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3 Form: the origin of complex cells

3 Form: the origin of complex cells

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20



PROCESS, FORM AND PATTERN



Table 1.1 Fundamental grades of organisation

Prokaryotes (Monera)

(includes both Archaebacteria and Eubacteria)



Eukaryotes (Eukarya)

(includes the four kingdoms: protists, plants,

fungi and animals)



r No membrane-bound nucleus

r DNA in circular chromosomes and without histones



r Membrane-bound nucleus

r DNA complexed with histones in



r Cell fission

r No cytoplasmic membrane-bound organelles (but



chromosomes

r Mitosis and cytokinesis

r Cytoplasmic membrane-bound organelles



mesosomes and membrane systems may be present)



Figure 1.17. Drawing of a

transmission electron micrograph

of the cell of the green alga

Chlamydomonas showing organelles

and membranes systems: (Ch)

chloroplast, (CV) contractile

vacuole, (Er) endoplasmic

reticulum, (G) Golgi body, (F)

flagellum, (M) mitochondrion, (V)

vesicle, (N) nucleus, (Py) pyrenoid,

(Cm) cell membrane, (Cw) cell

wall (from Lee, 1999).



(mitochondria, chloroplasts, Golgi

apparatus, endoplasmic reticulum)



is the extensive thylakoid membrane system of cyanobacteria and

chloroplasts: thylakoids are stacks of flat membranes in which the

photosynthetic pigments are located. In addition, most plant cells

have one or more vacuoles; liquid-filled sacs surrounded by a membrane called the tonoplast. An example of the complex internal structure of a eukaryote is shown in Figure 1.17.

The cytoplasmic membrane is quite fluid but is stabilised in the

Eukarya and methanotrophic Eubacteria by the presence of rigid flat

sterol molecules that are absent from most prokaryotes, although

some of these have similar molecules called hopanoids. The Archaebacteria have slightly different membranes from other organisms,

perhaps because they have a tendency to occupy high-temperature

environments that would disrupt a fluid cytoplasmic membrane:

the interior fatty acids are linked to the glycerol part of the membrane by ether links and, in addition, some Archaebacteria have a

membrane in which the interior hydrophobic part is stabilised as a

monolayer.

The cytoplasmic membrane is also stabilised, in many organisms

by the presence of a cell wall exterior to the membrane. The simplest

cell walls are found in Gram-positive Eubacteria and Archaebacteria.

They are called Gram-positive because during a particular staining

regime, devised by the microbiologist Christian Gram, they retain

a stain called crystal violet even when washed with ethanol. Their

cell wall is thick and composed of 90% peptidoglycan. Gram-negative

prokaryotes have a more complex multi-layered cell wall in which

peptidoglycan makes up only 10%. Different kinds of cell walls are

found in protists, fungi and plants. Animal cells and some protists

are normally naked. Fungi have a cell wall in which chitin is a major

component. The cell walls of protists are very diverse in chemical

composition and structure and many planktonic protists such as the

diatoms, dinoflagellates and desmids have remarkably sculptured cell

walls.

Plants and some kinds of protists have a type of cell wall where

cellulose is a major structural component. The evolutionary origin of



1.3 FORM: THE ORIGIN OF COMPLEX CELLS



cellulose cell walls is obscure. The basic structure of cellulose seems

relatively simple, essentially it is a polysaccharide with glucose as its

basic unit, but it is a very large polymer with many possible variations

in the degree of and kinds of bonding in its various parts. Cellulose

is also produced by the acetic acid bacteria (Acetobacter), forming an

outer coat or pellicle of cellulose that helps them to float at the surface where conditions are aerobic. Bacterial cellulose microfibrils are

isolated from each other and do not form the strong material seen

in plants. In plants and algae the microfibrils are closely associated

with each other. Cellulose is synthesised as scales in the Golgi apparatus by some algae but more usually cytoplasmic membrane-bound

cellulose synthase enzymes synthesise it. There are differences in the

form of cellulose microfibrils produced among different algal groups

and plants.

