosperm and angiosperm subsections. You acknowledge in the last few sentences that there is currently a disagreement between scholars about the classification of the central cell; this is very relevant, and you do a great job informing the reader of the current state of knowledge. Also, you included a variety of examples across species and orders. It seems to me that the content of your article is pertinent, current, and helpful to the reader.

I have a couple notes that I think would really improve your article which is already in good shape. I noticed some typos and grammatical errors in the draft that I'm sure can be fixed with a quick read through of the article. There were a couple parts that I thought might benefit from a little extra explanation including what tube cells, sterile cells, and prothallial cells are. Also, it might be helpful to add a short sentence defining microstrobilus versus megastrobilus; you have links to the conifer cone page here which is great, and I think a short explanation could help. In the second paragraph of the gymnosperms section, you note that mitosis occurs without cell division. The sentence after indicates that this is replication of the nucleus without replicating the whole cell, but I think you could reword or combine these sentences to minimize any possible confusion.

There are a lot of ways you could go in creating a graphic for this page. If you're looking for thoughts, some ideas that struck me while reading your draft include making something that shows the gametophyte fertilization process, a breakdown of parts of the gametophyte tissue in angiosperms or gymnosperms, or a comparison between the structure of seed plant gametophytes and other gametophytes. You used a lot of reliable sources, though it seems like you have some sources that aren't being used in yet in your bibliography.

All in all, I think your draft is in really good shape. I'm sure this is going to be a great article, and you've clearly put a lot of work and research into it!

Peer Review - Elena

[edit]

I think you did a great job at providing very detailed information about gametophytes and reproduction processes. I also think that your introduction was a solid overview of what a gametophyte is and what it consists of. I did see some typos throughout the article but those are easy to fix. I think it might be useful to provide some information in the intro (or wherever you think is most appropriate) about how gymnosperm and angiosperm gametophytes are different. You do talk about them separately but it might be helpful to the reader to see an overview of similarities/differences just so they know what to expect. I liked that you included how there is some debate among scholars about the gametophyte and I think if you can find more information about other points of contention, it might be worth creating a separate section just for that.

I liked how you provided examples of how different orders have different kinds of gametophytes. It might be useful to create a separate section for each of those orders, if you have enough information for each of them, within the gymnosperm or angiosperm section. This may make it easier for the reader to follow. There are also a lot of advanced terms, which is very helpful, but may seem confusing to someone who is new to the subject. If possible, I think it would be worth maybe simplifying the explanations but I definitely understand it will be very difficult to describe these processes you mention without using the exact terminology. You also provide a lot of links attached to most of the terms, which will be extremely helpful. Overall, well done!

-Hi Elena

Thank you for taking the time to do a peer review. I wanted to respond to a few of your comments that I appreciate getting, but that I have decided not to implement. The first was your comment to discuss gymnosperm and angiosperm differences in the introduction. While I did draft out a few sentences about those, after reviewing it, I decided I did not want to get into specifics in the introduction when I have four paragraphs of specifics that detail the differences. The other comment I chose not to utilize was to subdivide my edit into sections. While I agree with this thought process, the only comment I received on the talk page was about explicitly not subdividing the page any further. The commenter was a very active editor (in the top 400) and I decided that his knowledge of Wikipedia should not be glossed over and as such, I am not going to subdivide the page. Overall, thank you for your comments and even thought I am not going to use two of them, I appreciate your time nevertheless. -19345beta

  1. ^ a b c d e f Fernando, Danilo; Quinn, Christian; Bernner; Owens (2010). "Male Gametophyte Development and Evolution in Extant Gymnosperms". International Journal of Plant Developmental Biology. 4: 47–60.
  2. ^ a b Carmichael, J. S.; Friedman, W. E. (1995-12-01). "Double Fertilization in Gnetum gnemon: The Relationship between the Cell Cycle and Sexual Reproduction". The Plant Cell: 1975–1988. doi:10.1105/tpc.7.12.1975. ISSN 1040-4651.
  3. ^ a b c d Friedman, William (1993). "The evolutionary history of the seed plant male gametophyte". Trends in Ecology and Evolution. 8 (1): 15–21. doi:10.1016/0169-5347(93)90125-9.
  4. ^ a b Friedman, William; Carmichael, Jeffery (1996). "Double Fertilization in Gnetales: Implications for Understanding Reproductive Diversification among Seed Plants". International Journal of Plant Sciences. 157 (6): 77–94.
  5. ^ a b c d Friedman, William; Carmichael, Jeffrey (1998). "Heterochrony and Developmental Innovation: Evolution of Female Gametophyte Ontogeny in Gnetum, a Highly Apomorphic Seed Plant". Evolution. 52 (4): 1016–1030. doi:10.1111/j.1558-5646.1998.tb01830.x.
  6. ^ Friedman, William; Goliber, Thomas (1986). "Photosynthesis in the female gametophyte of Ginkgo biloba". American Journal of Botany. 73 (9): 1261–1266.
  7. ^ Williams, Claire G. (2009). Conifer reproductive biology. Dordrecht: Springer. ISBN 978-1-4020-9602-0. OCLC 405547163.
  8. ^ a b Baroux, Célia; Spillane, Charles; Grossniklaus, Ueli (2002). "Evolutionary origins of the endosperm in flowering plants". Genome Biology. 3: 1026.1–1026.5.
  9. ^ Mascarenhas, J. P. (1989-07-01). "The Male Gametophyte of Flowering Plants". The Plant Cell: 657–664. doi:10.1105/tpc.1.7.657. ISSN 1040-4651. PMC 159801. PMID 12359904.{{cite journal}}: CS1 maint: PMC format (link)
  10. ^ Khan, Aisha S. (2017). Angiosperms Structure and Important Products from Flowers in Industry. Somerset: John Wiley & Sons, Incorporated. ISBN 978-1-119-26278-7. OCLC 972290397.
  11. ^ Brewbaker, James (1967). "The distribution and phylogenetic significance of binucleate and trinucleate pollen grains in the angiosperms". American Journal of Botany. 54 (9): 1069–1083.
  12. ^ a b c d Singh, V. (2009–2010). Plant anatomy and embryology of angiosperms. Pande, P. C., Jain, D. K., ebrary, Inc. (1st ed ed.). Meerut, India: Global Media. ISBN 978-81-7133-723-1. OCLC 909120404. {{cite book}}: |edition= has extra text (help)CS1 maint: date format (link)
  13. ^ Borg, M.; Brownfield, L.; Twell, D. (2009-01-23). "Male gametophyte development: a molecular perspective". Journal of Experimental Botany. 60 (5): 1465–1478. doi:10.1093/jxb/ern355. ISSN 0022-0957.
  14. ^ a b Maheshwari, P. (1949). "The Male Gametophyte of Angiosperms". Botanical Review. 15 (1): 1–75.
  15. ^ a b Yadegari, Ramin; Drews, Gary (2004). "Female Gametophyte Development". The Plant Cell. 16: 133–141.
  16. ^ Rudall, Paula J. (2006). "How many nuclei make an embryo sac in flowering plants?". BioEssays. 28 (11): 1067–1071. doi:10.1002/bies.20488. ISSN 0265-9247.
  17. ^ Madrid, Eric N.; Friedman, William E. (2010). "Female gametophyte and early seed development in Peperomia (Piperaceae)". American Journal of Botany. 97 (1): 1–14. doi:10.3732/ajb.0800423.
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