Ploidy and Meiosis: _{The basis for recombination}

Ploidy

Almost all multicellular organisms in the animal and plant kingdoms use diploid cells as their building blocks.

Shu-Nog Bai Chinese biologist, 2015

“Ploidy” refers to how many versions of each chromosome, and thus of each gene, are contained in cells. Ancient organisms had just one copy of each chromosome in their cells. Because they reproduced asexually—duplicating themselves—they passed on an exact copy of the entire gene set (i.e., their full genome) to their offspring.

Today, organisms like bacteria, yeasts, and mosses continue to be “haploid,” reproducing through cloning with just one version of each gene in their genome (see our Reproduction module). But animals and plants evolved to have “diploid” body cells, meaning they have two sets of chromosomes, one from each parent, so every cell in the body has two versions of every gene (except those on sex chromosomes). Diploidy—two complete sets of DNA—is a feature of all “eukaryotes” (organisms whose cells have a nucleus; see our Cellular Organelles I module). The total number of chromosomes varies across organisms, but they all occur in pairs.

How many chromosome pairs do humans have?

If you count the pairs in Figure 3, you’ll see 23 total. American biologist John C. Venter and colleagues first sequenced the DNA of the entire diploid genome of one human individual in 2007. The sequence included the chromosomes inherited from the mother and the father. A map of the entire genome paved the way for tracking the source of particular traits and diseases to the father’s, mother’s, or both parents’ chromosomes.

Advantages of diploidy

Mutations can be good or bad. While many genetic changes will disrupt organism function in ways that are detrimental, mutation is ultimately the source of all adaptive variation.

Kevin Bao American geneticist, 2022

Genes tend to mutate as they’re exposed to “mutagens,” like chemicals, radiation, viruses, and more. Random, spontaneous changes also inevitably occur over time. Bad mutations (also called “deleterious”) are more common than good ones. Over time, the deleterious mutations accumulate, whether in bacteria, plants, or humans. For example, “diffuse gliomas” are tumors that develop in the central nervous systems of humans or other mammals as they age. Figure 4 shows data on how the “mutations” (genetic changes) are responsible for diffuse gliomas accumulating over the years of lifespan (Draaisma et al., 2015).

Note that the number of mutations increases over time, such that older patients tend to have higher-grade tumors than younger patients. The resulting high number of mutations takes a toll on the health and fitness of organisms. When mutations occur in the precursors to gametes (egg or sperm cells), they’re passed on to offspring. Fortunately, diploidy and sexual reproduction offer two ways out of this problem.

Backup gene copies

Diploidy ensures that the cell has a backup copy of every single gene. If a gene gets damaged, the cell may use the backup as a template to repair the damage or replace the function of the bad gene. Thus, diploidy is thought to make organisms more resilient to damage and mutation. This system of self-repair is a huge advantage, so this reason alone may explain why all groups of complex organisms—plants, animals, fungi, and protists—are diploid for at least part of their life cycle.

Genetic experimentation

Diploidy can also facilitate faster and easier genetic innovation. Besides being more tolerant of harmful mutations, diploid organisms can more easily develop beneficial ones. This is because one copy of the gene can maintain the original function, which may be crucial, while the second copy can be tweaked into a new function that may bring new or enhanced functions.

Having two copies of every gene allows for creative genetic experimentation, the raw material for adaptation and innovation.

This experimentation is a key reason why diploid organisms have tended toward innovation, sophistication, and complexity. Conversely, haploid organisms are more constrained and tend toward simplicity and efficiency. Compare single-celled bacteria (haploid) to the complex, multicellular vertebrates (diploid).

Polyploidy

A few organisms even have more than two sets of chromosomes. Such “polyploidy” (more than two sets of chromosomes) is common in plants and has occurred in a few animals. Highly invasive plants that spread rapidly in ecosystems are often polyploid.

For example, American ecologist Adam Green and colleagues (Green et al. 2014) examined two climbing ivy species in the genus Hedera. Although common ivy (H. helix) is native to North America, it is often overtaken by English ivy (H. hibernica), which European colonists first brought over in the 17th and 18th centuries. Green and his colleagues found that the English ivy is “tetraploid”—having four copies of every chromosome. This gives the ivy larger leaves and thicker stems, helping explain why it has successfully invaded forests and urban areas throughout the Northeastern United States, replacing common ivy.

Recognizing that polyploid plants are often hardier and grow faster, plant breeders often intentionally select for polyploidy, which provides even more buffering against harmful mutations. For example, standard grocery-store strawberries are “octoploid,” with eight complete chromosome sets! Bananas may be diploid, triploid, or tetraploid!

Recombination

Natural selection depends essentially on the cumulative augmentation of the most minute useful variations in the direction of their utility.

