Chapter 42 Lecture 22 Asexual and sexual reproduction Dr. Tony Michael

Asexual and sexual reproduction Aims: To examine reproduction without sex To examine reproduction with sex

Aims: To examine reproduction without sex To examine reproduction with sex These lecture aims form part of the knowledge required for learning outcomes 3 and 4. Describe mechanisms for life processes (LOC3). Appreciate how the physiology of an organism fits it for its environment (LOC4). Asexual and sexual reproduction

42 Animal Reproduction 42.1 How Do Animals Reproduce without Sex? 42.2 How Do Animals Reproduce Sexually? Essential reading Pages

42.1 How Do Animals Reproduce Without Sex? Asexual reproduction requires no mating but does not result in genetic diversity. Asexually reproducing species are mostly invertebrates, sessile, and live in constant environments. Three types of asexual reproduction are budding, regeneration, and parthenogenesis.

42.1 How Do Animals Reproduce without Sex? Budding produces new individuals that form from the bodies of older animals. A bud grows by mitotic cell division and cells differentiate before the bud breaks away. The bud is genetically identical to the parent.

42.1 How Do Animals Reproduce without Sex? Regeneration can replace damaged tissue or may form a complete individual. Example: Echinoderms

Figure 42.1 Asexual Reproduction in Animals

Asexual reproduction (budding) in hydra

42.1 How Do Animals Reproduce without Sex? Parthenogenesis is the development of offspring from unfertilized eggs. Parthenogenesis may determine the sex of the offspring. In some species, females can act as males depending on cyclic states of estrogen and progesterone.

Figure 42.2 Sexual Behavior May Be Required for Asexual Reproduction

42.2 How Do Animals Reproduce Sexually? In sexual reproduction two haploid cells form a diploid individual. Gametogenesis produces haploid gametes through meiotic cell division. Meiosis allows genetic diversity through crossing over, between homologous chromosomes, and independent assortment.

42.2 How Do Animals Reproduce Sexually? Three fundamental steps of sexual reproduction: Gametogenesis: making gametes Mating: getting gametes together Fertilization: fusing gametes

42.2 How Do Animals Reproduce Sexually? Gametogenesis occurs in the gonads. In males the gonads are the testes and the gametes are small, motile sperm. In females the gonads are the ovaries and the gametes are nonmotile ova, or eggs.

42.2 How Do Animals Reproduce Sexually? Gametes are produced from germ cells, which occur early in development and are distinct from the rest of the body. All other embryonic cells are called somatic cells. Germ cells migrate to the gonads when they begin to form.

42.2 How Do Animals Reproduce Sexually? Germ cells undergo mitosis and produce spermatogonia in males and oogonia in females. Mitosis continues and produces primary spermatocytes and primary oocytes, both diploid. Meiosis produces haploid cells that mature into sperm and ova.

42.2 How Do Animals Reproduce Sexually? Spermatogenesis: Male germ cells divide by mitosis to form spermatogonia and ultimately primary spermatocytes. The first meiotic division results in two secondary spermatocytes; the second division produces four haploid spermatids.

42.2 How Do Animals Reproduce Sexually? After each division the offspring of primary spermatocytes are connected by cytoplasmic bridges. Spermatids will differentiate into sperm.

Figure 42.3 Gametogenesis

42.2 How Do Animals Reproduce Sexually? Oogenesis Female germ cells divide by mitosis to form oogonia and then primary oocytes. A primary oocyte immediately begins prophase I of meiosis. After prophase I development stops in many species.

42.2 How Do Animals Reproduce Sexually? The primary oocyte grows larger and acquires nutrients. When meiosis resumes, the nucleus of the oocyte divides into two daughter cells of unequal sizes. The cell with more cytoplasm is the secondary oocyte and the smaller one forms the first polar body.

42.2 How Do Animals Reproduce Sexually? The secondary oocyte divides through meiosis and produces: A large, haploid ootid, which differentiates into a mature ovum A second polar body, which degenerates

Figure 42.3 Gametogenesis

42.2 How Do Animals Reproduce Sexually? Fertilization is the union of a haploid sperm and a haploid egg. It creates a single diploid cell, called a zygote, which will develop into an embryo.

42.2 How Do Animals Reproduce Sexually? Steps in fertilization: Recognition of sperm and egg Activation of sperm Plasma membranes fuse Additional sperm entry blocked Activation of egg Egg and sperm nuclei fuse

42.2 How Do Animals Reproduce Sexually? Species-specific sperm and egg interactions are controlled by specific recognition molecules. Eggs of aquatic species release chemical attractants to cause sperm to swim toward the egg. Sperm must go through two protective layers to reach an egg: a jelly coat and the vitelline envelope.

Figure 42.4 The Sea Urchin Egg

42.2 How Do Animals Reproduce Sexually? The acrosome is a membrane- enclosed structure on the sperm head. Egg and sperm contact causes substances in the jelly coat to trigger an acrosomal reaction. Membranes in the sperm head and acrosome break down, enzymes are released and digest the jelly coat.

Figure 42.5 Fertilization of the Sea Urchin Egg

42.2 How Do Animals Reproduce Sexually? An acrosomal process extends from the head of the sperm. The acrosomal process is coated with bindin, specific recognition molecules. Bindin acts on bindin receptors in the vitelline envelope. Sperm and egg plasma membranes fuse to form a fertilization cone.

