(a) Why is one species of nematode, C. elegans, a good model for studying development and genetics?
(b) Compare the body structure of nematodes, flatworms, and annelids.
(c) What is biological control? How are nematodes and bacteria used to help control insect pests of plants?

(a) Because of their evolutionary conservation of gene function and experimental tractability.

(b) Flatworms, Nematodes and Annelids differ from each other on the basis of presence and absence of body cavity called coelom.

(c) Biological control or biocontrol is a method of controlling pests such as insects, mites, weeds and plant diseases using other organisms.

A group of organisms that shows promise as biological control agents for soil pests are insect-parasitic nematodes.

Explanation:

(a) C. elegans is a small soil worm or nematode, and it shares a common ancestor with humans that lived in the pre-Cambrian era, 500-600 million years ago.   This ancestor is referred to as the urbilaterian ancestor, as it is the relative of all bilaterally symmetric, multicellular organisms on the planet, including invertebrates (insects, nematodes, sea urchins) and vertebrates (mammals, fish, birds, reptiles).

Most of the genes and genetic mechanisms that govern modern organismal development, including those involved in human development and disease, were present in the urbilaterian ancestor and are shared by existing animals, including humans and nematodes.

Evolution is a process by which existing genetic mechanisms are conserved but changed slightly to result in distinct species.  As the descendants of the urbilaterian ancestor evolved, core pathways governing development were generally conserved but slightly modified in each distinct species by this evolutionary “tinkering”.

As a result, C. elegans nematodes have neurons, skin, gut, muscles, and other tissues that are very similar in form, function, and genetics to those of humans.  The genes that control the development and function of these tissues have been “conserved” from our common ancestor.  While there are clear and important differences in human and nematode development, scientists can study developmental mechanisms in C. elegans to understand the basic, core mechanisms that also operate in humans.

This is extremely important, as many developmental and genetic experiments are impossible in humans, and are very time-consuming and expensive in mice and other vertebrates.  C. elegans are easily cultured in the laboratory, have a short (4-day) generation time, and come with a “toolkit” of genetic and molecular techniques by which gene function in development can be experimentally dissected.

(b) Flatworms, Nematodes and Annelids differ from each other on the basis of presence and absence of body cavity called coelom. The coelom is a fluid-filled cavity that lie between the digestive tube and the outer body tube which is surrounded by mesodermal tissues.

In flatworms, the body does not have any cavity and hence, they are flat. Due to the absence of coelom, these invertebrates are also called Acoelomates.

Nematodes, on the other hand, do have a space in the body between the digestive tract and the body wall. But it is not lined with tissue and hence, is not considered to be a true coelom. So, nematodes are sometimes referred to as Pseudocoelomates.

Unlike Platyhelminthes and Nematodes, Annelids have a true coelom, i.e. they possess a body cavity between the digestive tube and the external body wall which is lined with tissue.

(c) Biological control or biocontrol is a method of controlling pests such as insects, mites, weeds and plant diseases using other organisms. It relies on predation, parasitism, herbivory, or other natural mechanisms, but typically also involves an active human management role. It can be an important component of integrated pest management (IPM) programs.

There are three basic strategies for biological pest control: classical (importation), where a natural enemy of a pest is introduced in the hope of achieving control; inductive (augmentation), in which a large population of natural enemies are administered for quick pest control; and inoculative (conservation), in which measures are taken to maintain natural enemies through regular reestablishment.

Traditionally, soil-inhabiting insect pests are managed by applying pesticides to the soil or by using cultural practices, for example, tillage and crop rotation. Biological control can be another important way to manage soil-inhabiting insect pests. A group of organisms that shows promise as biological control agents for soil pests are insect-parasitic nematodes. These organisms, which belong to the families Steinernematidae and Heterorhabditidae, have been studied extensively as biological control agents for soil-dwelling stages of insect pests. These nematodes occur naturally in soil and possess a durable, motile infective stage that can actively seek out and infect a broad range of insects, but they do not infect birds or mammals. Because of these attributes, as well as their ease of mass production and exemption from EPA registration, a number of commercial enterprises produce these nematodes as biological "insecticides."

an individual’s sex (i.e., whether they are a male or female) is determined by the sex chromosomes. most people have two sex chromosomes, one that is inherited from their mother and one that is inherited from their father. typically, females have two x chromosomes (xx) and males have one x chromosome and one y chromosome (xy). conditions caused by changes (“mutations”) in genes located on the x chromosome are considered x-linked.

x-linked recessive inheritance

most x-linked conditions are recessive. this means that in a person with two x chromosomes (most females), both copies of a gene (i.e., one on each x chromosome) must have a change or mutation whereas in a person with one x chromosome (most males), only one copy of a gene must have a mutation. a female with a mutation in one copy of a gene on the x chromosome is said to be a “carrier” for an x-linked condition. a male with a mutation in a gene on the x chromosome is typically affected with the condition. because females have two copies of the x chromosome and males have only one x chromosome, x-linked recessive diseases are more common among males than females. however, x-linked recessive diseases can occur in both males and females.

for x-linked recessive disorders, an unaffected carrier mother who has a mutation in a gene on the x chromosome can transmit either the x chromosome with this mutation or a “normal” x chromosome to her children. if the father is unaffected, none of her daughters will be affected and all of her daughters will be unaffected—since they will inherit at least one normal x chromosome from their father. however, each daughter will have a 50% chance of being an unaffected carrier like her mother and a 50% chance of both x chromosomes being normal.