Este peixe é um híbrido descendente de um peixe-jacaré ( Atractosteus spatula ) e de um peixe-jacaré-pintado ( Lepisosteus oculatus ) — dois gêneros que compartilharam um ancestral comum pela última vez há pelo menos 100 milhões de anos. Solomon David
Uma versão desta história apareceu na Science, Vol. 383, Edição 6687. Baixar PDF
Em 1859, Charles Darwin cunhou o termo "fóssil vivo"
para descrever linhagens que se assemelham há dezenas de milhões de
anos, como o celacanto, o esturjão e o caranguejo-ferradura. O termo
cativou a imaginação popular, mas os cientistas têm lutado para entender se essas espécies apenas se assemelham a seus ancestrais longínquos ou se, de fato, evoluíram pouco ao longo das eras.
Agora, em um estudo publicado hoje na Evolution , pesquisadores confirmam que em alguns — mas não em todos — fósseis vivos, a evolução está praticamente estagnada
. Os exemplos mais impressionantes são os peixes de aparência
pré-histórica chamados gars, que apresentam a taxa de evolução molecular
mais lenta entre todos os vertebrados com mandíbula. A equipe também
propõe um mecanismo para explicar a atemporalidade dos gars: um
excelente mecanismo de reparo de DNA. Esse reparo provavelmente manteve
os genomas dos gars tão estáveis que espécies e até mesmo gêneros cujo
último ancestral comum viveu há mais de 100 milhões de anos divergiram
muito pouco, e alguns ainda conseguem hibridizar hoje para produzir
descendentes viáveis.
"Isso é incrível", diz Tetsuya Nakamura, biólogo evolucionista do
desenvolvimento da Universidade Rutgers, que não participou do trabalho.
"Este artigo traz muitos trabalhos interessantes sobre a questão do que
constitui um fóssil vivo, mas quando li isso, fiquei chocado."
Para verificar se vários supostos fósseis vivos evoluem mais
lentamente do que outros grupos de vertebrados, a equipe reuniu
sequências publicadas de mais de 1.100 éxons (as regiões codificadoras
do genoma) de 478 espécies. Utilizando árvores genealógicas existentes
para cada grupo, eles criaram uma enorme árvore evolutiva. Para cada
linhagem, os pesquisadores estimaram a taxa de mudança de cada base de
DNA nos éxons estudados ao longo do tempo — a chamada taxa de
substituição.
Surpreendentemente, eles descobriram que a evolução não estava em
pausa em todos os fósseis vivos. O celacanto, o tubarão-elefante e uma
ave chamada cigana — todos considerados antigos — apresentam taxas de
mutação mais rápidas do que o esperado, de cerca de 0,0005 mutações em
cada sítio por milhão de anos, embora ainda seja mais lenta do que a
taxa média para anfíbios (0,007 mutações por milhão de anos) e mamíferos
placentários (0,02 mutações por milhão de anos). As descobertas
corroboram a ideia de que algumas espécies que ainda se assemelham a
seus ancestrais antigos mudaram em nível molecular.
Mas os gars, grandes peixes de água doce com focinhos longos e
dentados, eram diferentes: em quase todos os éxons, os gars apresentavam
as taxas mais lentas de substituição molecular, muitas vezes em várias
ordens de magnitude, e apresentavam uma média de apenas 0,00009 mutações
por milhão de anos em cada sítio. De fato, dois gêneros que divergiram
há cerca de 20 milhões de anos apresentaram sequências idênticas em
quase todos os sítios analisados — uma descoberta que a equipe
inicialmente atribuiu a um erro de sequenciamento. "Entrei neste projeto
com cautela ao usar o termo fóssil vivo", diz o coautor do estudo,
Chase Brownstein, doutorando em biologia evolutiva na Universidade de
Yale. "Mas, pelo menos para os gars, é um termo apropriado."
Os autores postulam que, como as taxas de substituição em gars
parecem consistentemente baixas em todos os locais — inclusive em
regiões genômicas com pouca probabilidade de estarem sob pressão
seletiva —, é provável que haja um mecanismo global impulsionando a
lenta substituição. Eles sugerem que os gars são extremamente eficientes
no reparo do DNA após mutações ou danos, impedindo a evolução dos
animais mesmo com a mudança dos continentes ao seu redor. Uma hipótese
semelhante já foi proposta por outros pesquisadores para o esturjão, que
apresentou a segunda menor taxa de substituição entre os vertebrados do
estudo.
