Environmental Regulation of Animal Development

SUMMARY:

Regulation of Normal Development

Disruption of Normal Development

Environmental Regulation in Animal Development

•Larval Settlement-Red abalone- must bind coralline red algae to begin metamorphosis

•Mosquito- blood meal triggers egg production**In Aedes aegypti , the digested product of blood meal stimulate the brain to secrete EGG DEV'T NEUROSECRETORY HORMONE (EDNH)

1. Many examples of environmental regulation exist

•Developmental symbiosis•A bacterium multiplies only in leaf hopper CYTOPLASM

•No bacterium= embryonic death due to lack of abdomen development

1. environmental regulation (cont.)

•Seasonality and sex in Aphids- hatch only reproducing females in the spring, but male and female are hatched in the autumn (mechanism unknown)

•Many insects use diapause- a suspension of development due to harsh conditions (e.g winter condition)

•Nitrogen fixing bacteria provide nitrogen to legumous plants (e.g. bean plants)

•Diapause is not triggered by harsh conditions, but before the harsh conditions arrive

•Gravity/pressure

•affects development of bones in chicken

2. Phenotypic plasticity- (cont.)

c. Environment-dependant sex determination

Crocodiles- extremes temperature result in femaleModerate temperature result in Male

Advantage- increase sexual reproduction if 10 females to each male ratio/ male:female ratio

Disadvantage- less adaptable to slight environment change (e.g. Pollution)

• Blue headed Wrassse (reef fish) -depends on other fish it encounters

•If wrasse reaches reef with males, it develops into Female

•If wrasse reaches reef without males, it develops into male

•If male dies, largest female becomes male within a day

d. PREDATOR -induced polyphenism

a. Organisms change shape in response to soluble factors released by predators

•The immune system “destroy ” its foreign invaders

3. Predator-induced polyphenism (cont.)

b. Mammalian immunity

•A foreign object (antigen) is recognized by a one B-lymphocyte (B-cells)

The basis for the immune response is summarized in the fivemajor postulates of the clonal selection hypothesis (Burnett 1959)

1. Each B lymphocyte (B cell) can make one, and only one, type of antibody. It is specific for one shape of antigen only2. Each B cell places the antibodies it makes into its cell membrane with the specificity-bearing side outward.3. Antigens are presented (usually by macrophages) to the antibodies on the B cell membranes.4. Only those B cells that bind to the antigen can complete their development into antibodysecreting plasma cells. These B cells divide repeatedly, produce an extensive rough endoplasmic reticulum, and synthesize enormous amounts of antibody molecules. These antibodies are secreted into the blood.5. The specificity of the antibody made by the plasma cell is exactly the same as that which was on the cell surface of the B cells.

4. Learning is an environmentally induced system

a. New neurons are produced – in male songbirds/juvenile females learning a song

b. Visual pathway development is affected by experience

• A flash of light seen by a cortical neuron right eye is also “seen” by the corresponding cortical neuron in the left eye

•If sew right eye of newborn closed for 3 months, they were functionally blind in right eye

•Kitten experiments

•If sew both eyes of newborn closed for 3 months, vision remains (though weak) in one eye or the other.

4. Learning is an environmentally induced system

•Critical window of development is 4 weeks to 3 months

Thus, not all development is encoded in the Genome Some is the result of learning

•Thus, synapses have been stabilized by 3 months

2. experience plays key role in determining whether neuronal connections remain

Two key principles:

1.neuronal connections are made prior to when the animal sees

Also, the adult functions phenomenon is due to learning

5. Environmental disruptions to development

•About One-half/two-thirds of human conceptions survive to term

•About 5 percent of human babies have a recognizable deformation (normally in limbs, lungs or face)

•Abnormalities caused by exogenous agents are called DISRUPTIONS

Examples

Skunk cabbage induced Sonic hedgehog in sheep

In Humans

likely to have infants that are smaller than those born to women who do not smoke

•Quinine cause deafness

•Smoking (>20/day)