August Weismann German evolutionary biologist, 1904

Let’s think back to that kitchen equipment. You have too much of it, so you decide to give a few sets away. Each set will include a frying pan, a saucer, a steamer, etc. But if you keep your favorite pieces, your remaining set will differ from the one you started with when you moved in. Moreover, each giveaway set will likely also be from a mix of sources (Figure 5).

When organisms are diploid, there’s an opportunity for “recombination”—a DNA exchange between chromosomes that creates new sets altogether. This is a key feature of sexual reproduction. Swapping out DNA breaks up any accumulations of deleterious mutations and creates new gene combinations. Over time, some combination of genes may fare better than others as the environment changes. This is the basis of natural selection.

For example, UK biologist Matthew R. Goddard and colleagues used experimental populations of yeast to assess how sexual reproduction relates to natural selection (Goddard et al., 2005). Because yeast can reproduce both sexually and asexually, the researchers set up conditions to observe how quickly the yeast would adapt using one or the other mode of reproduction (see our Reproduction module). In an environment with favorable temperatures for yeast, the two strains grew about the same. But in a harsh environment—a little too hot and a little too salty—the two strains reacted differently.

Consider Figure 6. Which strain had higher fitness (better growth) in the harsh environment?

Over time (shown in yeast generations on the x-axis), the sexually reproducing strain (shown in red) adapted better to the harsh conditions than the asexually reproducing strain (shown in blue). In one simple experiment, the researchers showed the value of sexual reproduction—greater adaptability to new environmental challenges.

But there’s an apparent conundrum in sexual reproduction:

How can two genomes be combined without doubling the number of chromosomes every time?

There is a limit to how much DNA and how many chromosomes a cell can handle. Research has shown that polyploid organisms tend to gradually reduce their genome size after the sudden increase (Jaillon et al., 2009). If roommates keep combining their collections of pots and pans, they will eventually start to throw some of them away, or else their cabinets will overflow with redundant cookware.

While polyploidy does occasionally occur, most sexual reproduction involves two diploid organisms creating offspring that are also diploid, with no increase in chromosome number or DNA content. But how is this ensured?

Meiosis

By 1905 there was already a clear understanding that in the life cycles of sexually reproducing eukaryotes there is, associated with nuclear fertilization, a compensating process of nuclear/chromosomal reduction.

UK bioscientist Gareth Jones and colleagues 2024

By the turn of the 20th century, scientists predicted that chromosome reduction must occur to prevent the doubling of DNA during sexual reproduction. In 1887, after observing salamander sperm under a microscope, German anatomist Walther Flemming published a paper proposing that two cell divisions occurred and that they were different (Flemming, 1887). Those divisions are now known as Meiosis I and Meiosis II.

“Meiosis” is a form of cell division that yields haploid egg and sperm cells with just one set of chromosomes each. When two haploid gametes combine, they result in an organism with the diploid complement of chromosomes, avoiding the doubling with each generation. This is like two roommates selecting just one of each type of pan before they move in together so that their combined collection will have just two of each.

Meiosis looks a lot like mitosis, the division that occurs in regular body cells (see our Cell Division II module); however, with sex cells, there are two divisions in a row, producing four haploid egg or sperm cells from one original diploid cell (see Figure 7).

Haploid-Diploid Cycle

Sexual reproduction as we know it today in higher plants and animals can be regarded as a haploid-diploid cycle …These haploids are not viable unless two of them fuse and once more make a diploid cell, which then multiplies by mitosis and eventually forms the mature individual.

Erkan TüzelTurkish biophysicist, 2001

In every instance of sexual reproduction, “fertilization” occurs, like when a sperm cell unites with an egg cell (see our Reproduction module). They combine into a single cell (the “zygote”), which will divide into more cells and differentiate into new individual organisms.

Recalling that the sperm and egg gametes are haploid, what ploidy does the zygote have?

Consider Figure 8. Diploidy has been restored in the zygote and the organism that develops from it. Those new organisms may go on to reproduce, prompting the process of meiosis, which leads to more haploid gametes, which then may fuse into diploid zygotes. Therefore, sexual reproduction is a cycle—the “sexual reproduction cycle,” also called alternation of generations (Bai and Xu, 2013).

Alternation of generations in plants

German botanist Wilhelm Hofmeister first proposed this “Alternation of Generations” (Hofmeister, 1851). Hofmeister studied the most primitive kind of plants, the “bryophytes,” or mosses. But the alternation of generations he described is most dramatic in another ancient family of plants, the ferns.

Consider Figure 9. Which fern lifestage is the most familiar to you?