DNA repair is “a reasonable hypothesis, but there’s probably more
than just one explanation,” says Elise Parey, an evolutionary genomicist
at University College London. Experts have noted, for example, that
gars have slow metabolic rates and long generation times, features that could reduce mutation rates. Gars have also preserved the arrangement of DNA in their chromosomes and dampened the effects
of so-called jumping genes that can cause genetic reshuffling as they
move from place to place in the genome. “This goes not just to sequence
changes, but also to chromosome evolution, which would be an interesting
avenue to explore,” Parey says.
Para testar suas descobertas, os autores acompanharam relatos de
peixes-agulha incomuns que poderiam ser híbridos naturais em rios por
todo Oklahoma e Texas. Eles analisaram amostras de tecido de dezenas
desses peixes para rastrear sua ancestralidade, descobrindo que dois
gêneros de peixes-agulha — Atractosteus e Lepisosteus
— estão se cruzando para produzir filhotes férteis e híbridos. Esses
grupos compartilharam um ancestral comum pela última vez há cerca de 105
milhões de anos, tornando sua separação a mais antiga entre eucariotos
que podem produzir descendentes viáveis. Os peixes-agulha superaram os
recordistas anteriores — duas espécies de samambaia — em cerca de 60
milhões de anos. (Mentes perspicazes podem se lembrar do peixe- esturddel
, um híbrido de peixe-espátula e esturjão, que divergiu há ainda mais
tempo, mas esses híbridos acidentais provavelmente eram estéreis e não
ocorrem naturalmente.)
O próximo passo será comprovar que os mecanismos de reparo do DNA dos
peixes-agulha são de fato responsáveis pela ausência de alterações
genéticas. Ao equipar o peixe-zebra — um modelo animal padrão — com
genes de reparo do DNA dos peixes-agulha, os pesquisadores poderão
observar os genes em ação em laboratório. "Este será um experimento
desafiador, no entanto, porque [os genes de reparo do DNA] são
fundamentais", e alterá-los pode ter consequências indesejadas, diz
Nakamura.
Mas os autores afirmam que entender como os gars mantêm sua taxa de
mutação tão baixa pode trazer benefícios que vão além da compreensão dos
fósseis vivos em nível molecular. Também pode ajudar os humanos a
entender melhor nossas próprias vias de reparo do DNA, que podem levar
ao câncer quando falham.
Evolution is change over time. Under this broad
definition, evolution can refer to a variety of changes that occur over
time—the uplifting of mountains, the wandering of riverbeds, or the
creation of new species. To understand the history of life on Earth
though, we need to be more specific about what kinds of changes over time we're talking about. That's where the term biological evolution comes in.
Biological evolution refers to the changes over time that occur in living organisms.
An understanding of biological evolution—how and why living organisms
change over time—enables us to understand the history of life on Earth.
They key to understanding biological evolution lies in a concept known as as descent with modification.
Living things pass on their traits from one generation to the next.
Offspring inherit a set of genetic blueprints from their parents. But
those blueprints are never copied exactly from one generation to the
next. Little changes occur with each passing generation and as those
changes accumulate, organisms change more and more over time. Descent
with modification reshapes living things over time, and biological
evolution takes place.
All life on Earth shares a common ancestor. Another
important concept relating to biological evolution is that all life on
Earth shares a common ancestor. This means that all living things on our
planet are descended from a single organism. Scientists estimate that
this common ancestor lived between 3.5 and 3.8 billion years ago and
that all living things that have ever inhabited our planet could
theoretically be traced back to this ancestor. The implications of
sharing a common ancestor are quite remarkable and mean that we're all
cousins—humans, green turtles, chimpanzees, monarch butterflies, sugar
maples, parasol mushrooms and blue whales.
Biological evolution occurs on different scales.
The scales on which evolution occurs can be grouped, roughly, into two
categories: small-scale biological evolution and broad-scale biological
evolution. Small-scale biological evolution, better known as
microevolution, is the change in gene frequencies within a population of
organisms changes from one generation to the next. Broad-scale
biological evolution, commonly referred to as macroevolution, refers to
the progression of species from a common ancestor to descendent species over the course of numerous generations.