In Rhodophyta (red algae) the cell wall has two layers: the inner

layer has cellulose or another polysaccharide and the outer layer is

mucilaginous with a sulphated polymer of galactose. This gives the

red algae their characteristic slipperiness. Red algal cell walls are harvested to provide agar or carrageen (carragheen). Agar is used not only

as a culture medium but also in cosmetics and to produce capsules for

drugs. Agarose is a purified form used in electrophoresis. Carrageen

from Euchema is used as a stabiliser in dairy products, paints and

cosmetics. In nature, continually sloughing this mucilaginous layer

prevents other organism colonising the surface of red algae. Some

coralline red algae (Corallinaceae) also deposit calcium carbonate in

the cell wall and they may have a jointed or crustose form. Red algae

are important components of coral reefs, and are also common on

rocky shores.



1.3.2 The domains of life

The profound differences in membranes and cell wall types of the

Eubacteria, Archaebacteria and Eukarya have encouraged some writers to speculate that cellular life has originated three times, once for

each domain of life (Table 1.2).

Eukaryotes differ in two key respects from prokaryotes: the presence of membrane-bound organelles in the cytoplasm, such as mitochondria, and in photosynthetic organisms, plastids (chloroplasts and

others); and the presence of a nucleus, itself a membrane-bound structure (a double membrane) containing the genetic material organised

into chromosomes. The genetic material undergoes mitotic division

controlled by the action of the cytoskeleton in eukaryotes.

One important advantage that eukaryotes have is that their cells

are larger than prokaryotes. Their greater size is accompanied by

greater internal structural complexity that compartmentalises different cell functions. The largest prokaryotic unicellular organisms

are symbionts of surgeonfish called Epulipiscum fithelsoni that can be

more than 0.5 mm long, but this is a very exceptional prokaryote.

Most prokaryotes are in the range 1.0--4 μm long with a diameter

of 0.25--1.5 μm. The Cyanobacteriaceae, on average, exceed this range



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PROCESS, FORM AND PATTERN



Table 1.2 The domains of life

Archaebacteria



Eubacteria



Eukarya



Prokaryote organisation

DNA-binding proteins HMf and HMt

with homology to HU-1 and HU-2

1 RNA polymerase transcription

factors not required



Prokaryote organisation

DNA-binding proteins HU-1

and HU-2

Several RNA polymerases

transcription factors not

required

Not normally inhabitants of

extreme environments



Eukaryote organisation

Histones



Not methanogens

Muramic acid in cell wall



Not methanogens

No muramic acid in cell wall



Membrane lipids ester-linked,

unbranched

Ribosomes 70S

Initiator tRNA

formylmethionine

Introns mostly absent

Operons

No capping and poly-A tailing

of mRNA



Lipids ester-linked,

unbranched

Ribosomes 80S

Initiator tRNA methionine



Commonly inhabitants of extreme

environments: high salt, low pH, or

high temperature

Includes methanogens

No muramic acid in cell wall

membrane

Membrane lipids ether-linked, some

branched

Ribosomes 70S

Initiator tRNA methionine

Introns sometimes present

Operons

No capping and poly-A tailing of

mRNA



3 RNA polymerases

transcription factors

required

Not normally inhabitants of

extreme environments



Introns commonly present

Operons absent

Capping and poly-A tailing of

mRNA



with a mean length of about 50 μm. In contrast most eukaryotic unicellular organisms have cell diameters 2--200 μm but some are much

larger than this.

Geochemical evidence such as the presence of steranes, especially

cholestane and its analogues, indicate the existence of ‘eukaryotes’ at

least 500 million to 1 billion years before fossil eukaryotes are found.

The earliest fossil evidence of probable eukaryotes is provided by the

dark curl or spiral of Grypania, up to 0.5 m in length and 2 mm in

diameter, in rock cores first observed in rocks dated at about 2100 Ma

(Negaunee Iron Formation, Michigan, USA). However, the diversity

of eukaryotes up to 1000 Ma in the early Phanerozoic was very

limited.



