11.2 Sexual Reproduction

It can be hard to remember that some of the characteristics of organisms are so fundamental.

Biologists have trouble testing hypotheses about how Meiosis may have evolved because it is so complex.
It is important to separate the questions of the evolution of meiosis and the evolution of sex because early meiosis may have been beneficial for different reasons than it is now.
One way to find out how it may have evolved is to think outside the box.

It makes sense that the two diseases share the same cellular processes.

There are clear differences between meiosis I and mitosis.
There were unique events that needed to occur for the evolution of meiosis.
There are steps that suppression of DNA replication in interphase is one of.

They argue that understanding how it evolved would make the evolutionary process clearer.

There are genetic experiments that might shed light on the evolution of synapsis.

There are other ways to understand the evolution of meiosis.

There are some forms of meiosis that are simpler.
The evolution of meiosis may be revealed by comparing the meiotic divisions of different protists.
Marilee and colleagues compared the genes involved in meiosis in protists to understand when and where it might have evolved.
Recent scholarship into meiosis in protists suggests that some aspects may have evolved later than others.
This kind of genetic comparison can show us what parts of meiosis are the oldest and what cellular processes they may have borrowed from earlier cells.

By the end of this section, you will be able to explain meiosis and sexual reproduction, identify variation among offspring as a potential evolutionary advantage of sexual reproduction, and describe the three different life-cycle types among sexually reproducing multicellular organisms.

After the appearance of eukaryotic cells, sexual reproduction is likely to have been an early evolutionary innovation.

It appears to have been very successful because most eukaryotes are able to reproduce sexually and in many animals, it is the only mode of reproduction.
Scientists recognize some disadvantages to sexual reproduction.
Creating offspring that are genetic clones of the parent appears to be a better system on the surface.
The offspring with the same traits should be successful if the parent is successful.
There is an obvious benefit to an organisms ability to produce offspring when there is asexual budding, or when eggs are produced asexually.

Some organisms that lead a solitary lifestyle have the ability to reproduce asexually.

Every individual is capable of reproduction in asexual populations.

Adam S. Wilkins and Robin Holliday wrote about the evolution of meiosis.

Multicellular organisms that depend on asexual reproduction are very rare.

These are important questions in biology, even though they have been the focus of a lot of research in the last half of the 20th century.
One of the possible explanations is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of the population.
A sexually reproducing population will leave more descendants than an asexually reproducing population.
There is only one source of variation in asexual organisms.
Germ cell lines are the ultimate source of variation in sexually reproducing organisms.
During sexual reproduction, different parents combine their unique genomes and the genes are mixed into different combinations by random assortment, but in contrast to this, the same parents combine their unique genomes and the genes can be shuffled from one generation to the next.

The Red Queen hypothesis was first proposed by Leigh Van Valen in 1973.

Parasites evolve with their hosts.

Each tiny advantage gained by favorable variation gives a species a reproductive edge.
The reproductive fitness of other genotypes or phenotypes within a given species is what determines the survival of any given genotype or phenotype in a population.
As one species gains an advantage, this increases selection on the other species, they must also develop an advantage or they will be outcompeted.
All species have a mechanism to improve rapidly because of genetic variation among the progeny of sexual reproduction.
The species that can't keep up are extinct.
Coevolution between competing species is described in this way.

The process reduces the number of chromosomes.

Fertilization restores the diploid condition.
Some organisms have a multicellular diploid stage that only produces haploid reproductive cells.
Humans and animals have this type of life cycle.
The multicellular haploid stage is most obvious in organisms such as fungi.
Plants and some algae have different life stages that can be seen to different degrees depending on the group.

Most animals use a diploid-dominant life-cycle strategy in which the only haploid cells produced are the gametes.

Germ cells are capable of producing haploid gametes.
The ability to divide is lost once the haploid gametes are formed.
There isn't a multicellular haploid life stage.

Haploid gametes are formed from diploid germ cells in animals.