Life on Earth has been changing at various rates since our common ancestor first appeared more than 3.5 billion years ago.
To better understand the changes that have taken place, it helps to
look for milestones in the history of life on Earth. By grasping how
organisms, past and present, have evolved and diversified throughout the
history of our planet, we can better appreciate the animals and
wildlife that surround us today.
The first life evolved more than 3.5 billion years ago.
Scientists estimate that the Earth is some 4.5 billion years old. For
nearly the first billion years after the Earth formed, the planet was
inhospitable to life. But by about 3.8 billion years ago, the Earth's
crust had cooled and the oceans had formed and conditions were more
suitable for the formation of life. The first living organism formed
from simple molecules present in the Earth's vast oceans between 3.8 and
3.5 billion years ago. This primitive life form is know as the common
ancestor. The common ancestor is the organism from which all life on
Earth, living and extinct, descended.
Photosynthesis arose and oxygen began accumulating in the atmosphere about 3 billion years ago.
A type of organism known as cyanobacteria evolved some 3 billion years
ago. Cyanobacteria are capable of photosynthesis, a process by which
energy from the sun is used to convert carbon dioxide into organic
compounds—they could make their own food. A byproduct of photosynthesis
is oxygen and as cyanobacteria persisted, oxygen accumulated in the
atmosphere.
Sexual reproduction evolved about 1.2 billion years ago, initiating a rapid increase in the pace of evolution.
Sexual reproduction, or sex, is a method of reproduction that combines
and mixes traits from two parent organisms in order to give rise to an
offspring organism. Offspring inherit traits from both parents. This
means that sex results in the creation of genetic variation and thus
offers living things a way to change over time—it provides a means of
biological evolution.
The Cambrian Explosion is the term given to the time period between 570 and 530 million years ago when most modern groups of animals evolved.
The Cambrian Explosion refers to an unprecedented and unsurpassed
period of evolutionary innovation in the history of our planet. During
the Cambrian Explosion, early organisms evolved into many different,
more complex forms. During this time period, nearly all of the basic
animal body plans that persist today came into being.
The first back-boned animals, also known as vertebrates, evolved about 525 million years ago during the Cambrian Period.
The earliest known vertebrate is thought to be Myllokunmingia, an
animal that is thought to have had a skull and a skeleton made of
cartilage. Today there are about 57,000 species of vertebrates that
account for about 3% of all known species on our planet. The other 97%
of species alive today are invertebrates and belong to animal groups
such as sponges, cnidarians, flatworms, mollusks, arthropods, insects,
segmented worms, and echinoderms as well as many other lesser-known
groups of animals.
The first land vertebrates evolved about 360 million years ago.
Prior to about 360 million years ago, the only living things to inhabit
terrestrial habitats were plants and invertebrates. Then, a group of
fishes know as the lobe-finned fishes evolved the necessary adaptations
to make the transition from water to land.
Between 300 and 150 million years ago, the first land
vertebrates gave rise to reptiles which in turn gave rise to birds and
mammals. The first land vertebrates were amphibious tetrapods
that for some time retained close ties with the aquatic habitats they
had emerged from. Over the course of their evolution, early land
vertebrates evolved adaptations that enabled them to live on land more
freely. One such adaptation was the amniotic egg. Today, animal groups including reptiles, birds and mammals represent the descendants of those early amniotes.
The genus Homo first appeared about 2.5 million years ago.
Humans are relative newcomers to the evolutionary stage. Humans
diverged from chimpanzees about 7 million years ago. About 2.5 million
years ago, the first member of the genus Homo evolved, Homo habilis. Our species, Homo sapiens evolved about 500,000 years ago.
Fossils are the remains of organisms that lived in the distant past.
For a specimen to be considered a fossil, it must be of a specified
minimum age (often designated as greater than 10,000 years old).
