viernes, junio 27, 2008

Filogenia Rara de las Aves




Phylogenomic Study of Birds Reveals Their Evolutionary History
Shannon J. Hackett, Rebecca T. Kimball, Sushma Reddy, Rauri C. K. Bowie, Edward L. Braun, Michael J. Braun, Jena L. Chojnowski, W. Andrew Cox, Kin-Lan Han, John Harshman, Christopher J. Huddleston, Ben D. Marks, Kathleen J. Miglia, William S. Moore, Frederick H. Sheldon, David W. Steadman, Christopher C. Witt, and Tamaki Yuri
Science Jun 27 2008: 1763-1768.

miércoles, junio 25, 2008

La historia de la biología evolutiva en Chile... según Medel

(ATENCIÓN: NO OLVIDAR LEER ADEMÁS ACLARACIÓN AL PIE DE ESTA NOTA)

Hace poco me llegó un PDF de un trabajo "en prensa" de Rodrigo Medel (Facultad de ciencias de la universidad de chile). Aunque no sé donde se publicará, entiendo que Medel mismo autorizó la distribución de este material y que no existen contraindicaciones para discutirlo acá; el PDF lo encontrarán donde siempre.

El trabajo es lo más reciente en lo que han sido una serie de opininones publicadas sobre el estudio de la biología evolutiva en Chile. Medel encaja claramente en el perfil de lo que el historiador de la biología Ron Amundson (2005) describe como "synthesis historiographer", es decir, alguien cuya narrativa de la historia se entrelaza con una defensa del neodarwinismo. Presenta ciertas distorsiones típicas, especialmente la tendencia a ver la genética de poblaciones como la iniciativa más valiosa: Debido a esto, los logros de personajes de esta área (como Danko Brncic) son enfatizados, mientras que falla en documentar o evaluar apropiadamente el desarrollo e importancia logrado desde otras áreas de la evolución en Chile. Adicionalmente, Medel confronta la crítica al énfasis en la selección natural en Chile como una especie de aberrante enfermedad local, sin reconocer que se trata de un fenómeno internacional que desde los 60's viene desestabilizando la hegemonía alguna vez gozada por el neodarwinismo ( su "edad dorada" fue la postguerra de los 40's-50's)

Una notable omisión histórica hecha por Medel es la corta pero influyente estadía del prestigioso paleontólogo y biólogo argentino Osvaldo Reig, quien trabajó en la facultad de medicina de la U. de Chile, y que fue nada menos que el fundador del instituto de ecología y evolución de la Universidad Austral (según al menos dos obituarios, en J Mammalogy y Genetica). La estadía de Reig fue corta, debiendo escapar para el golpe militar, pero dejó huella , especialmente en el acercamiento citogenético al estudio de la evolución en Chile. Reig también trabajó en el museum of comparative zoology en Harvard, y es un inolvidable de la paleontología de vertebrados tras estudiar el "clásico" dinosaurio temprano Herrerasaurus.

Es notable, además, que Medel no hiciera investigación alguna del interesante conflicto que se dió en la U. de Chile entre los naturalistas-profesores y médicos-experimentalistas, y las fracturas que ante el desprecio mutuo han sido la única solución: la separación del pedagógico de la U. de Chile, y la separación del departamento de ecología y biología al interior de la facultad de ciencias. La patente existencia de los naturalistas-profesores como tradición aparte no es rescatada por Medel. Pero el instituto de entomología de la UMCE sigue ahí. El profesor Daniel Frías, que trabaja en la genética de insectos, ha publicado un interesante libro sobre sus estudios de evolución de insectos, un libro muy serio, de escasa afinidad con la ortodoxia darwinista, y que Medel por supuesto, no incluyó en su censurada "foto" de la diversidad intelectual chilena en materias de evolución.

El siguiente "cuento" muestra los prejuicios torpemente disimulados de Medel:

"When democracy was re-established in 1990, after 17 years of military dictatorship, teaching and research in genetics and evolutionary biology had not changed substantially neither in scope nor depth, in part due to the political isolation of Chile, which restricted the chance of academic and student exchange with foreign countries. The development of evolutionary biology in Chile during that period was kept constant at best. With only a couple of exceptions, the few courses of evolution available emphasized non-scientific topics that provided fertile ground for epistemological discussions but not for scientific research"

De qué se hablaba en la mayoría de las clases de evolución en los 80's? De astrología? Creacionismo?

