Reproduction: _{Asexual to sexual}

Staking your odds

Imagine you are at the horse races with $100 to wager. You can either stake your entire $100 on the favored horse named Secretariat. Or, you could spread it out to bet $20 each on five different horses. What should you do? Betting the whole amount on Secretariat means you will get a big payout if he wins. But if conditions change in a way that dampens Secretariat’s advantage, like if it rains on the track or he gets a bad start, you risk losing all your money. If you spread the investment, you have better odds of winning, although the payoff won’t be as big (Figure 1).

This classic gambling dilemma parallels a dilemma in nature—how best to reproduce. Asexual reproduction is like going all-in on one horse, while Sexual reproduction is like spreading out your wager among multiple horses, which may increase your chance of winning if conditions change. In sexual reproduction, genetic recombination occurs, yielding offspring with various genomes that may fare differently in different environments.

However, sexual reproduction requires male and female sex cells—egg and sperm—to come together, which can involve a costly and prolonged path to generating offspring. Asexual reproduction, on the other hand, is almost always quicker, easier, more efficient, and more predictable.

All organisms on Earth reproduce, but how they reproduce varies, reflecting the different risk-benefit ratios of asexual versus sexual reproduction.

Asexual reproduction

Asexual reproduction—a simple “copy and divide” strategy—at first glance seems both less messy and more efficient.

Culotta and Hines, 1998

The least costly way to reproduce is asexually, i.e., just betting on the horse that you’re pretty sure will win. In asexual reproduction, an organism simply makes copies of itself, such that every offspring is a genetic clone of the parent. Although asexual reproduction is a speedy and easy way to multiply, it’s not the best way to generate diversity. It’s like betting on the same horse over and over again.

Cloning is the most straightforward and ancient form of reproduction. For the first two billion years of life on Earth, all organisms were prokaryotes: single-celled bacteria and archaea (see our Discovery and Structure of Cells module). Because single-celled organisms don’t leave fossils, we can only speculate about how they reproduced. However, given that most aspects of reproduction are universal among even the most distantly related microbes, these ancient ancestors almost certainly reproduced similarly to the bacteria and archaea that scientists study today.

Caveat: Some mixing of genetic information occurs when prokaryotes release and uptake DNA through their environments. In such “parasexual” genetic exchange, a bacterium may take free DNA into its cell and then make it part of its chromosome. This is not reproduction, but it does generate some genetic diversity, which can facilitate adaptation and evolution.

Modes of asexual reproduction

Most single-celled organisms reproduce through one of several possible modes of asexual reproduction.

Binary fission

Bacteria and some yeast reproduce through a process called “binary fission,” where a bacterium doubles in size and then splits into two cells. Because each offspring requires a complete copy of the genes, the cell must first copy all of its DNA and send each copy to opposite sides of the cell before it splits. How a bacterium clones itself is similar to how cells in our body multiply (see our modules Cell Division I and Cell Division II). 

Budding

I believe that alcoholic fermentation never occurs without either the simultaneous organization, development and multiplication of cells or the continued life of cells already formed.

Louis Pasteur, Alba-Louis et al., 2010

“Budding” is similar to binary fission, except that one new offspring sprouts off the parent cell and breaks away to become an independent clone of the original parent cell.

What do you notice in the drawing of yeast cells atop the surface of wine in Figure 2?

In Figure 2, notice how the yeast cells are in clusters, sometimes with smaller cells attached to larger cells. Since such observations by French chemist and microbiologist Louis Pasteur, scientists have known that yeasts reproduce by budding (Pasteur, 1857). For thousands of years, humans have capitalized on the fast reproduction of yeasts, using them to cultivate fermented products such as beer, wine, bread, and vinegar. As yeasts convert sugar into alcohol (see our Chemical Reactions I module), grape juice transforms into wine.

Intracellular budding

Evolution has come up with odd variations on budding. American microbiologist David Miller and his colleagues study large bacteria living in the surgeonfishes' gut, Epulopiscium viviparus (Miller et al., 2012). The bacteria feed on food eaten by the fish and, in exchange for their bed and breakfast, help the fish digest their meals.

Figure 3a is an image of these big bacteria. Inside them, you can see multiple offspring that form and then get released from the mother cell. This, too, is a form of budding called “intracellular budding,” in that the parent is producing clones of itself.

Asexual reproduction is an effective way to generate offspring in stable conditions, like the belly of a sturgeon fish. However, when we examine multicellular organisms, things get a bit more complicated.