•Alcohol (>2oz/day)cause physical and mental retardation in the infant

•Abnormalities caused by genetic mutations agents are called MALFORMATIONS (e.g Down’s syndrome, aniridia (Pax6 mutation))

Experimental Evidence

Mutations in normal developmental pathways lead to malformations in embryonic development

a. Retinoic acid as a teratogenAccutane-for treating severe cystic acne

About 160,000 women of childbearing age have used Accutane

1985 study of pregnant Accutane users

26“ normal”12 aborted spontaneously

21with anomalies- absent ears, small jaws, cleft palate, and/or CNS problems

5. Disruptions (cont.)

FAS children have mean IQ of 68; at 16.5 yrs, they have vocabulary of 6.5 yrs

Estimated that 30 to 40 percent of children born to alcoholic mothers will have FAS

b. Alcohol is a teratogen

Fetal alcohol syndrome (FAS) is 3rd most prevalent type of mental retardation (behind fragile X and Down Syndrome) affects 1 out ofevery 500 750 children born children in USA

CH3CH2OH

Fetal Alcohol Syndrome

Huge problem in South Africa due to alcohol-based economy

Misshapen eyes, flat nose, long upper lip

c. Pathogens as teratogens

. Rubella (German measles) can cause deafness, heart malformations

. Herpes simplex and cytomegalovirus can cause deafness, blindness, mental retardation. Treponema pallidum, the cause of Syphilis can cause deafness or death

d. Chemicals as teratogens

lead, mercury, and zinc are common ones

In Kazakhstan (former Soviet Union) nearly half of population have extensive chromosome breakage due to “industrial production at all costs”

Other teratogens

• Radiation (fallout)

• Infectious agents- Herpes simplex II, cytomegalovirus

• Drugs- alcohol, aminopterin, chlorophenyls, tetracyclines, thalidomide

• Other possible teratogens- heat fron high fever and zinc deficiency

e. Estrogen

DDT is a banned insecticide that can act like estrogenic compound

DDT is linked to increased incidence of breast cancers, and decreased alligator populations in Florida and the worldwide decline of human sperm counts

e. Estrogen (cont.)

•p-nonylphenol(used to harden plastics used in plumbing and milk/orange juice containers)- replaces role of estrogen to induce certain cultured cell to divide

•PCBs (previously used as refrigerants); banned in 1970 due to cancer-causing ability in rats; yet remain in environment

•Blamed for reduced reproductive capacities in seals, mink and fish

Dramatic increase in deformed frogs in US- Due to toxins in water?

Summary: The Environmental Regulation of Development

1. The environment can affect development in several ways. Development is sometimes cued to normal circumstances that the organism can expect to find in its environment. The larvae of many species will not begin metamorphosis until they find a suitable substrate. In other instances, symbiotic relationships between two or more species are necessary for the complete development of one or more of the species.

2. Developmental plasticity makes it possible for environmental circumstances to elicit differentphenotypes from the same genotype. Many species have a broad reaction norm, wherein the genotype can respond in a graded way to environmental conditions.3. Some species exhibit polyphenisms, in which distinctly different phenotypes are evoked by differentenvironmental cues.4. Seasonal cues such as photoperiod, temperature, or type of food can alter development in ways that make the organism more fit. Changes in temperature also are responsible for determining sex in several organisms, including many types of reptiles and insects.

5. Predator-induced polyphenisms have evolved such that the prey species can respond morphologically to the presence of a specific predator. In some instances, this induced adaptation can be transmitted to the progeny of the prey.

6. The differentiation of immunocompetent cells and the formation of synapses in the visual system are examples where experience influences the phenotype.

7. Compounds found in the environment (teratogens) can disrupt normal development. Teratogens can be naturally occurring substances or synthetic ones.

8. Alcohol and retinoic acid are two of the most intensively studied human teratogens. They may produce their teratogenic effects through more than one pathway.

9. It is possible that numerous compounds may be acting as hormone mimics or antagonists disrupt normal development by interfering with the endocrine system.

10. Genetic differences can predispose individuals to being affected by teratogens.

THE END....