La calificación de "no-ciencia" nos dice más sobre la visión "positivista simple" de Medel, que separa del todo epistemología de ciencia.
Aunque culpando a los alcances negativos (innegables) de la dictadura imperante, Medel reconoce que en los 70's y 80's hubo un estancamiento dentro de la genética de poblaciones. Sin embargo, en otras áreas (específicamente en el área de la neurobiología) en Chile se desarrollaba un nuevo marco teórico para la biología, basado en la teoría de sistemas, del cual se desprenderían importantes implicaciones evolutivas.
Este nuevo esquema no se construye pensando en la genética de poblaciones, sino para la biología en general. Desde un punto de partida completamente distinto, se recuperan conclusiones sobre la evolución, esbozadas algunas en los libros clásicos Maturana y Varela ("De máquinas y seres vivos" 1973, "El árbol del conocimiento" 1982). Se trata de los únicos libros de biología teórica escritos por chilenos que han disfrutado de verdadero éxito académico y editorial, nacional e internacional. Pero medel decidió no mencionarlos en su historia. Medel se nos hace el loco con el éxito de la noción de autopoiesis introducida por Maturana y Varela para caracterizar a los sistemas vivientes. Desconoce la innegable influencia teórica y experimental que ha tenido la noción de autopoiesis en el campo del origen de la vida. Desconoce los vínculos de Maturana con Heinz Von Foerster, y su categoría como uno de los pioneros de la teoría de sistemas en su aplicación a la biología.

Y por supuesto desconoce todo valor al trabajo de Maturana y Mpodozis 1992, publicación especial del Museo Nacional de Historia Natural de Chile, el cual es un trabajo que Medel aún no es capaz de entender. Si leyera con cuidadito, sabría que este es un trabajo realmente adelantado a su tiempo, que diagnosticó con anticipación la actualmente revindicada importancia de la plasticidad fenotípica, además de introducir nociones sistémicas de herencia y desarrollo, todo esto muy en línea con lo más recientemente expresado por destacados autores como Piggliuccci, West-Eberhardt, Oyama, Weiss y otros.

Medel continúa contando cuentos:

"More specifically, by questioning not only the prevalence but the entire existence of natural selection as a force driving evolutionary change, a peculiar all-inclusive perspective elaborated in the 70s gained adherents in young and uninformed undergraduate students (see Maturana-Romesín & Mpodozis 2000). Although this doctrine stimulated an interesting debate among Chilean naturalists (e.g., Gallardo 1997, Camus 2000, Nespolo 2003), it had a strong detrimental impact on several cohorts of students, ultimately retarding the advance of evolutionary biology research in Chile. Fortunately, the situation has changed in the last few years as an increasing number of students are involved in scientific research on a diverse array of evolutionary questions. For example, although still in their infancy, the areas more developed in Chile are evolutionary ecology, population and quantitative genetics, systematic biology, and biogeography. Areas less represented include genomics-bioinformatics, and paleobiology. The areas still absent include experimental evolution and evodevo (Roberto Nespolo, pers. comm.)"

Este tipo de tonterías son transparentes en su mera motivación denostadora, porque son completamente falsas. Medel intenta tapar el sol con un dedo. La verdad es que, dentro de esas cohortes de pobres alumnos que se "perdieron" en el maligno lavado de cerebro Maturánico, han surgido los únicos exponentes del Evo-Devo en Chile (Que sí, Medel, que el evo-devo en Chile existe, diga lo que te diga nespolito). El más antiguo Evo-Devo de Chile es Francisco Aboitiz, quien estuvo en el lab de Maturana. Aboitiz fue mi tutor de tesis. Estoy yo, colaborador cercano del lab (donde ya pronto estaré instalado!) y también , otro colaborador del lab, Carlos Guerrero (Catelo). En pocas palabras: los pocos chilenos que han encabezado publicaciones en revistas como Evolution & Development y JEZ part B, son todos vinculados al rayo. La solución de Medel? Sencillamente, no existimos. Flor de historiador!

