Development of Plant Gametes - Online Tutor, Practice Problems & Exam Prep

Plants exhibit various sexual differentiation terms: unisexual (dioecious) organisms have either male or female parts, while bisexual (monoecious) organisms possess both. Gamete formation involves two life stages: the haploid gametophyte and the diploid sporophyte. The sporophyte produces haploid spores through meiosis, which then undergo mitosis to form gametes. In algae like Platydomonas, haploid organisms can fuse under specific conditions to create a diploid form, highlighting the complexity of plant reproduction and gamete formation.

Plant Gamete Terminology

So when it comes to plants, there's a lot of different terminology. So let's make sure we understand this terminology, and different books use different combinations, and sometimes they use one word, and sometimes they use another. So let's make sure we are all on the same page with what people are talking about. The first one is unisexual, dioecious, or gonochoric, and all three of these terms mean the same thing, and it's that the organism contains either male or female organs. So these are humans. Right? So the majority, the overwhelming majority of humans are either male or female. So that means that we're unisexual, dioecious, or gona court, depending on the term your book and professor uses.

Then you have bisexual, monoecious, or hermaphroditic, and this means that the organisms contain both male and female organs. And so, sometimes some organisms will switch between the two. There are cases of that in humans, where certain chromosomal abnormalities result in organisms having both male and female organs, but they typically, in human settings, are sterile. But in a lot of different plants, these are very common.

So there's a third term called intersex, and this generally refers to organisms with an intermediate sexual condition. So usually, these are people that are or plants; we're talking about plants. They're supposed to have one or the other, but instead, they have some weird mixture that's really not normal, and these are generally sterile.

Now there are two types of sexual differentiation, and this depends on how we determine what sex a plant is. Primary sexual differentiation refers to the gametes produced. So, do they produce eggs or do they produce, in the case of plants, pollen. Right? But for secondary sexual differentiation, this refers to what we can see, so the overall sexual appearance. This is referring to the actual sexual organs that the plant has, whereas primary sexual differentiation, we can't really see. We can look at a flower and see, you know, what does it have male parts or female parts? But we can't look and see what gametes it's producing. We can infer that from the secondary sexual differentiation, but these are two different ways of classifying the sex of the organism.

Now, I don't really have a great example for this. Hopefully, it's just clear, but I really make sure you understand the difference between male or female and male and female. So, with that, let's now move on.

Formation of Plant Gametes

Okay, so now let's talk about the formation of plant gametes. Plants, like animals, have two life stages. There's a haploid stage and a diploid stage. These are given fancy terms. The haploid is called the gametophyte and the diploid is called the sporophyte. Both the gametophyte and the sporophyte are used to create gametes. How this works is you start off with haploid spores. Haploid spores are created from meiosis of the sporophyte. Remember, the sporophyte is diploid, so it undergoes meiosis and creates the spores. The spores themselves are not gametes, but instead, they undergo mitosis to create a lot of themselves to become gametes. If we were to look at what this looks like, we start off with spores here. So, the spores were created, the sporophyte here, this is diploid. It undergoes meiosis, creates the spores. Then, mitosis happens, that creates a bunch of copies called the gametophyte. These are where the gametes come from; they can fuse together to create a zygote, which again is diploid, and undergo mitosis to create the sporophyte. Hopefully, I wasn't in the way, hopefully, you can see all that. And so, this is super important.

The structures of the sporophyte and the gametophyte are different, which makes sense because one is diploid and one is haploid, so different types of cells. Typically what you see is the flower, you can actually see the sporophyte in it, and the gametophyte is actually only a few haploid cells, so it's not a ton. It's small, you can't really see it, and it resides within the flower.

So, the plant typically contains both male and female organs. The male organ is called the stamen, and this has microsporocytes. These are diploid. These undergo meiosis to form haploid microspores, and microspores undergo mitosis to produce the haploid pollen, which is the gametophyte. So when we talk about male, we're using "micro." When we talk about female, that is called the stigma, and this is megaspores, and it's the same thing that happens. Megasporesites undergo meiosis to create megaspores. Most of these megaspores die; usually one survives, and it divides by mitosis. What happens is it actually creates a single 8 haploid nuclei. This 8-nuclei complex is called the gametophyte. So this is a little bit different. The microspores, there can be tons of them and they're released as pollen. We all know that pollen, there's a whole lot of pollen that's released. Whereas the female creates this much smaller structure. Here's an example of this: we have our microspores, it's diploid, it undergoes meiosis to create the microspores which are haploid, undergoes mitosis to create a lot of them, this is pollen because we're talking about microspores. Megaspores are a little different, but this is also haploid and considered the gametophyte stage of the plant. Now, fertilization occurs when the pollen lands on the stigma, and the pollen and the female gametophyte, which is given a term called the embryo sac, come together and create a diploid zygote. So that is just an example of the development of plant gametes and how they are fertilized. So with that, let's now move on.