1.3.3 The nucleus, the cytoskeleton and cell division

Cell division provides another trace of the presence of living organisms in rocks of a great age. The simplest kind, carried out by prokaryotic organisms is binary fission. The cell enlarges and then splits into

two. There are many examples of fossils of Archaean age showing

these stages.

A defining feature of eukaryotes is the presence of a nucleus and

cytoskeleton, and with it a particular kind of organisation of the

genetic material. The nucleus has a double membrane surrounding

a matrix containing the chromosomes. The chromosomes are highly



1.3 FORM: THE ORIGIN OF COMPLEX CELLS



structured packages of the genetic material, DNA, complexed with

proteins called histones, to form a material called chromatin. The

DNA is wrapped around the histones forming bead-like structures

called nucleosomes. Nucleosomes are linked like a string of beads

by the chain of DNA running between them. This string is coiled

and supercoiled into tightly condensed chromatin to form a chromosome. Each nucleus has several to many chromosomes, depending

upon species and each chromosome carries different genes.

A peculiar chromosomal organisation is present in the dinoflagellates, planktonic algae with armour-like coats. They have 12--400

‘chromosomes’ attached to the nuclear membrane that unwind only

slightly between cell divisions and they are the only group of eukaryotes that lack histones. Current thinking is that these peculiarities

are highly derived features.

The nucleus divides with the aid of the cytoskeleton. The cytoskeleton is a network of protein filaments, called microtubules and actin

filaments, extending through the cell. The cytoskeleton is involved

in many aspects of cell movement and growth, for example directing vesicles towards the growing cell wall and aligning the growing cellulose microfibrils. Microtubules about 24 nm wide are built

up from the protein tubulin in a helical structure at special places

in the cytoplasm called microtubule organising centres. Sometimes

microtubules are associated with contractile actin filaments 5--7 nm

wide.

Perhaps the most important role of the cytoskeleton is in cell division. Microtubules arising from an area called the centrosome form

the spindle or phragmoplast that controls the movement of chromosomes to daughter nuclei. Microtubules attach to chromosomes that

have already replicated into two chromatids and are held together at

their centromeres. By the action of the cytoskeleton the chromatids

are separated, one to each pole of the spindle. In this way a regular and highly organised division of the genetic material occurs.

Following nuclear division the cytoplasm divides by a process called

cytokinesis, also controlled by the cytoskeleton. Either the cytoplasm

furrows until the two cells are separated or a cell plate is formed

across the cytoplasm.



1.3.4 Organelles

Organelles are intra-cellular structures that are either like mitochondria and chloroplasts, which are membrane bound, or centrioles, which are not. Mitochondria and chloroplasts have their own

genome, DNA in circular chromosomes like those of bacteria. Almost

all eukaryotes have mitochondria while plant cells also have plastids

including chloroplasts. A few eukaryotic organisms, the Archezoa,

lack mitochondria. This may be because they are truly primitive or

that they have lost mitochondria because of their peculiar lifestyle as

extra- or intra-cellular parasites. They also lack Golgi bodies or have

peculiar kinds.



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PROCESS, FORM AND PATTERN



Endosymbiont = organism living

symbiotically inside a host cell.



Heterotroph = an organism

requiring organic molecules to

provide energy.



Endosymbiosis

The modern consensus among biologists is that the cells of eukaryotes

have a fundamentally chimeric origin, from the fusion of two or more

distinct organisms, and their organelles arose as endosymbionts.