Vale la pena destacar además que en el ex- departamento de morfología (hoy anatomía y desarrollo) de la fac. de medicina de la universidad de Chile ha trabajado por muchos años David Lemus, quien es legítimamente un precursor del evo-devo en el país, estudiando la embriología de reptiles chilenos. Especialmente, a partir de los 80's desarrolló investigación en dientes quiméricos en combinaciones heteroespecíficas de tejidos embrionarios del pollo y lagartijas chilenas, que continúa siendo muy citada en el área Evo-Devo. Ha sido por largo tiempo uno de los pocos preocupados de la enseñanza de la embriología comparada en Chile, manteniendo para esto un notable pequeño museo zoológico- didáctico. Otro de "los olvidados" en el esquema neodarwinista de Medel.

En la incompleta y cuentera historia de Medel, el Chile de las últimas décadas queda parado como un país más patético, más chato, menos diverso, y más dependiente de la influencia del "primer mundo", de lo que realmente es.

Referencias:

Amundson, R. 2005 The changing role of the embryo in evolutionary thought. Cambridge University

ACLARACIÓN: He recibido un email de Roberto Nespolo reclamando por la mención de su nombre en este blog. Asumiendo que este blog sólo lo leían los regulares, me referí a él en tono humorístico como "Nespolito"(supongo que los rumores vuelan!). Usar ese tono humorístico y cómplice (pensando que no necesariamente llegaría a oídos del afectado) fue un error licencioso de mi parte y pido diculpas por ello; por otra parte el incidente produjo en e-mail de Nespolo que sirve para aclarar su propia opinión , ya que él reconoce que la aproximación al Evo-Devo en Chile existe, encontrándose al tanto del trabajo de gente como Francisco Aboitiz y Miguel Concha. Que se entienda entonces claramente que lo que aparece citado por Medel como "Nespolo, pers comm" no refleja la opinión de Nespolo sobre las diferentes áreas de la biología evolutiva representadas en Chile. Aclaración agregada 21/07/08.

sábado, junio 21, 2008

Is everything OK with Olfactores? A call to properly assess morphological implications

When molecular phylogenies do not coincide with the morphological phylogenies, this is a serious problem. Experience tells us that its not a matter of assuming the morphological data is equivocal; specially if it is completely uncontroversial within (morphological) phylogenetic systematics. Many times the conflict thereafter disappears, specially upon better taxon sampling of molecular data (Note: increasing the number of adequate taxa seems to be of much greater consequence than increasing the number of genes). We all remember that "sharks are teleosts" thing. This is why it is good to know the exact "morphological cost", or extra morphological transformations, that are implied by molecular hypotheses that openly conflict with morphological phylogenies. If the conflict shows no resolution by further studies, it is not a matter of assuming the molecular data must be "the correct one"; or at least, if you are going to make that assumption, KNOW the implications for the evolution of morphology!

Is something smelling fishy about the phylogeny of the chordates? Or is everything OK? At record speed, the evo-devo community has accepted the results of the latest molecular phylogenies, that vertebrates are closer to urochordates than to cephalochordates. Olfactores = Urochordates + Vertebrates. This term was born in the context of a marginal theory of Jefferies (1981) within his interpretation that the fossil Homalozoa ("calcichordates") are the ancestors of chordates (other paleontologists consider Homalozoa to be basal echinoderms).

The recent publication of the entire genome of Amphioxus has further repeated this result for the comparison of an astounding 1090 genes in a phylogenetic analysis (Putnam et al 2008) that included important groups that are frequently left out, such as an acorn-worm, and a larvacean urochordate.
Given that the Olfactores are becoming accepted as some "new truth", it is interesting to review why never before had this notion attained popularity, the most common assumption being that cephalochordates and vertebrates were closest, conforming a clade Euchordata (also called "Myomerozoa" for the presence of somites). Actually, just a glance at amphioxus, a very fish-looking creature, should make us immediately suspect that the olfactores is probably not consistent with the most parsimonious morphological history; that is, that we may have the proverbial case of a clash of molecular vs morphological phylogenies.

Everybody agrees that urochordates have secondarily lost traits. This is obvious when the traits absent in urochordates are present even in hemichordates , such as coelomic cavities, and several (not just one) pairs of branchial openings. However, the list of things lost in urochordates increases substantially if we consider Olfactores to be real: for instance, the loss of somites, and several vertebrate-like gene expression patterns in the developing neural tube.