Together, all fossils—when considered in the context of the
rocks and sediments in which they are found—form what is referred to as
the fossil record. The fossil record provides the foundation
for understanding the evolution of life on Earth. The fossil record
provides the raw data—the evidence—that enables us to describe the
living organisms of the past. Scientists use the fossil record to
construct theories that describe how organisms of the present and past
evolved and relate to one another. But those theories are human
constructs, they are proposed narratives describing what happened in the
distant past and they must fit with fossil evidence. If a fossil is
discovered which does not fit with current scientific understanding,
scientists must rethink their interpretation of the fossil and its
lineage. As science writer Henry Gee puts it:
"When people discover a fossil they have enormous
expectations about what that fossil can tell us about evolution, about
past lives. But fossils actually don't tell us anything. They are
completely mute. The most the fossil is, is an exclamation that says:
Here I am. Deal with it." ~ Henry Gee
Fossilization is a rare occurrence in the history of life.
Most animals die and leave no trace; their remains are scavenged soon
after their death or they decompose quickly. But occasionally, an
animal's remains are preserved under special circumstances and a fossil
is produced. Since aquatic environments offer conditions more favorable
to fossilization than those of terrestrial environments, most fossils
are preserved in freshwater or marine sediments.
Fossils need geological context in order to tell us valuable information about evolution.
If a fossil is taken out of its geological context, if we have the
preserved remains of some prehistoric creature but don't know what rocks
it was dislodged from, we can say very little of value about that
fossil.
Descent with Modification
A page from one of Darwin's notebooks depicting his first tentative
ideas about the branching system of descent with modification.
Public domain photo.
Biological evolution is defined as descent with modification.
Descent with modification refers to the passing on of traits from
parent organisms to their offspring. This passing on of traits is known
as heredity, and the basic unit of heredity is the gene. Genes
hold information about every conceivable aspect of an organism: its
growth, development, behavior, appearance, physiology, reproduction.
Genes are the blueprints for an organism and these blueprints are passed
from parents to their offspring each generation.
The passing on of genes is not always exact, parts of the blueprints
may be copied incorrectly or in the case of organisms that undergo
sexual reproduction, genes of one parent are combined with the genes of
another parent organism. Individuals that are more fit, better suited
for their environment, are likely to transmit their genes to the next
generation than those individuals that are not well-suited for their
environment. For this reason, the genes present in a population of
organisms is in constant flux due to various forces—natural selection,
mutation, genetic drift, migration. Over time, gene frequencies in
populations change—evolution takes place.
There are three basic concepts that are often helpful in clarifying how descent with modification works. These concepts are:
genes mutate
individuals are selected
populations evolve
Thus there are different levels at which changes are taking place,
the gene level, the individual level, and the population level. It is
important to understand that genes and individuals do not evolve, only
populations evolve. But genes mutate and those mutations often have
consequences for individuals. Individuals with different genes are
selected, for or against, and as a result, populations change over time,
they evolve.
"As buds give rise by growth to fresh buds ..." ~ Charles Darwin In 1837, Charles Darwin sketched a simple tree diagram in one of his notebooks, next to which he penned the tentative words: I think.
From that point on, the image of a tree for Darwin persisted as a way
to envision the sprouting of new species from existing forms. He later
wrote in On the Origin of Species:
"As buds give rise by growth to fresh buds, and these,
if vigorous, branch out and overtop on all sides many a feebler branch,
so by generation I believe it has been with the great Tree of Life,
which fills with its dead and broken branches the crust of the earth,
and covers the surface with its ever-branching and beautiful
ramifications." ~ Charles Darwin, from Chapter IV. Natural Selection of On the Origin of Species
Today, trees diagrams have taken root as powerful tools for
scientists to depict relationships among groups of organisms. As a
result, an entire science with its own specialized vocabulary has
developed around them. Here we'll look at the science surrounding
evolutionary trees, also known as phylogenetics.
Phylogenetics is the science of constructing and evaluating
hypotheses about evolutionary relationships and patterns of descent
among organisms past and present. Phylogenetics enables
scientists to apply the scientific method to guide their study of
evolution and assist them in interpreting the evidence they collect.
Scientists working to resolve the ancestry of several groups of
organisms evaluate the various alternate ways in which the groups could
be related to one another. Such evaluations look to evidence from a
variety of sources such as the fossil record, DNA studies or morphology.
Phylogenetics thus provides scientists with a method of classifying
living organisms based on their evolutionary relationships.
A phylogeny is the evolutionary history of a group of organisms.
