Module 5 Lesson 5 - 3

Sexual Reproduction

---

Sexual reproduction always involves two important processes: meiosis and fertilization. Recall that diploid cells have two sets of chromosomes present while haploid cells only have one. Haploid cells do not contain homologous chromosomes.

Sexual reproduction in animals and plants requires more energy than asexual reproduction does. To reproduce sexually, the organism must search for and select a mate. Because sexual reproduction takes much longer and requires more energy, organisms are unable to take maximum advantage of favourable environmental conditions. However, sexual reproduction allows for genetic variations. In addition to seeking out a mate with specific phenotypes, genetic recombination and independent assortment of chromosomes during meiosis provides further opportunities for variations called mutations. Mutations within a population are advantageous during harsh environmental conditions and epidemic disease because those organisms with mutations may be adapted better for survival.

Alternation of Generations

---

Meiosis and fertilization act as gate keepers between life as a diploid organism and life as a haploid organism. Animals and plants can alternate between diploid and haploid generations. However, the animal life cycle is dominated almost completely by the diploid generation. The haploid side (the gametes) does not grow and produce large cell masses through mitosis.

Plants alternate between haploid and diploid generations with growth on both sides. This variance between generations is known as alternation of generations. The diploid generation is referred to as the sporophyte generation because it produces haploid spores through meiosis to start the next generation. Each haploid spore grows through mitosis into a multicellular structure referred to as the gametophyte. It produces male and female gamete cells that fertilize to form a new sporophyte generation.

Example: Moss

Ferns and mosses are examples of an organism that spends much time in both parts of the life cycle. Consider the moss life cycle in the image or in your text on page 577. Can you tell which generation is haploid and which is diploid?

The most familiar form of the moss is at the bottom of the image in the haploid form called the gametophyte. The gametophyte can be either a male or a female. The male plant produces male gametes (sperm) through mitosis. Similarly, the female plant produces female gametes (eggs) through mitosis. Both gametes, eggs and sperm, are haploid.

In an aquatic or wet environment, sperm cells move toward egg cells and fertilize the egg. The sexual reproduction yields a diploid zygote called sporophyte.

The sporophyte grows through mitosis. This is the brown stalk with a capsule that is developing within the green, leafy gametophyte. The diploid sporophyte undergo meiosis and produces haploid spores within the capsule. 

The spores are released into the surrounding atmosphere and the wind transports them. A spore that lands in a favourable, moist environment germinates and grows into a haploid gametophyte.

Remember that haploid means only one set of chromosomes (n), and diploid means to have two sets or (2n).

• What process changes an organism from haploid to diploid?
  (If you guessed fertilization, you were right.)
• Which process reduces the organism back to haploid?
  (Meiosis is correct.)

Example: Yeast

Some life forms reproduce either sexually or asexually. Bacteria can grow extensions into neighbouring bacteria and exchange genetic information. This is called conjugation, and it results in a new, genetically-varied daughter cell.

Yeast cells reproduce many times asexually through budding during the diploid and haploid cell cycles. This is the most common method of reproduction for yeast cells.

If conditions are harsh, a haploid cell fuses with another yeast cell and form a diploid cell. Spore are produced through meiosis by these diploid cells. This spore can resist drying out and may lay dormant for a long time. This ensures the survival of the generation throughout the absence of nutrient and favourable living conditions.

When conditions are right, the spore germinates into haploid yeast cells that likely colonize through budding (mitosis). The haploid yeast cells can conjugate to form diploid yeast cells.

Advantages and Disadvantages of Reproductive Strategies

---

The jellyfish has a very complex life cycle. It can reproduce both sexually and asexually. It takes on two body forms: polyp and medusa. The adult jellyfish is called a medusa (shown as #1 in the image), and this is the most familiar form. The female and male medusa jellyfish reproduce sexually by releasing eggs and sperm into surrounding water to be fertilized. Upon fertilization, a larvae forms. The larvae attaches to a firm surface and becomes a polyp. The polyps cannot move, but they reproduce asexually  by budding. These polyps can transform into medusa jellyfish through a budding process shown in the image below.

Why do some organisms reproduce asexually and / or sexually? Why do organisms reproduce sexually when the conditions are favourable? Consider the advantages of these reproductive strategies.

Advantages of Sexual Reproduction:

• Increases genetic diversity
  
• Decreases competition within species
  
• Increases the survival rate of the species in a changing environment
  

• Crossing over during meiosis may replace damaged chromosomes

Advantages of Asexual Reproduction:

• Requires less time than sexual reproduction
  
• Does not require two organisms to find each other
• Allows the species to take advantage of unfavourable conditions by reproducing quickly
  

• Requires less energy than sexual reproduction does
  

Watch and Listen

1. Watch this presentation on the Yeast Life Cycle.



2. Watch the following video on asexual reproduction and the alternation of generations. It reviews mitosis and meiosis and goes through the reproductive strategies introduced above. Note their list of advantages and disadvantages and keep this for studying.

Although several strategies may seem unusual, each has been developed by one species to gain advantage over other species. Balancing concerns over variation and speed, they generally alternate between mitosis for colonization and meiosis to overcome harsh conditions.