In this sense, it is important to point out that the exact morphological cost of the olfactores, in terms of assuming extra steps beyond parsimony, has not being adequately discussed or investigated yet. An important conflict with morphology and gene expression is looming, but people are failing to see it.

For instance, a recent comment (Swalla and Smith 2006) says "an extensive cladistic reanalysis of morphological data found strong support for Olfactores ( Ruppert 2005)". However, the cited work of Ruppert is not a cladistic analysis at all, but a "homology analysis", the mere mapping of proposed events assuming that Olfactores is real, for a limited set of traits. Further, the morphological cost of the olfactores is hardly rescued by Ruppert 2005, which cites only a few of the most notorious losses we must assume occurred in the urochordates (for instance, Ruppert fails to mention any of the similarities of gene expression between the neural tube of cephalochordates and vertebrates)

In good faith, we must assume that Swalla and Smith were thinking about another morphological analysis that is cited as support for the olfactores, namely the cladistic analysis of all metazoa made by Zrzávy et al (1998). While indeed Zrzávy et al. is an extensive analysis (238 traits), it does not specifically address the question of the phylogeny of chordates , but of metazoa in general. So actually only a minority of these traits are bound to be relevant to the question of chordate phylogeny. Further, despite the non-traditional placement of the urochordates retrieved by this analysis, Zrzávy et al did not make any mention of this result in their discussion, concentrating on other aspects of animal phylogeny. No list of "Olfactorian" synapomorphies was discussed. The analysis by Zrzávy has been criticized at length by Jenner 2001 on various grounds, such as the assumptions made in the definition and polarization of traits.

With no explicit cladistic analysis of chordates for an alternative phylogeny , the review by Rowe (2004) of chordate phylogeny does not even mention the work by Zrzávy or the possibility of the olfactores. Indeed, the best references for a cladistic analysis specifically focused on the chordates are earlier works (Maisey 1986, and Schaeffer 1987), which support the euchordata, and are largely accepted within the community of phylogenetic systematics. However, these studies will fail to collect numerous newly described traits shared by cephalochordates and vertebrates, from gene expression patterns to fine structure of the nervous system. So, in fact, a new updated cladistic phylogenetic analysis of the cephalochordates is needed to establish the precise consequences of the Olfactoria for the morphological history of the chordates.

It is certainly possible that this new analysis may show the history implied by olfactores to be too absurd, implying too many reversals or convergences. If this is the case, it is not just a matter of going with the molecular phylogeny over the morphological; rather, the possibility of an artifact in the molecular studies must be taken into account (yes, even with 1090 genes! )


This is the tree of the 1090 genes (Putnam et al 2008). Some observations:

1) As usual, the longest branches of the chordates are the Urochordates. Also, notice that this effect is not mitigated by the large amount of genes; that is, abnormally high substitution rates is a genome-wide phenomenon. Is it possible that high substitution rates may distort the phylogenetic signal of entire genomes? Also, notice the low 76.4 % bootstrap value support for the monophyly of chordates. Traditionally, molecular evidence has had some problems retrieving this node, which is, from a morphological perspective, a very straight-forward conclusion

2) Hagfish, morphologically the most basal vertebrates , were not included in this analysis. In fact, the position of hagfishes haunts this entire issue, since it is a case in which a clear conflict of molecules vs morphology still lives on

3) Despite the general trend of nuclear genes to support the olfactoria, the comparison of entire mitochondrial genomes supports the classic hypothesis of euchordata (Bourlat et al. 2006). Further, this is also the case when discussing the relationships of Hagfishes, supporting them as basalmost vertebrates (Yu et al. 2008). Why this diference ? Is it possible that the mitochondrial genomes have been spared from some source of artifact affecting the phylogenetic signal of nuclear genes?

References.

Bourlat et al 2006 Nature 444:85-8

Jefferies 1981 Zool. J. of Linnean Soc. 73, 351-396

Jenner 2001 Syst. Biol. 50(5):730-742

Putnam et al 2008 Nature 453: 1064-1070 doi:10.1038/nature06967

Rowe 2004 In: Cracraft & Donoghue, Ed. Assembling the tree of life. Oxford. pp 384-409

Ruppert 2005 Can. J. Zool. 83: 8–23

Swalla & Smith 2006 Phil. Trans. R. Soc. B doi:10.1098/rstb.2007.2246

Yu et al. 2008 J. Genet. Genomics 35: 285-290

Zrzávy et al. 1998 Cladistics 14, 249 -285

miércoles, junio 18, 2008

Evo-Devo: The Good, the Bad, and the Ugly.