A phylogeny is a 'family history' that describes the temporal sequence
of evolutionary changes experienced by a group of organisms. A phylogeny
reveals, and is based on, the evolutionary relationships among those
organisms.
A phylogeny is often depicted using a diagram called a cladogram.
A cladogram is tree diagram that reveals how lineages of organisms are
interconnected, how they branched and re-branched throughout their
history and evolved from ancestral forms to more modern forms. A
cladogram depicts relationships between ancestors and descendants and
illustrates the the sequence with which traits developed along a
lineage.
Cladograms superficially resemble the family trees used in
genealogical research, but they differ from family trees in one
fundamental way: cladograms do not represent individuals like family
trees do, instead cladograms represent entire lineages—interbreeding
populations or species—of organisms.
There are four basic mechanisms by which biological evolution
takes place. These include mutation, migration, genetic drift, and
natural selection. Each of these four mechanisms are capable of
altering the frequencies of genes in a population and as a result, they
all are capable of driving descent with modification.
Mechanism 1: Mutation. A mutation is a change in the
DNA sequence of a cell's genome. Mutations can result in various
implications for the organism—they can have no effect, they can have a
beneficial effect, or they can have a detrimental effect. But the
important thing to keep in mind is that mutations are random and occur
independent of the organisms' needs. The occurrence of a mutation is
unrelated to how useful or harmful the mutation would be to the
organism. From an evolutionary perspective, not all mutations matter.
The ones that do are those mutations that are passed on to
offspring—mutations that are heritable. Mutations that are not inherited
are referred to as somatic mutations.
Mechanism 2: Migration. Migration, also known as
gene flow, is the movement of genes between subpopulations of a species.
In nature, a species is often divided into multiple local
subpopulations. The individuals within each subpopulation usually mate
at random but might mate less often with individuals from other
subpopulations due to geographic distance or other ecological barriers.
When individuals from different subpopulations move easily from one
subpopulation to another, genes flow freely among the subpopulations and
the remain genetically similar. But when individuals from the different
subpopulations have difficulty moving between subpopulations, gene flow
is restricted. This may in the subpopulations becoming genetically
quite different.
Mechanism 3: Genetic Drift. Genetic drift is the
random fluctuation of gene frequencies in a population. Genetic drift
concerns changes that are driven merely by random chance occurrences,
not by any other mechanism such as natural selection, migration or
mutation. Genetic drift is most important in small populations, where
the loss of genetic diversity is more likely due to their having fewer
individuals with which to maintain genetic diversity.
Genetic drift is controversial because it creates a conceptual
problem when thinking about natural selection and other evolutionary
processes. Since genetic drift is a purely random process and natural
selection is non-random, it creates difficulty for scientists to
identify when natural selection is driving evolutionary change and when
that change is simply random.
Mechanism 4: Natural selection. Natural selection is
the differential reproduction of genetically varied individuals in a
population that results in individuals whose fitness is greater leaving
more offspring in the next generation than individuals of lesser
fitness.
In 1858, Charles Darwin
and Alfred Russel Wallace published a paper detailing the theory of
natural selection which provides a mechanism by which biological
evolution occurs. Although the two naturalists developed
similar ideas about natural selection, Darwin is considered to be the
theory's primary architect, since he spent many years gathering and
compiling a vast body of evidence to support the theory. In 1859, Darwin
published his detailed account of the theory of natural selection in
his book On the Origin of Species.
Natural selection is the means by which beneficial variations
in a population tend to be preserved while unfavorable variations tend
to be lost. One of the key concepts behind the theory of
natural selection is that there is variation within populations. As a
result of that variation, some individuals are better suited to their
environment while other individuals are not so well-suited. Because
members of a population must compete for finite resources, those better
suited to their environment will out-compete those that are not as
well-suited. In his autobiography, Darwin wrote of how he conceived this
notion:
"In October 1838, that is, fifteen months after I had
begun my systematic inquiry, I happened to read for amusement Malthus on
Population, and being well prepared to appreciate the struggle for
existence which everywhere goes on from long-continued observation of
the habits of animals and plants, it at once struck me that under these
circumstances favourable variations would tend to be preserved, and
unfavourable ones to be destroyed." ~ Charles Darwin, from his
autobiography, 1876.
Natural selection is a relatively simple theory that involves five basic assumptions.