Chances are good that you’ve seen the “sporophyte” diploid stage since that is what is commonly known as a “fern.” It has green leaves (fronds) and, when reproductive, it produces brown spores that you can see as dots on the underside of the fronds. Each spore can grow into a “gametophyte,” a life stage that tends to be much smaller, like a delicate heart-shaped moss.

Looking at Figure 9 again, what is the ploidy of the spores? What about the gametophyte?

The spores, which the fern makes by meiosis, are haploid. And the gametophyte that results is also haploid. The gametophyte then produces male and female gametes.

What ploidy are these gametes?

They’re also haploid! After a male and female gamete fuse during fertilization, diploidy is restored. This alternation of “sporophyte” (spore-making) and gametophyte (gamete-making) generations is the pattern of sexual reproduction in all plants and algae. A similar cycle is seen in fungi, slime molds, and even some one-celled organisms. Some scientists even argue that haploid-diploid cycles in animal reproduction constitute an alternation of generations.

Alternation of generations in animals

In animals, the haploid stage is limited to the single-celled gamete (sperm and egg), and the diploid stage is the complete multicellular organism.

American botanist Charles J. Chamberlain argued that “animals exhibit an alternation of generations comparable with the alternation so well known in plants,” in a paper published in 1905 (Chamberlain, 1905). He argued that an egg is like a female gametophyte in plants, and the sperm is comparable to the male gametophyte. He then explained that all other animal cells are like the sporophyte generation in plants since they are diploid. Since there is a haploid-diploid cycle, biologists consider this a form of alternation of generations.

Across sexually reproducing organisms, the relative size and independence of the haploid (gametophytes) and diploid (sporophytes) stages can vary a lot. Recall that the sporophyte is the dominant life stage in ferns, but there is still an easily visible, multicellular haploid stage called the gametophyte that lives independently from the sporophyte. In flowering and cone-bearing plants, the free-standing plant is the diploid sporophyte, whether it's a rosebush, a mighty oak, or a pine tree.

Where are the gametophytes?

In flowering and cone-bearing plants, the haploid stages (gametophytes) are found entirely within the flowers or cones and grow from the gametes (pollen and eggs). The male and female gametophytes inside a flower or cone are small, even microscopic, but multicellular. They eventually produce either a sperm or an egg, which fuse to create a diploid zygote. The zygote gets packaged into a seed to grow into a new plant, the sporophyte. As in ferns, both the diploid sporophyte and the haploid gametophyte are multicellular. But, as in animals, the haploid stages depend entirely on the sporophyte and cannot live independently.

Canadian zoologists Sarah P. Otto and Aleeza C. Gernstein (Otto and Gernstein, 2008) examined the variation in how one ploidy life stage dominates over another. They found a tremendous diversity across types of organisms (Figure 10).

• In some organisms, such as yeasts (A), cell division occurs during the haploid stage. So, growth happens before the egg and sperm fuse to make a diploid organism. The result is a dominant haploid life stage, and the diploid stage is tiny and present only during sexual reproduction.
• In seed plants and most animals, including humans (B), the haploid cells are the small gametes or gametophytes. The egg and sperm don’t grow or multiply on their own. Cell growth and division only happen during the diploid stage, after eggs and sperm fuse, making this the dominant life stage.
• In some plants, such as ferns and green and red algae (C), the haploid and diploid phases both experience growth and cell division. So, the haploid gametophyte and diploid sporophyte are both large, multicellular, and independent.

ADD FIGURE 10

Why is there so much variation in haploid-diploidy dominance in sexually reproducing organisms?

Perhaps because meiosis has been around for a long time! Over long periods, nature tends to produce variety and diversity.

How did meiosis evolve?

The stereotypical reductive division in meiosis is a major evolutionary innovation in eukaryotic cells.

American geneticist Marilee Ramesh and colleagues2005

The advent of meiosis—and its role in genetic mixing—transformed the landscape of evolution. Despite its seeming complexity, meiosis is not new. Evidence indicates that meiosis evolved around the time eukaryotes first emerged, and the two events are likely connected. Perhaps the ancient origin of meiosis is not surprising, given that it is the primary source of genetic variety and recombination.

But if meiosis, leading to the recombination of duplicate genes, is such a positive force in adaptation, why are there organisms around that don’t do it?

Diploidy and meiosis produce variety that is beneficial for adaptation. However, asexual reproduction is much faster, easier, and more efficient. Therefore, for organisms already well adapted to their environment, asexual reproduction helps them maintain dominance in their particular environment, such as the bacteria that live in our intestines. All organisms on Earth are the product of a long evolutionary process that has generated multiple life cycles, each with its own costs and benefits. While many diploid organisms favor increasing complexity and sophistication, most haploid organisms have tended toward simplicity, streamlining, and efficiency. Although the resulting organisms are very different (e.g., bacteria versus humans), both strategies are successful.