Chlamydomonas

Okay. So, now, let's talk about Platydomonas. Platydomonas is an algae, and it has an interesting life cycle. Not all of you will see this in your book, but some of you will see it in lab, some of it is in your book, so I thought I would include it here for those of you that will see it either in lab or in class. A Caudydomonas is an algae, and for most of its life, it's haploid, meaning that it asexually reproduces via mitosis. It just creates exact copies of itself. However, there are certain conditions that the algae encounters. It can be high salt, weird pH, or temperature; whatever the conditions are, but there are certain conditions that result in a fusion of two of the haploid Plaidammoni. And this, when they fuse together, creates a diploid organism that exists. Now when those conditions are removed, so the salt concentration goes back to normal, pH is fine, temperature drops, or you know it goes back to how it's supposed to be, meiosis can occur in this diploid organism, and, this organism will actually produce these haploid zoospores. These are haploid cells and have reverted back to how they are normally. Right? But the interesting part of this that scientists have found is that not all Cladidymis organisms are able to fuse together. Only certain combinations of them can fuse. So, they've discovered that there are 2 mating types, classified by a positive or a negative sign, MT⁺ or MT⁻. Those are the 2 types. And the MT⁺ will only fuse or mate with the MT⁻, so it has to be the opposite type in order for them to mate. However, when you look at Cladidymis, if you have a bunch of Cladidymis and you're looking under a microscope at it, they are called isogametes because you can't tell them apart by looking at them. They're not morphologically distinguishable. They look exactly the same, but something inside of them is different, and that is what allows the different mating types to be able to mate. So here's what Platydomonas looks like. You can see there are a few different ones here. Obviously, if you don't know what you're looking for, but even if you did, you wouldn't be able to tell which one is mating type positive or mating type negative because they're isogametes, and that means that they look exactly the same, but they do have something different about them that allows them to fuse the appropriate type. So with that, let's now move on.

Which of the following terms can be used to describe an organism that contained either male or female sex organs, but not both.

• A. Bisexual
• B. UnIsexual
• C. Intersex
• D. Sexually differentiated

Which of the following is the diploid life cycle stage in plants?

• A. Gametophyte
• B. Spores
• C. Sporophyte
• D. Gametes

In the alternations of generations life cycle, which cellular division process is used to create gametes?

• A. Meiosis
• B. Mitosis

Microsporocytes undergo which cellular process to form microspores?

• A. Meiosis
• B. Mitosis

Megaspores are created in the male stamen.

• A. True
• B. False

Here’s what students ask on this topic:

What are the differences between unisexual (dioecious) and bisexual (monoecious) plants?

Unisexual (dioecious) plants have either male or female reproductive organs, meaning each individual plant is either male or female. Examples include holly and kiwifruit. Bisexual (monoecious) plants, on the other hand, possess both male and female reproductive organs within the same individual. This allows them to self-pollinate or cross-pollinate with other plants. Examples include corn and cucumbers. Understanding these differences is crucial for studying plant reproduction and breeding strategies.

How do the haploid and diploid stages contribute to plant gamete formation?

Plants have two life stages: the haploid gametophyte and the diploid sporophyte. The diploid sporophyte undergoes meiosis to produce haploid spores. These spores then undergo mitosis to form the gametophyte, which produces gametes. The male gametophyte produces pollen, while the female gametophyte produces the embryo sac. Fertilization occurs when pollen lands on the stigma, leading to the formation of a diploid zygote, which grows into a new sporophyte. This cycle ensures genetic diversity and adaptation.

What is the role of meiosis and mitosis in the life cycle of plants?

Meiosis and mitosis play crucial roles in the plant life cycle. Meiosis occurs in the diploid sporophyte to produce haploid spores, which ensures genetic diversity. These spores then undergo mitosis to form the haploid gametophyte, which produces gametes. Mitosis allows for the growth and development of both the gametophyte and the sporophyte stages. Together, these processes enable plants to reproduce sexually and maintain genetic variation within populations.

How does the life cycle of Platydomonas differ from that of higher plants?

Platydomonas, an algae, primarily exists in a haploid state and reproduces asexually via mitosis. Under specific conditions like high salt or unusual pH, two haploid cells can fuse to form a diploid organism. When conditions normalize, meiosis occurs, producing haploid zoospores. Unlike higher plants, which have distinct gametophyte and sporophyte stages, Platydomonas can switch between haploid and diploid states based on environmental conditions, showcasing a simpler yet adaptive reproductive strategy.

What are the primary and secondary sexual differentiations in plants?

Primary sexual differentiation in plants refers to the type of gametes produced: pollen (male) or eggs (female). Secondary sexual differentiation involves the visible sexual organs, such as stamens (male) and pistils (female). While primary differentiation is about the reproductive cells, secondary differentiation is about the physical structures that produce and house these cells. Understanding both types is essential for studying plant reproduction and identifying plant sexes.