The theory of natural selection can be better understood by identifying
the basic principles on which it relies. Those principles, or
assumptions, include:
Struggle for existence - More individuals in a population are born each generation than will survive and reproduce.
Variation - Individuals within a population are variable. Some individuals have different characteristics than others.
Differential survival and reproduction
- Individuals that have certain characteristics are better able to
survive and reproduce than other individuals having different
characteristics.
Inheritance - Some of the characteristics that influence an individual's survival and reproduction are heritable.
Time - Ample amounts of time are available to allow for change.
The result of natural selection is a change in gene frequencies
within the population over time, that is individuals with more favorable
characteristics will become more common in the population and
individuals with less favorable characteristics will become less common.
Sexual selection is a type of natural selection that acts on traits related to attracting or gaining access to mates.
While natural selection is the result of the struggle to survive,
sexual selection is the result of the struggle to reproduce. The outcome
of sexual selection is that animals evolve characteristics whose
purpose do not increase their chances of survival but instead increases
their chances of reproducing successfully.
There are two kinds of sexual selection:
Inter-sexual selection occurs between the sexes and acts on characteristics that make individuals more attractive to the opposite sex.
Inter-sexual selection can produce elaborate behaviors or physical
characteristics, such as the feathers of a male peacock, the mating
dances of cranes, or the ornamental plumage of male birds of paradise.
Intra-sexual selection occurs within the same sex and acts on characteristics that make individuals better able to outcompete members of the same sex for access to mates.
Intra-sexual selection can produce characteristics that enable
individuals to physically overpower competing mates, such as the antlers
of an elk or the bulk and power of elephant seals.
Sexual selection can produce characteristics that, despite increasing
the individual's chances of reproducing, actually diminish the chances
of survival. The brightly colored feathers of a male cardinal or the
bulky antlers on a bull moose might make both animals more vulnerable to
predators. Additionally, the energy an individual devotes to growing
antlers or putting on the pounds to outsize competing mates can take a
toll on the animal's chances of survival.
Coevolution
The relationship between flowering plants and their pollinators can offer a classic examples of coevolutionary relationships.
Photo courtesy Shutterstock.
Coevolution is the evolution of two or more groups of organisms together, each in response to the other.
In a coevolutionary relationship, changes experienced by each
individual group of organisms is in some manner shaped by or influenced
by the other groups of organisms in that relationship.
The relationship between flowering plants and their pollinators can
offer a classic examples of coevolutionary relationships. Flowering
plants rely on pollinators to transport pollen among individual plants
and thus enable cross-pollination.
The term species can be defined as a group of individual
organisms that exist in nature and, under normal conditions, are capable
of interbreeding to produce fertile offspring. A species is,
according to this definition, the largest gene pool that exists under
natural conditions. Thus, if a pair of organisms are capable of
producing offspring in nature, they must belong to the same species.
Unfortunately, in practice, this definition is plagued by ambiguities.
To begin, this definition is not relevant to organisms (such as many
types of bacteria) that are capable of asexual reproduction. If the
definition of a species requires that two individuals are capable of
interbreeding, then an organism that does not interbreed is outside of
that definition.
Another difficulty that arises when defining the term species is that
some species are capable of forming hybrids. For example, many of the
large cat species are capable of hybridizing. A cross between a female
lions and a male tiger produces a liger. A cross between a male jaguar
and a female lion produces a jaglion. There are a number of other
crosses possible among the panther species, but they are not considered
to be all members of a single species as such crosses are very rare or
do not occur at all in nature.
Species form through a process called speciation. Speciation takes
place when the lineage of a single splits into two or more separate
species. New species can form in this manner as a result of several
potential causes such as geographic isolation or a reduction in gene
flow among members of the population.
When considered in the context of classification, the term species
refers to the most refined level within the hierarchy of major taxonomic
ranks (though it should be noted that in some cases species are further
divided into subspecies).
domingo, 29 de abril de 2018
New life form answers question about evolution of cells
The left panel shows: EM image of a normal E. coli cell.