Symbiosis between closely related bacteria enables bacteria to

adapt rapidly to local conditions. The first stage of cooperation may

have been the production of highly stratified bacterial mats where

the physical conditions of light quality and oxygen concentration control the ecological transition from one dominating bacterial species

to another, but with each relying on the transformation of conditions created by the species above. A more significant cooperation

is seen in the so-called consortium species that consist of a symbiotic relationship between anaerobic heterotrophs and photosynthetic

green sulphur bacteria: they cluster together in aggregates in anaerobic sulphide-rich mud. It is a short step from this to the formation

of a chimera by endocytosis or horizontal gene transfer. One important example within the bacteria is the presence of two photosystems in the cyanobacteria; this is thought to indicate that they have

evolved from a genetically chimeric prokaryote, something related to

the Heliobacteriaceae fused with something related to the filamentous green non-sulphur bacteria.

Eukaryotic cells may therefore be perceived as a special case of

the general phenomenon of microbial associations, their plastids and

other organelles such as mitochondria having arisen by a series of

endosymbioses involving different lineages of prokaryotes. Originally,

symbiosis may have resulted from endocytotic ingestion by the host

cell. Endocytosis is the folding of the cell membrane around materials from the environment to make a small pocket lined by the plasma

membrane, which is eventually sealed off to make a vesicle. Phagocytosis is endocytosis of a large solid particle. Mitochondria and plastids

have a double membrane; one derived from the host cell and one from

the ingested endosymbiont.

The emergence of partner species and their coevolution must have

begun by at least 3500 million years ago. There is some evidence that a

member of the prokaryotic Archaebacteria, an eocyte (a highly thermophilic and sulphur-metabolising archaebacterium), was the host

cell in the endosymbiosis of two eubacterial species, which became

the mitochondrion and plastid respectively. Archaebacteria are closer

to eukaryotes in some respects than the Eubacteria. However, it is

clear that even after the first eukaryotic lineage had arisen there

was substantial horizontal gene transfer between different lineages

so that the relationships and origins of the different components has

become somewhat obscured.

Both mitochondria and chloroplasts contain circular DNA

genomes and are capable of independent protein synthesis. In

dinoflagellates the chloroplast genome is peculiar because each gene

is on its own mini-circle chromosome. It is apparent that after

endosymbiosis many of the previous functions of the prokaryote



1.3 FORM: THE ORIGIN OF COMPLEX CELLS



genome were subsequently lost or transferred to the nucleus of the

host cell. For example, rubisco, the enzyme involved in carbon fixation, is a simple multimeric enzyme composed of small and large

subunits: in plants the small-subunit of rubisco, has transferred from

the chloroplast to the nucleus. Also there is some evidence that

nuclear genes coding for mitochondrial proteins in higher plants are

more similar in sequence to prokaryotic than to eukaryotic genes.

The transfer of functions to the nucleus could be viewed as a move

towards more efficiency. The dependency of the organelle upon the

expression of nuclear genes, because of the loss or transfer of the

majority of organellar genes to the nucleus, distinguishes organelles

from obligate endosymbionts.

Nevertheless some genes have been retained within the organelle.

These code mainly for proteins that maintain redox balance, which

must be synthesised where they are needed to counteract the deadly

side effects of ATP generating electron transport. Evolutionary divergence in mitochondria occurred very early in the evolution of eukaryotes. For example, plants and animals have flattened cristae compared

to the tubular or discoid cristae found in many kinds of protists.

Mitochondria are thought to have arisen from formerly free-living

purple non-sulphur eubacteria. These are the only Eubacteria apart

from the cyanobacteria, which are both photosynthetic and not

strictly anaerobic, although in the purple non-sulphur Eubacteria

photosynthesis is inhibited by oxygen at relatively low concentrations.

However, it was not their photosynthetic ability, which was important

in the evolution of mitochondria, but their ability to utilise organic

compounds in aerobic respiration.

The origin of chloroplasts

The origin of chloroplasts from something like free-living cyanobacteria is supported by evidence of the similar DNA sequences they

contain. The Prochlorophyceae, which are a derived group from the

cyanobacteria, seem to be strong contenders as ancestors because,

like chloroplasts, they have chlorophyll a and b and carotenoids but

do not have phycobilisomes. They also have stacked thylakoid membranes where the photosynthetic pigments are located. However, they

are not direct ancestors of chloroplasts though they share common

ancestry with them. A more direct cyanobacterial origin of chloroplasts in the red algae (Rhodophyta) is supported because they too

have phycobilisomes like the cyanobacteria.

