sexual reproduction in vertebrates. zoology

1. Sexual reproduction in
vertebrates.
By: Muhammad Basit Ali
Roll# 17301514-108

2. Vertebrates
 Vertebrates are those animals which contain vertebral column.
 The first vertebrates evolved in the oceans about 470 million years
ago.They were jawless fishes with a single caudal fin.
 All vertebrates have a heart and a closed circulatory system.
 All vertebrates have a heart and a closed circulatory system.
 Feed on a variety of organic materials.

3. Vertebrates

4. Reproductive strategies
▪ Ovipary refers to the development of an embryo within an egg
outside the mother’s body.This occurs in
most amphibians and reptiles and in all birds.
▪ Ovovivipary refers to the development of an embryo inside an egg
within the mother’s body until it hatches.The mother provides no
nourishment to the developing embryo inside the egg.This occurs in
some species of fish and reptiles.
▪ Vivipary refers to the development and nourishment of an embryo
within the mother’s body. Birth may be followed by a period
of parental care of the offspring.This reproductive strategy occurs in
almost all mammals.

5. Sexual reproduction in vertebrates
▪ Vertebrates reproduce sexually, and almost all have separate male
and female sexes.
▪ Aquatic species generally have external fertilization,
whereas terrestrial species usually have internal fertilization.
▪ Vertebrates have one of three reproductive strategies, known as
ovipary, ovovivipary, or vivipary.

6. Continue…
Sexual reproduction involves specialized reproductive cells or gametes.
 These cells are haploid, and are usually produced by meiosis. The male cell
is called a sperm, it is smaller than the female cell (the egg) and it is motile
(it can move).
 The egg is larger than the sperm, it is non-motile, and it carries nutrients to
sustain the embryo until it can feed or absorb nutrition from the mother.
 The gametes are produced in the gonads or reproductive organs; the male
organs are the testes and the female organs are called the ovaries. Either
sex may have accessory sex organs such as the penis or the vagina; the
accessory sex organs are usually involved in the mating process itself.

7. Continue…
Sexual reproduction involves fertilization, the union of gametes to form a
zygote.
 The zygote is a single cell, and it is genetically different from either of the
parents due to recombination during meiosis, and due to the union of
genetic material from two different organisms.
There are several patterns of sexual reproduction in vertebrates.
Most familiar is biparental reproduction (sometimes called dioecious
reproduction). Here there are two parents, one male and the other female.
 Dioecious reproduction leaves the greatest genetic variability among
offspring (an advantage under changing conditions or in competition or
when faced with disease), but the organisms must find mates.

8. Parthenogenesis
▪ A second pattern is parthenogenesis. Here a viable offspring is produced by the parent (a female) without mating or
fertilization. Some people call this asexual reproduction, but it does not meet the strict definition used above since the sex
organs are used to produce the egg. There are 2 forms of parthenogenesis:
Ameiotic
▪ no meiosis
▪ egg formed by mitosis
▪ offspring clones of the parent
Meiotic
▪ egg produced via meiosis
▪ sperm do not fuse genetic material
▪ offspring are haploid
▪ crossing-over usually results in some genetic diversity
▪ no need to mate

9. Hermaphroditism
▪ A third pattern of sexual reproduction is hermaphroditism.
▪ This is the monecious pattern. The hermaphrodite posses both male
and female organs, and, in some cases may fertilize itself.
▪ Other hermaphrodites mate. Some exhibit what is known as
sequential hermaphroditism; here the organism starts out either
male or female and later changes its sex.

10. Advantages and disadvantages
▪ Benefits of sexual reproduction revolve around increased genetic diversity.
▪ This increased diversity is good when conditions change and a population
needs new combinations of genes to do well under the new conditions;
▪ when predators or competitors are evolving rapidly and the new gene
combinations may help the population "keep up", and when the population
is threatened with disease.
▪ In the latter case, at least some members of a genetically diverse
population will be able to resist the disease.
▪ Overall, asexual reproduction seems to be favored when rapid population
growth is needed, and sexual reproduction is favored under changing
conditions. There are, in fact, populations of animals (mostly invertebrates)
which modify their reproductive pattern is response to environmental
conditions.

11. Disadvantages…
▪ The need to find a mate (hard to do for sessile organisms or organisms with low
population densities;
▪ 2.The time and energy required to find a mate and to copulate (this is time that
could be spent eating or acquiring resources);
▪ 3.The exposure to predation (sometimes from the mate!) that comes with mating;
▪ 4. Exposure to disease from the mate.
▪ 5. when an animal mates, only 1/2 of its genes go into each of the offspring, so the
genes are "diluted";
▪ 6. Because some proportion of the population is male, not all adult organisms are
directly producing offspring, thus lowering the rate of population growth; and
▪ 7. Adaptive combinations of genes are lost.