Fragmentation (and regeneration)

Regeneration has the widest range of biological implications ...allowing the rapid colonization of new habitats through the production of multiple clones well adapted to local conditions.

Yousra Ben Khadra, 2018

Asexual reproduction is not limited to single-celled organisms. Larger organisms that can reproduce asexually include some simple plants such as mosses, invertebrates such as worms and insects, and even a few vertebrates.

Suppose you’ve accidentally chopped a piece off an earthworm and seen it survive and grow into a new worm. In that case, you’ve witnessed “fragmentation,” where a body fragment regenerates into a whole organism. A diverse set of eukaryotes can reproduce by fragmentation, including sea sponges, molds, worms, and sea stars. Figure 4 shows the progression of a sea star’s arm into a new (cloned) individual.

The fragmentation of a multicellular organism must be more complicated because a detached fragment will not contain all the various body structures. How can a whole new organism grow from just one part? It’s not like a bacterium or a yeast that makes an entire copy of itself, which can stand alone. A sea star, for example, has a mouth, eyes, gonads, and skeleton, each contained in a different arm.

Tunisian geneticist Yousra Ben Khadra has been studying regeneration in sea stars and some of their closest relatives to determine how a body fragment can lead to a new organism. Khadra identifies three phases:

1. Repair: The fragment's wound heals as cells stretch over it and are backfilled with other cells.
2. Early regeneration: A clump of undifferentiated cells forms and orients so new arm structures grow at the right points.
3. Advanced regeneration: The cells differentiate to form the body tissues that comprise the tube feet, exoskeleton, and even eyes (Ben Khadra et al., 2018).

Whether by fission, budding, or fragmentation, asexual reproduction allows organisms to create multiple clones of themselves. By the numbers, if you include bacteria and other single-celled organisms (with estimates ranging from 1.0 billion to 1.0 trillion species; Dykhuizen, 2005; Locey & Lennon, 2016), asexual reproduction is the oldest and still the dominant mode of reproduction on the planet.

Sexual reproduction

Consider this drawing - Figure 5. Which of these organisms do you suppose reproduce sexually?

All of the organisms shown in Figure 5 reproduce sexually: vertebrates such as birds, small mammals, and fish, as well as flowering plants and grasses.

Vertebrates (animals with backbones) typically reproduce sexually. A survey of the approximately 70,000 known vertebrates found asexual reproduction in just 0.1%: 22 fish, 23 salamanders, and 29 reptiles (Vrijenhoek et al., 1989). Despite being able to reproduce asexually, most of these 70 or so species still reproduce sexually most of the time.

It is the same story with plants. The oldest and simplest plants (the mosses) are capable of asexual and sexual reproduction. The largest and most complex plants (seed plants with flowers or cones) heavily favor sexual reproduction, even if they can reproduce asexually. In fact, sexual reproduction has come to dominate nearly all of the most successful plant, animal, and fungi lineages.

Figure 6 depicts the reproductive cycle of a bacteria (asexual reproduction) and a flowering plant (sexual reproduction). Which looks more complicated?

The molecular and cellular events of sexual reproduction are much more complicated than those of asexual reproduction. For instance, animals dedicate time and energy to finding a mate and assessing their suitability. They risk exposure to predators and sexually transmitted diseases, all in the uncertain hope of uniting eggs with sperm (Engelstädter, 2008; Weedall & Hall, 2015). Even though sexual reproduction is more time-consuming, risky, and less reliable, more than 95% of “eukaryotic organisms” (those with nuclei in their cell; see our module Discovery and Structure of Cells) reproduce sexually, including 99.9% of animals and 90% of plants.

For centuries, scientists have questioned why many organisms evolved to reproduce sexually despite its many costs. It is relatively easy to see how asexual reproduction would beat sexual production in a head-to-head match-up. Half a century ago, mathematician John Maynard Smith recognized that unless a sexual pair produces twice as many successful offspring as an asexual individual, the pair will have lower reproductive output per capita. Maynard Smith dubbed the phenomenon the “cost of males” (Smith, 1971).

Later, German evolutionary ecologist Manfred Milinski modeled Maynard Smith’s hypothesis. In Figure 7, you can see the theoretical output after four generations of sexual versus asexual reproduction, with the original breeder, or breeding pair, producing just two offspring each. With sexual reproduction, two offspring are only enough to replace the breeding pair, and no population growth will occur. Asexual reproduction, on the other hand, leads to swift growth.