In eukaryotes endosymbiosis of a cyanobacteria-like organism

is seen most clearly in a small group of fresh-water algae, the

Glaucocystophyta, which contain a photosynthetic organelle called

a cyanelle. Two features of the cyanelle show a direct link to

cyanobacteria. There is a persistent peptidoglycan cell wall between

its two plasma membranes and it has genes for both subunits of

rubisco. Cyanelles were, therefore, thought to be the result of recent

endosymbiosis and two have even been given names as species of



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26



PROCESS, FORM AND PATTERN



(a)



(b)



(c)



Figure 1.18. Photosynthetic

apparatus of (a) a Cyanobacterium;

(b) a red alga; (c) a chloroplast

(from Lee, 1999).



cyanobacteria. Other groups of photosynthetic protists have acquired

their plastids by secondary (or tertiary) endosymbiosis, with an

endosymbiont eukaryote already equipped with a chloroplast. For

example, in the photosynthetic euglenoids, three membranes surround the photosynthetic organelles.

Centrioles and flagella

There is a third organelle called the centriole, which may also have

an origin as a highly modified endosymbiont. Centrioles are practically identical to the basal bodies of the characteristic flagella of

eukaryotes, which are sometimes called undulipodia to distinguish

them from the flagella of prokaryotes. Each flagellum has a characteristic structure of an axoneme, a ring of nine pairs of microtubules

running as a core inside the flagellum membrane, and extending as

nine triplet microtubules in the basal body where it is attached to

the main part of the cell. Centrioles have an identical triplet microtubule structure as the flagella basal bodies. Centrioles, if present,

are found in pairs perpendicular to each other in a region called the

centrosome. There are two centrosomes near the nucleus. They function as microtubule organising centres and are associated with the

production of the spindle in cell division. Centrioles are not present

in conifers, flowering plants, and some other organisms that never

produce motile cells.

There is considerable variation in the form and arrangement of

flagella in algae. Flagella may be smooth (whiplash flagellum) or

hairy (tinsel flagellum). One large and important group of algae the

Ochrophyta (or Heterokontophyta), which include the brown algae

and diatoms, have one of each kind. The hairs (mastigonemes) may

be either non-tubular or tubular. The latter consist of a hollow shaft

with terminal filaments. The tinsel flagella in the Heterokontophyta

are tubular, a feature they share with ‘fungal’ oomycetes and several

other non-algal groups that are placed with them in a group called

the stramenopiles (= straw hair). The number, orientation and distribution of flagella differs among groups of algae.

The endosymbiont origin of flagella and centrioles is hypothesised

to be from a spirochaete bacterium; a motile eubacterium with a

flagellum that vibrates between the protoplasm and an outer flexible

sheath, even though nothing like an undulipodium or centriole is

seen in any prokaryotes, not even in spirochaetes. Because centrioles

lack their own DNA there are few data on whether their evolutionary origin is through symbiosis. However, an endosymbiotic origin

of undulipodia and centrioles is attractive for one very important

reason. It may explain how mitotic cell division in eukaryotes arose.

Symbiotic associations between motile and non-motile organisms are

well known. To become well established, cell division between the

symbiotic partners would have to be coordinated to ensure both

partners divided at the same time. Perhaps the spindle first arose

in the primeval eukaryote as a derived flagellum basal body as a

means of coordinating cell division between a spirochaete and its



1.3 FORM: THE ORIGIN OF COMPLEX CELLS



archaebacterial partner. An alternative hypothesis is that the centrioles arose endogenously first to control mitosis and only later permitted production of undulipodia.

The basal bodies of undulipodia extend as various microtubule

root systems. Different root systems delineate different algal groups.