The gametes give rise to a fertilized egg cell.
Multiple rounds of mitosis will be required to produce a multicellular offspring.
Early in the development of the zygote, germ cells are generated.

The "body" of the organisms is haploid, which is the important part of the life cycle.

The tissues of the dominant multicellular stage are formed by haploid cells.
During sexual reproduction, specialized haploid cells from two individuals join to form a diploid zygote.
Four haploid cells are formed by the meiosis of the zygote.
There is a new genetic combination from two parents in these spores.
For a variety of time periods, the spores can remain inactive.
The multicellular haploid structures form when conditions are favorable.

Black bread mold (Rhizopus nigricans) has a haploid multicellular stage that produces specialized haploid cells.

haploid spores are produced by the meiosis of the zygote.
A multicellular haploid organisms is formed by each spore.
There are different types of hyphae that join to form a zygospore.

The third life-cycle type is a blend of the haploid-dominant and diploid-dominant extremes.

Both haploid and diploid multicellular organisms are part of the life cycle of some species.
The production of gametes is not affected by Meiosis because the organisms that produce the gametes are already haploid.
A diploid zygote is formed by fertilization between the gametes.
Haploid spores will be produced by specialized cells of the sporophyte.
The gametophytes will develop into the spores.

Plants have a life cycle that alternates between multicellular haploid organisms and multicellular diploid organisms.

The haploid and diploid plant stages are free-living in some plants.
The diploid plant produces haploid spores.
Gametophytes are multicellular, haploid plants that produce gametes.
The gametes of two people will form a diploid zygote.

The relationship between the sporophyte and the gametophyte varies greatly, even though all plants use some version of the alternation of generations.

The gametophyte is the freeliving plant and the sporophyte is dependent on the gametophyte.
The gametophyte and sporophyte plants are free- living in other plants.
In magnolia trees and daisies, the gametophyte is composed of a few cells and the female gametophyte is completely retained within the sporophyte.

Sexual reproduction can take many forms.

The benefits of producing offspring with unique genes are shown to be strong by the fact that nearly every multicellular organisms on Earth uses sexual reproduction.

Sexual reproduction requires that organisms only produce one set of chromosomes, instead of the two sets that can be fused during fertilization to produce offspring.

The main difference between most animals and humans is that meiosis is used to produce haploid eggs and between the two nuclear division processes take place during sperm from diploid parent cells so that the fusion of an egg the first division of meiosis.
During the S-phase of the segments, there is an exchange of non sister chromatids.
The reduction of chromatids is caused by the separation of the homologous chromosomes so that each one becomes a pair of sister different nuclei.
There are two rounds of nuclear ploidy in the first division.
The cells with half the number of chromosomes as the daughter cells don't have the same genes are not identical to the parent cells.

Meiosis causes variation in the daughter's sexual reproduction nuclei during Sexual crossover in prophase I as well as during the Nearly all eukaryotes undergo sexual reproduction.

The cells that variation is introduced into the reproductive cells are unique.

Single nuclear fertilization is alternate in sexual life cycles.

Each daughter nucleus has the same number of chromosomes as the original cell, and each gamete has half the number of chromosomes as the parent cell.
The timing between meiosis and reproducing organisms varies greatly.

Meiosis usually produces daughter cells.

The pea plants used in the studies were diploid with 14 chromosomes.

There are homologous chromosomes at metaphase I.

At the end of telophase I, cells are diploid, but at the end of telophase II, they are haploid.

The daughter cells form a cell plate.

There is no phase after telophase II.

The part of meiosis that is similar is called _____.

Sexual reproduction requires fewer steps.

A given environment has a lower chance of using up resources.

Sexual reproduction is more cost-effective than living in a freshwater lake.

Which type of life cycle has a haploid and diploid plant built upstream?

Consumers don't trust produce that looks the same.

List and describe the three processes.

Both plants and animals have diploid and haploid cells.

There is a comparison of the stages of meiosis to the stages of offspring.