THE GOOD: Homology assesment evo-devo, and epigenetic evo-devo. Evo-Devo that works on the pretty empirical task of assessing problematic homologies, with tree-based inferences on the evolution of development, continues to greatly help the reconstruction of the evolutionary history of life on earth. Epigenetic evo-devo's, in turn, understand that developmental biology is not genetics. They have realistically confronted the role of higher level and environmental epigenetic interactions in development, and thus also in the origin of evolutionary novelties. Both of these tend to emphasize how standing developmental mechanisms, and not natural selection alone, are essential to the pathway taken by evolution.

THE BAD: Reductionist "regulatory" evo-devo. Dangerous, because it is is upheld by important figures of evo-devo, presenting itself as a triumph and empirical conclusion. Yes, mutations in cis-regulatory regions are commonplace in evolution, but these people seem to have reductionist difficulties in understanding there is anything more beyond finding such a mutation. The notion that only non-coding "regulatory" sequence changes can produce localized expression (in time or space) simply makes no good developmental sense. As Lillie pointed out, all cells have the same DNA content, including the "regulatory" elements; whether a gene is expressed or not still varies from cell type to cell type depending on something else as well. In other words, Lillie's "paradox" forces the question: "who regulates the regulators"? This question reveals that "regulation" is nothing but a sloppy, semi-nonsensical wastepaper-basket term. Both coding and non-coding sequences can be "regulatory". Even environment can "regulate" gene expression. Genes are expressed differentially in cells, NEVER because of their "regulatory" sequences alone, but ALWAYS including the higher-level and environmental interactions at the cell and tissue level, which explain "Lillies paradox". (again: This is why developmental biology is different from genetics!). Focusing only on one type of mutations (cis-regulatory) is just a re-strengthened version of the old reductionist fallacy that genotype=phenotype. This false equivalence ultimately downplays the role of understanding development, the actual mechanisms that relate genotype to phenotype. Without really introducing developmental mechanisms, no serious challenge is made to the hegemony of population genetics as a way of understanding evolution. Yes: The bad is a traitor of development, for love of genetics. Evo-Devo can now become a mere footnote: The largely uninteresting filling-in of superfluous data on "what the specific mutations were".

THE UGLY. Just plain wrong or artifactual topics, mostly born from the lack of proper integration of different fields of research. Specially silly is the "conflict" between microevolution and macroevolution. Doubtless, the study of microevolution offers many advantages. But this does not mean at all that a macroevolutionary study will not be able to derive sound conclusions: when the evidence is there, there is nothing to say about the micro or macro level in which a question is satisfactorily answered. That comparable experiments can only be tested between closely related species is a myth: gene expression experiments can produce the same phenotypic alteration despite hundreds of millions of years of separation.
This follows in an old lab-bench tradition of being purely "experimental" negating any need to know much about natural history and macroevolution. It also relates to "blind" faith in molecular phylogenies, that is, with little capacity for critically evaluating these studies (such as by morphologica implications). As a result, plain artifacts of the tree become the basis for many weird hypotheses (I have argued before this is happening right now, with new supposed clades such as "urochordates+vertebrates"). Studies continue to emerge where well-established facts of natural history are swept aside in favor of some "groundbreaking hypothesis".
Many Evo-Devos have thought that macroevolution is some kind of truly radical "body plan" change with mechanisms quite different from those observed at a microevolutionary level, appealing to some mysterious happenstance of the past for the origin of phyla or higher "grades". However, Evo-devo's are slowly wisening up to the fact that macro and micro evolutionary change proceeds pretty much in the same way, with the simple fact being that lineages diverging earlier can accumulate greater differences (as pointed out before)

martes, junio 17, 2008

Answering Cuvier: Notes on the systemic/historical nature of living beings

Más literatura subversiva y underground; por fin tenemos el trabajito de Cecchi, Vargas, Villagra y Mpodozis 2004 en pdf (donde siempre!). Releyendo esta joyita (a la cual sólo contribuí afinando el inglés y con modestas pizcas de aliño) surge una bonita reflexión: ¿Qué tan diferente es el desafío de entender el desarrollo al de entender la evolución? Hay muchas cosas en común: La constante coherencia interna y con el medio, a través de un proceso radical de transformación biológica que incluye aumentos de complejidad. Incluso, el desarrollo puede librarse de algunas presunciones que son adhosadas incorrectamnete a la evolución; por ejemplo no es muy defendible que el desarrollo sea un proceso de cada vez mejor ajuste al medio; más bien, contemplando un ciclo de vida, es evidente que cada etapa del desarrollo está ajustada a su medio. En efecto, ambos evolución y desarrollo son procesos de deriva sistémica-histórica, con continua participación del medio y en conservación de la adaptación.