In some green alga, and in plants, the basal bodies are distinctive

multi-layered structures that may function as microtubule organising

centres.



1.3.5 Reproduction

Prokaryotic organisms are uniparental and partly because of this they

potentially have very rapid rates of reproduction. Many are motile

but most rely on passive means of dispersal. A few produce resistant

dormant cells, endospores, with improved survivability while being

dispersed. The cyanobacteria produce akinetes, enlarged, thick-walled

resting cells that survive harsh conditions. Although prokaryotes are

mainly asexual some kinds do have the ability to exchange genetic

information from one parental lineage to another. Two cells come

together and part of the DNA of one cell is transferred to the other,

where it can become incorporated into its own genome. This process

is called conjugation. Alternatively, in a process called transduction,

DNA is carried from one cell to another by a bacterial virus, a bacteriophage, which first infects one cell and then the other. In laboratory experiments it has even been shown that DNA that has been

released to the medium by the lysis of one cell, can be taken up, in

a process called transformation, and incorporated by a recipient cell.

Although all these processes have been demonstrated in laboratory

cultures it is uncertain how important they are in nature. Conjugation does not seem to occur in the cyanobacteria but transduction

might, since cyanobacterial viruses do exist. Prokaryotes potentially

have a very fast rate of reproduction and new mutations are multiplied and rapidly propagated through the population.

Eukaryotes have a much greater size and complexity in their cells

generally and in particular in their genetic material. They reproduce

much more slowly and although, like prokaryotes, they rely on mutation as the primary source of variation, sexual reproduction is their

main source of new genetically distinct individuals. The evolution of

the cytoskeletal spindle and cytokinesis enabled regular highly organised mitotic cell divisions. Following division of the nuclear material

the cell divides. Different groups of algae and plants differ in the

form of spindle formation and cytokinesis (Figure 1.20).

Eukaryotes undergo asexual fission to multiply the individual,

rather like the prokaryotes, but following a mitotic division. In

most, including algae and plants, there is also sexual reproduction,

which involves another kind of cell division called meiosis. Like

mitosis, meiosis utilises the cytoskeletal spindle and cytokinesis, but

whereas mitosis produces daughter cells with an identical duplicate

genome, meiosis halves the number of chromosomes, producing haploid daughter cells (Figure 1.21).



(a)



(b)



(c)



(d)



(e)



Figure 1.19. Flagellum

structure: (a) 9 doublet + 2

singlet microtubule axoneme

structure; (b) 9 triplet microtubule

basal body and centriole structure;

(c) whiplash flagellum; (d)

non-tubular tinsel flagellum; (e)

tubular tinsel flagellum.



27



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PROCESS, FORM AND PATTERN



Cladoph r



a



Figure 1.20. Spindle formation.

There is considerable variation in

spindle formation. In the ‘green

algae’ group the Chlorobionta

coenocytic organisms like

Cladophora have nuclear division

without cell division (cytokinesis).

In other Chlorobionta the spindle

soon collapses and a new system

of microtubules called the

phycoplast is formed perpendicular

to the spindle. In the Streptobionta

(Charophyceae and Land Plants)

the spindle (phragmoplast) is

persistent and survives until

cytokinesis and a cell plate is

formed.



Allele = version of a gene.



Sex

Eukaryotes alternate between haploid and diploid phases of the life

cycle linked by the sexual cell division called meiosis. In meiosis haploid daughter cells are produced with half the number of chromosomes as the diploid meiotic mother cell. The sexual fusion (syngamy

or fertilisation) of two haploid cells to produce a zygote restores the

diploid condition. The zygote carries the genetic material from two

different parents. It has pairs of homologous sister chromosomes,

one from each parent, each homologue bearing the same genes, but

perhaps bearing different alleles. In this way each zygote carries a

different combination of different parts of the genetic material from

different parents. When meiosis occurs, this pooled genetic variation

is assorted randomly, so that each haploid cell produced, called a

gamete, carries a different set of alleles. In these two ways, random

mating and random assortment of genetic variation in meiosis, the

range of genetic variation is greatly increased.