42.2 How Do Animals Reproduce Sexually? Internal fertilization involves species- specific mating behaviors and egg- sperm recognition mechanisms. The egg is surrounded by the cumulus, cells in a gelatinous matrix. The zona pellucida is a glycoprotein envelope beneath the cumulus.

42.2 How Do Animals Reproduce Sexually? Sperm deposited in the reproductive tract are activated and can penetrate the cumulus. Species-specific glycoproteins in the zona pellucida attach to the head of the sperm and trigger the acrosomal reaction. Enzymes digest the zona pellucida and egg and sperm membranes fuse.

42.2 How Do Animals Reproduce Sexually? Fusion and entry of a sperm into the egg lead to: Blocks to polyspermy: mechanisms to prevent more than one sperm from entering an egg.

42.2 How Do Animals Reproduce Sexually? Fast block to polyspermy: Transient Caused by change in membrane potential as sodium ions enter plasma membrane of egg after contact with a sperm.

42.2 How Do Animals Reproduce Sexually? Slow block to polyspermy – Initiated by release of calcium ions. Cortical granules fuse with plasma membrane and release enzymes. Bonds between vitelline envelope and egg plasma membrane dissolve. H 2 O is absorbed and enzymes remove sperm-binding receptors.

42.2 How Do Animals Reproduce Sexually? H 2 O is absorbed by proteins into the space between the vitelline envelope and the plasma membrane. The vitelline envelope rises to form a fertilization envelope. Enzymes remove sperm-binding receptors and cause the envelope to harden.

42.2 How Do Animals Reproduce Sexually? In mammals sperm entry does not cause membrane depolarization. Calcium influx occurs and cortical granules fuse with the egg plasma membrane. No fertilization envelope forms but enzymes destroy the sperm-binding molecules in the zona pellucida.

Figure 42.6 A Mammalian Egg is Surrounded by Barriers to Sperm

42.2 How Do Animals Reproduce Sexually? Sexual reproduction requires: Production of haploid gametes Formation of a diploid zygote Mating brings eggs and sperm together so that fertilization occurs.

42.2 How Do Animals Reproduce Sexually? External fertilization occurs in an aquatic environment when gametes are released into the water. Reproductive behaviors to ensure fertilization include: Release of gametes synchronized by day length, temperature, or weather Congregating with potential mates

42.2 How Do Animals Reproduce Sexually? Internal fertilization occurs when sperm is released directly into the female reproductive tract. Gonads are the primary sex organs. Accessory sex organs include the penis and vagina, and copulation is the joining of the male and female accessory organs.

42.2 How Do Animals Reproduce Sexually? Transfer of sperm in internal, fertilization can also be indirect. Males can deposit spermatophores— packets of sperm—in the environment and females position themselves to allow sperm entry. Genitalia are external sex organs and are often species-specific.

42.2 How Do Animals Reproduce Sexually? Dioecious species have separate male and female members. In a monoecious, or hermaphroditic, species, a single individual may produce sperm and eggs.

42.2 How Do Animals Reproduce Sexually? An earthworm is an example of a simultaneous hermaphrodite; male and female at the same time. An anemone fish is a sequential hermaphrodite and may function as a male or a female at different times.

42.2 How Do Animals Reproduce Sexually? Reptiles and birds evolved an amniote egg to overcome a dry environment. An egg contains food and water and the hard shell reduces water loss. Sperm must reach the egg before the shell forms, requiring internal fertilization and accessory sex organs.

42.2 How Do Animals Reproduce Sexually? Snakes and lizards have paired hemipenes that fill with blood and protrude from the male’s body. Birds may have erectile penises or bring their genital openings close enough to transfer sperm. All mammals practice internal fertilization and the embryo develops in the female reproductive tract.

Figure 42.8 The Shelled Egg

42.2 How Do Animals Reproduce Sexually? Oviparity: egg-laying Oviparous animals lay eggs in the environment and embryos develop outside the mother’s body. Viviparity: live-bearing Viviparous animals retain the embryo in the mother’s body during early development.

42.2 How Do Animals Reproduce Sexually? Viviparity differs in mammals as they have a specialized female reproductive tract: Uterus (or womb): holds the embryo Placenta: develops in the uterus and enables exchange of nutrients and waste

42.2 How Do Animals Reproduce Sexually? Nonmammalian viviparous animals retain fertilized eggs in the mother’s body until they hatch. Ovoviviparity means that the embryo still receives nutrients from the egg.

Check out 42.1 Recap, page CHAPTER SUMMARY, page Recap, page CHAPTER SUMMARY, pages 918, See WEB/CD Activity 42.1 Self Quiz page 919: Chapter 42, questions 1-4 Asexual and sexual reproduction For Discussion page 919: Chapter 42, questions 1 and 2

Key terms: acrosome, acrosomal reaction, asexual reproduction, bindin, budding, copulation, embryo, fertilization, gametes, gonads, mating, meiotic cell division, mitotic cell division, oocytes, oogenesis, oogonia, ova, ovaries, parthenogenesis, penis, polyspermy, regeneration, sessile, sexual reproduction, sperm, spermatid, spermatocytes, spermatogonia, spermatophores, somatic cells, testes, Udx1 (urchin dual oxidase), vagina, vitelline envelope, zona pellucida Asexual and sexual reproduction