Question

Compare and contrast the life histories (life cycles) of invertebrate animals from different phyla using specific animal examples. Include in your answer: a. a description of fertilization, cleavage, and embryonic development of eggs with different amounts of yolk b. the many advantages of life histories that include a larval stage for marine animals and for insects c. a description of the life history of parasitic trematode worms and the features that increase reproductive success even though the life histories are complex and must pass through multiple hosts d. the advantages of asexual reproduction and the advantages of sexual reproduction

Answer 1

Fertilization events:
1. Fertilization accomplishes two separate activities: sex (the combining of genes derived from
two parents) and reproduction (the creation of a new organism).
2. The events of conception usually include: (1) contact and recognition between sperm and egg;
(2) regulation of sperm entry into the egg; (3) fusion of genetic material from the two gametes;
and (4) activation of egg metabolism to start development.
3. The sperm head consists of a haploid nucleus and an acrosome. The acrosome is derived from
the Golgi apparatus and contains enzymes needed to digest extracellular coats surrounding the
egg. The neck of the sperm contains the mitochondria and the centriole that generates the
microtubules of the flagellum. Energy for flagellar motion comes from mitochondrial ATP and a
dynein ATPase in the flagellum.
4. The egg contains a haploid nucleus, and an enlarged cytoplasm storing ribosomes, mRNAs,
and nutritive proteins. Other mRNAs and proteins, used as morphogenetic factors, are also stored
in the egg. Cortical granules lie beneath the egg's plasma membrane. Many eggs also contain
protective agents needed for survival in their particular environment.
5. Surrounding the egg plasma membrane is an extracellular layer often used in sperm
recognition. In most animals, this extracellular layer is the vitelline envelope. In mammals, it is
the much thicker zona pellucida.
6. In many species, eggs secrete diffusible molecules that attract and activate the sperm.
7. In sea urchins, the acrosome reaction is initiated by compounds in the egg jelly. The acrosomal
vesicle undergoes exocytosis to release its enzymes. Globular actin polymerizes to extend the
acrosomal process. Bindin on the acrosomal process is recognized by a protein complex on the
sea urchin egg surface.
8. In mammals, sperm must be capacitated in the female reproductive tract before they are
capable of fertilizing the egg.
9. Mammalian sperm bind to the zona pellucida before undergoing the acrosome reaction. In the
mouse, this binding is mediated by ZP3 (zona protein 3) and one or many sperm proteins that
recognize it. The mammalian acrosome reaction is initiated on the zona pellucida, and the
acrosomal enzymes are concentrated there.
10. Fusion between sperm and egg is mediated by protein molecules whose hydrophobic groups
can merge the sperm and egg plasma membranes. In sea urchins, bindin may mediate gamete
fusion. In mammals, fertilin proteins in the sperm bind to integrins in the egg and allow the
membranes to fuse.
11. Polyspermy results when two sperm fertilize the egg. It is usually lethal, since it results in
three sets of chromosomes divided among four cells.
12. There are often two blocks to polyspermy. The fast block is electrical and is mediated by
sodium ions: the egg membrane resting potential rises, and sperm can no longer fuse with the egg.
The slow block is physical and is mediated by calcium ions. A wave of calcium ions propagates
from the point of sperm entry, causing the cortical granules to fuse with the egg cell membrane.
The released contents of the granules cause the vitelline membrane to rise and to harden into the
fertilization envelope.
13. In mammals, blocks to polyspermy include the modification of the zona proteins by the
contents of the cortical granules. Sperm can no longer bind to the zona.
14. Inositol 1,4,5-triphosphate (IP3) is believed to be responsible for releasing calcium ions from
storage in the endoplasmic reticulum. DAG (diacylglycerol) is thought to initiate the rise in egg
pH. The free calcium ions, supported by the alkalization of the egg, activate egg metabolism,
protein synthesis, and DNA synthesis.
15. The male pronucleus and the female pronucleus migrate toward each other, replicating DNA
as they move.
16. In sea urchins, the two pronuclei merge and a diploid zygote nucleus is formed. In mammals,
the pronuclei disintegrate as they approach each other, and their chromosomes gather around a
common metaphase plate.
17. Some genes are transmitted differently depending on whether they are from the egg or the
sperm. Methylation differences determine if these genes are to be expressed in the early embryo.
18. Microtubular changes cause cytoplasmic movements. These rearrangements of cytoplasm can
be critical in specifying which portions of the egg are going to develop into which organs.

b) The presumed advantages of such larvae include:

the avoidance of competition for resources with adults,

temporary reduction of benthic mortality while in the plankton,

decreased likelihood of inbreeding in the next generation, and

increased ability to withstand local extinction.

d)

The advantages of asexual reproduction include:
•   the population can increase rapidly when the conditions are favourable
•   only one parent is needed
•   it is more time and energy efficient as you don't need a mate
•   it is faster than sexual reproduction
The advantages of sexual reproduction:
•   produces genetic variation in the offspring
•   the species can adapt to new environments due to variation, which gives them a survival advantage
•   a disease is less likely to affect all the individuals in a population