An important theoretical advantage sexual organisms have over

asexual lineages arises from the way asexual organisms have a tendency to gain slightly deleterious mutant alleles by chance, by what is

called genetic drift. Genetic drift is more likely in small populations

or those that go through population bottlenecks. Like a cog that can

only move forward because it is held in place by a ratchet, the number

of slightly deleterious alleles, or genetic load, accumulates over time.

This possibility was first pointed out by H. J. Muller. Sexual organisms

have the ability to escape Muller’s ratchet because, by sexual recombination, new lineages with novel combinations of non-deleterious



1.3 FORM: THE ORIGIN OF COMPLEX CELLS



Figure 1.21. Mitosis and

meiosis.



alleles are recreated in each generation of sexual reproduction.

Another very important evolutionary aspect is that it is differences in

sexual reproduction that reproductively isolate evolutionary lineages

enabling them to diverge and become differently adapted.

There are various forms of sexual reproduction. In the vast majority of eukaryotes, including plants, gametes from two different parents fuse to form the zygote, a condition enforced by self-sterility.

These organisms are called heterothallic. However, some organisms

are self-fertile or homothallic. Note that this distinction between

homo- and heterothallism is different from the distinction between

male and female. In some heterothallic algae, for example, there is

no distinction between male and female. The gametes look identical;

they are isogamous (Figure 1.22). However, even in isogamous organisms sometimes there are different mating types and only some combinations of gametes will fuse together in fertilisation. Isogamy is

quite widespread in the algae.

Alternatively, the two gametes that fertilise have the same form,

but one, designated as the female, is larger than the male. This condition is called anisogamy. There is a third condition called oogamy.

Oogamous species have a clear distinction between a small motile

male gamete, called the sperm or spermatozoid, and a large immobile female gamete, called the egg or ovum. Some species of the

green alga Chlamydomonas are isogamous, some are anisogamous and



29



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PROCESS, FORM AND PATTERN



Figure 1.22. Syngamy, the

fusion of gametes: isogamy,

anisogamy and oogamy compared.



gamet s

eg



gamet s



sper



zygote



zygote



isogam



y



anisogam



y



m



zygote

o gam



y



a few are oogamous. The evolution of differences from isogamy via

anisogamy to oogamy represents a specialisation; between the male

gamete, which is motile and produced in large numbers to maximise

its chances of finding a female gamete, and a female gamete, which

is large with food reserves for the zygote and produced in smaller

numbers. This specialisation has evolved on many different occasions

in different evolutionary lineages. The male gamete is normally flagellate and motile, although red algae, conifers and flowering plants

have non-flagellate male gametes. In those species that have male and

female gametes, these may be produced both by the same bisexual

individual, a state called monoecy, or by separate male and female

individuals, a state called dioecy. For example, in the brown algae,

Fucus is dioecious and Pelvetia is monoecious.

It was sex that created huge potential advantages to eukaryotic

organisms. As with the evolution of photosynthesis it was possibly the

existence of a high-UV light environment, because of the lack of any

significant ozone shield, which provided the spur for the evolution of

sex. A high-UV environment is potentially highly mutagenic. Haploid

organisms, which have a single copy of each gene, are disadvantaged

if UV causes a deleterious mutation in an essential gene. Those organisms with multiple copies of genes could survive the destruction of

one of the copies by mutation.

However, many aspects of cell life are determined by the relative

concentration of gene products. A disordered, or unbalanced, multiplication of genes is potentially disadvantageous, because it could

result in unfavorable proportions of gene products. Although nuclear

division without cell division, endopolyploidy, does not unbalance

the genome, the first time it happens it creates a diploid nucleus

in which each chromosome has a homologous chromosome with

exactly the same genes; however, if endopolyploidy is repeated several times, it results in a bloated genome and consequently a slow

rate of cell division. Meiosis may have first evolved as a mechanism to



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