What do you see in the numbers?

In asexual reproduction, there are no sexes. In sexual reproduction, every reproductive event requires both a male and a female. Assuming every reproduction yields two offspring (see Figure 7), asexual reproduction produces far more offspring after just four generations.

Despite the apparent drawbacks, sexual reproduction remains dominant in all of the most complex organisms.

Why sexual reproduction?

Sexual reproduction is an extraordinarily widespread phenomenon that assures the production of new genetic combinations in nearly all eukaryotic lineages.

Laure Mignerot and Susana M. Coelho, French geneticists, 2016

In every case of sexual reproduction, a sperm cell unites with an egg cell. The sperm and eggs are known as the “gametes” and most often come from males and females, respectively. However, many invertebrates are “hermaphrodites,” which make both male and female gametes. Regardless, gametes have half the number of chromosomes in the organism’s other cells. That way, when the eggs and sperm combine, the resulting individual will have a complete set of chromosomes.

This complex set of events accomplishes the real advantage of sexual reproduction: genetic diversity. Sexual reproduction produces genetically unique individuals instead of creating clones, like asexual reproduction. This constant generation of new genetic combinations is thought to be a key value in sexual reproduction, outweighing the costs and drawbacks. Sexual reproduction is also thought to carry advantages in breaking up accumulations of bad genetic material and escaping pathogens by evolving to evade them.

Asexual or sexual: take your pick?

Facultative sexuality, being able to reproduce both sexually and asexually, has been deemed evolutionarily favourable as the benefits of either mode may be fully realized.

Hannah Koch and Lutz Becks, German evolutionary biologists, 2017

Studies of organisms that can reproduce both asexually and sexually have shed light on tradeoffs. Having the flexibility to switch back and forth is rare in animals, but small insects called “aphids” can do just that. Aphids typically reproduce asexually in the spring and summer when resources are abundant, allowing them to increase their populations rapidly. Each offspring develops from an unfertilized egg, a process called “parthenogenesis.” But things change later in the year.

French geneticist Gaël Le Trionnaire and colleagues found that dwindling sunlight causes aphids to switch from asexual to sexual reproduction. In winter, a female’s eggs are fertilized by a male, then laid and buried underground, yielding offspring that hatch in the spring (Trionnaire et al., 2012, Figure 8).

Why might aphids switch seasonally between asexual and sexual reproduction?

This switch from asexual to sexual reproduction allows aphids to capitalize on the advantages of both modes. When plants are growing and food is abundant, a single female aphid can generate about 80 offspring every ten days. Cloned offspring hatch right from unfertilized eggs and get straight to work on feeding and growth. But when resources become limited, they can shuffle the genetic deck and promote diversity. A fertilized egg takes 100 days to hatch and, unlike the grown aphid, can survive the winter underground. When Spring arrives, a more diverse population hatches, and the cycle continues.

Other organisms, like small aquatic crustaceans (Daphnia) and “duckweed” water plants, show a similar pattern of switching reproduction types as conditions become seasonally unfavorable, providing further evidence for the distinct benefits of the two modes.

Although no asexual mammals are known, researchers find that even some vertebrates can switch reproductive modes when needed (Booth et al., 2012), keeping in mind that sexual reproduction takes more energy and requires a mate. Vertebrates typically reproduce sexually, but some snakes, lizards, birds, and sharks can, on rare occasions, reproduce asexually. For instance, captive female sharks with no male contact demonstrated parthenogenesis, and a lone female Komodo dragon at a zoo laid eggs that hatched without being fertilized. In some cases, parthenogenesis is a last-ditch strategy for reproduction when mates are not available.

Remember the horse race betting? There are two strategies to consider: bet it all on the horse you think is the best overall or spread your bet across multiple horses with different strengths and weaknesses. Both approaches have merit, and you won’t know which was the best strategy until the race ends and the bets are paid. The same is true for reproduction. The speed and efficiency of asexual reproduction make it the clear winner for a well-suited organism with abundant resources. But if and when conditions change, resources become scarce, and competition gets fierce, the diversity that sexual reproduction generates makes it the best choice.

Environmental cycles and patterns create an ever-changing world in which billions of species compete to survive and thrive. Through the creative diversity that sexual reproduction provides, we can be sure that Earth will always be teeming with “endless forms most beautiful” (Darwin, 1859).