ASEXUAL REPRODUCTION

Alternation of Generations

reproductive cycle of certain vascular plants, fungi, and protists.

The Haploid-diploid Life Cycle

The haploid-diploid life cycle is the most complex life cycle and thus has lots of variation.

It is also the most common life cycle among plants.

The Haploid-diploid

Life Cycle

A haploid plant of the gametophyte generation produces gametes by mitosis.

Two gametes (originating from different organisms of the same species or from the same organism) combine to produce a zygote, which develops into a diploid plant of the sporophyte generation.

This sporophyte produces spores by meiosis, which germinate and develop into a gametophyte of the next generation.

Moss If you’ve ever looked closely at a

moss, you may have noticed a tiny leafy green mat from which a stalk protrudes at certain times of the year.

The stalk is the sporophyte. From its cap, spores are cast that land on the ground and develop into the gametophyte—the leafy green mat.

Special structures within the mat produce sperm and egg. The sperm swim to the eggs and fertilize them. A stalk, which remains attached to the mat, results from each fertilized egg.

The moss life cycle thus requires ground water in order to be completed—this is why mosses are always found in moist environments.

Ferns In ferns, the different generations exist

as distinct individuals. The graceful fronds, or leaves, that we see adorn the sporophytes.

If you look under the fronds of a mature plant, you’ll see structures where the spores are produced. The spores are cast from these structures onto the ground, where they develop into gametophytes.

The gametophytes are tiny heart-shaped structures that are nearly invisible to the naked eye. They require a moist environment to develop and, once mature, produce sperm and egg. Like the mosses, the sperm require water to swim to the eggs, with each fertilized egg developing into the familiar, frond-bearing sporophyte.

Conifers In the conifers, the

stately needle- and cone-bearing trees are the sporophytes.

Conifers actually have two different types of cones. The female cone is probably what you are familiar with, bearing hard, woody scales.

In a structure on top of each scale of the female cone, female spores are produced, which develop into the microscopic female gametophyte

Conifers The male cones are much

smaller than the female cones and are the structures that produce copious amounts of yellow “dust” in the Spring.

On the underside of each tiny scale are structures that produce numerous male spores, which develop into gametophytes that consist of just four cells. The gametophyte and its covering are the pollen, which is carried by wind to the female cone.

Conifers• Pollination occurs

when pollen lands at the sticky base of the scale and the sperm grows to and fertilizes an egg, which eventually forms a papery seed on top of the scale.

• In the conifers, the stately needle- and cone-bearing trees are the sporophytes.

Binary Fission

Binary fission is used by most prokaryotes for asexual

reproduction. This process replicates the original, or mother, cell, to produce two identical daughter cells.

Binary Fission Binary fission begins with

DNA replication Each circular DNA strand then

attaches to the cell membrane. The cell elongates, causing the two chromosomes to separate.

The cell membrane then invaginates (grows inwards) and splits the cell into 2 daughter cells, separated by a newly grown cell plate. This process is called cytokinesis.

Binary Fission Binary fission offspring are

supposed to be genetically identical to the parent cell, but due to mutation, daughter cells may have slightly differed genetic makeups.

Bacterial DNA has a relatively high mutation rate. This rapid rate of genetic change is what makes bacteria capable of developing resistance to antibiotics and helps them exploit invasion into a wide range of environments.

Organisms that reproduce through binary fission generally grow exponentially. E coli cells are able to divide every 20 minutes under optimum conditions.

           

Binary Fission Many organisms reproduce by

binary fissions, such as: Bacteria (for example,

Rickettsia species, that cause diseases such as Rocky Mountain spotted fever)

Most protists (for example, Amoeba proteus)

Entamoeba histolytica (a protozoan that is a human intestinal parasite)

Pyrodictium abyssi (an anaerobic hyperthermophilic archaea of deep-sea hydrothermal vents)

Schizosaccharomyces pombe (a fungal organism that is a species of yeast)

           

Binary FissionA second protozoan (lower centre) approaches the protozoan (centre) undergoing binary fission

           

Aiding the binary fission process in a rather unorthodox way!

The second protozoan forces it's way between the two daughter cells

Parthenogenesis

Parthenogenesis (from the Greek παρθενος parthenos, "virgin", + γενεσις genesis, "birth") describes the growth and development of an embryo or seed without fertilization by a male. Parthenogenesis occurs naturally in some species, including lower plants (called agamospermy), invertebrates (e.g. water fleas, aphids, some bees and parasitic wasps), and vertebrates (e.g. some reptiles, fish, and, very rarely, birds).

ParthenogenesisLONDON, England (Reuters) -- Flora, a

pregnant Komodo dragon living in a British zoo, is expecting eight babies in what scientists said on Wednesday could be a Christmas virgin birth.

Flora has never mated, or even mixed, with a male dragon, and fertilized all the eggs herself, a process culminating in parthenogenesis, or virgin birth. Other lizards do this, but scientists only recently found that Komodo dragons do too.

Parthenogenesis has occurred in other lizard species, but Buley and his team said this was the first time it has been shown in Komodo dragons -- the world's largest lizards.

ParthenogenesisParthenogenesis in wild Komodo dragons could be adaptive, given that viable offspring are always male and that sexual reproduction can resume, albeit between related individuals, in a colony founded by a single unfertilized female. Fewer than 4,000 Komodo dragons remain in the wild, of which perhaps fewer than 1,000 are mature females

Parthenogenesis

A parthenogenetically activated human egg 15 hours after activation.

Parthenogenesis

While human eggs sometimes activate on their own, they never go on to complete embryonic development. Here, a four-cell human egg.