Right panel: an engineered cell with a mixed membrane, which shows an
elongated form. Credit: Photo's University of Wageningen / Van der Oost
laboratory
Bacteria and Archaea are two of the three
domains of life. Both must have evolved from the putative last universal
common ancestor (LUCA). One hypothesis is that this happened because
the cell membrane in LUCA was an unstable mixture of lipids. Now,
scientists from the University of Groningen and Wageningen University
have created such a life form with a mixed membrane and discovered it
is, in fact, stable, refuting this hypothesis. The results will be
published in the journal Proceedings of the National Academy of Sciences in the week of 19 March.
There are
many ideas on how cellular life could have evolved billions of years
ago. Protocells may have formed in clay minerals, or as simple lipid
vesicles. In the latter scenario, something called the 'lipid divide'
would have occurred, creating the separate domains of bacteria and
archaea, explains University of Groningen Professor of Molecular
Microbiology Arnold Driessen. "The lipid membranes of both domains are
different, composed of phospholipids that are each other's mirror
image."
Mixed membranes
In technical terms, the lipids in the membranes of bacteria are made
up of straight-chain fatty acids that are ester-linked to a backbone of
glycerol-3-phosphate. But the lipids in the archaeal membrane have a
backbone of glycerol-1-phosphate, to which isoprenoids are linked by
ether bonds.
The idea behind the lipid divide is that a common ancestor of both bacteria and archaea had a cell membrane
in which both types of lipid were mixed. "This mixed membrane would be
less stable than a homogenous membrane of just one type of phospholipid,
so eventually a split occurred, resulting in the two domains of
Bacteria and Archaea," says Driessen.
All this would have happened over 3.5 billion years ago, so hard
evidence is lacking. Driessen and his co-workers therefore decided to
"reverse-engineer" a micro-organism with a mixed membrane. "This has
been tried before, but these experiments resulted in bacteria with only
very small amounts (< 1 percent) of archaeal lipids."
Left panel: EM image of a normal dividing E. coli cell.
Right panel: an engineered cell with high archaeal lipid production,
showing lobular irregularities in the cell membrane. Credit: Photo's
University of Wageningen / Van der Oost laboratory
However, in the new study, this increased
to 30 percent. Two key breakthroughs made this possible: "In previous
research we discovered an enzyme crucial to the production of archaeal
membrane lipids. This takes three steps, and before that, only two of
the enzymes involved were known." The other breakthrough came from
scientists at Wageningen University, explains Driessen. "They managed to
increase the production of isoprenoids in the bacterium E. coli."
Isoprenoids are a ubiquitous class of organic compounds which includes
many natural flavours, colours or fragrances.
Both discoveries were transferred to a normal E. coli bacterium,
which was a major feat of genetic engineering.
"And we didn't know if
the end result would be a viable cell," says Driessen. But in the end,
it worked out well. With some fine tuning, the scientists created a cell
in which all phosphatidylglycerol, the lipids forming the basic bilayer
of the bacterial membrane,
were replaced by their archaeal equivalent (archaetidylglycerol).
This
accounts for 30 percent of the lipids in the membrane. "This bacterium
grew at normal speed and was stable," says Driessen. "So this result
does not support the hypothesis that a mixed membrane is inherently
instable and could thus have created the lipid divide."
Driessen notes that the archaeal enzymes for membrane lipid
production are less specific in the reactions they catalyze than their
bacterial equivalents. "They appear to be more "primordial." So the
evolution of enzyme specificity could have been a driver for the
divide." There is, of course, one major caveat—the experiments were done
in modern E. coli bacteria, which have evolved 3.5 billion years beyond
the original split with the archaea.
"The robustness of these mixed cells surprised us. We expected more
problems keeping them alive. After all, what we have engineered does
amount to a new life form." The engineered cells were more elongated
than the original E. coli. And when the production of archaeal lipids
was very high, growth slowed and the membrane developed lobular
appendages.
Apart from the evolutionary implications, this discovery could spawn
new research: "For example, we could engineer a bacterial expression
system for archaeal membrane proteins, such as those produced by hyperthermophiles that grow at extremely high temperatures and pressure."
The work by the two Dutch universities is part of the Origins Center,
a national programme dedicated to research into the origin of life on
our planet.
More information:
Antonella Caforio el al.,
"Converting Escherichia coli into an archaebacterium with a hybrid
heterochiral membrane," PNAS (2018). www.pnas.org/cgi/doi/10.1073/pnas.1721604115
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