Mientras la visión estructuralista de Cecchi et al. lleva a reconocer a desarrollo y evolución como procesos del mismo tipo, la visión ortodoxa, en vez de preocuparse por entender el desarrollo (la VERDADERA relación genotipo-fenotipo), se contentan con una metáfora de que el fenotipo está programado en el genotipo tal que genotipo = fenotipo. Con este ardid reduccionista, el adaptacionista darwiniano se ha convencido a sí mismo de que no es necesario entender de desarrollo para entender de evolución.

Tsk,tsk.

Cecchi C, Vargas A, Villagra C, Mpodozis J. 2004 Answering Cuvier: Notes on the systemic/historical nature of living beings. Cybernetics and human knowing 11(4): 1-19

jueves, junio 12, 2008

contingencia histórica waaaa

Historical contingency and the evolution of a key
innovation in an experimental population of
Escherichia coli


Zachary D. Blount, Christina Z. Borland, and Richard E. Lenski*
Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI 48824
This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on April 25, 2006.
Contributed by Richard E. Lenski, April 9, 2008 (sent for review March 26, 2008)


The role of historical contingency in evolution has been much
debated, but rarely tested. Twelve initially identical populations of
Escherichia coli were founded in 1988 to investigate this issue. They
have since evolved in a glucose-limited medium that also contains
citrate, which E. coli cannot use as a carbon source under oxic
conditions. No population evolved the capacity to exploit citrate
for >30,000 generations, although each population tested billions
of mutations. A citrate-using (Cit) variant finally evolved in one
population by 31,500 generations, causing an increase in population
size and diversity. The long-delayed and unique evolution of
this function might indicate the involvement of some extremely
rare mutation. Alternately, it may involve an ordinary mutation,
but one whose physical occurrence or phenotypic expression is
contingent on prior mutations in that population. We tested these
hypotheses in experiments that ‘‘replayed’’ evolution from different
points in that population’s history. We observed no Cit
mutants among 8.4 1012 ancestral cells, nor among 9 1012 cells
from 60 clones sampled in the first 15,000 generations. However,
we observed a significantly greater tendency for later clones to
evolve Cit, indicating that some potentiating mutation arose by
20,000 generations. This potentiating change increased the mutation
rate to Cit but did not cause generalized hypermutability.
Thus, the evolution of this phenotype was contingent on the
particular history of that population. More generally, we suggest
that historical contingency is especially important when it facilitates
the evolution of key innovations that are not easily evolved
by gradual, cumulative selection.


PNAS  June 10, 2008  vol. 105  no. 23  7899–7906


www.pnas.org/cgi/doi/10.1073/pnas.0803151105

Sorry el desorden

miércoles, junio 11, 2008

Looking for Darwin in all the wrong places

Looking for Darwin in all the wrong places: the misguided quest for positive selection at the nucleotide sequence level

AL Hughes

Heredity (2007) 99, 364–373

Recent years have seen an explosion of interest in evidence for positive Darwinian selection at the molecular level. This quest has been hampered by the use of statistical methods that fail adequately to rule out alternative hypotheses, particularly the relaxation of purifying selection and the effects of population bottlenecks, during which the effectiveness of purifying selection is reduced. A further problem has been the assumption that positive selection will generally involve repeated amino-acid changes to a single protein. This model was derived from the case of the vertebrate major histocompatibility complex (MHC), but the MHC proteins are unusual in being involved in protein–protein recognition and in a co-evolutionary process of pathogens. There is no reason to suppose that repeated amino-acid changes to a single protein are involved in selectively advantageous phenotypes in general. Rather adaptive phenotypes are much more likely to result from other causes, including single amino-acid changes; deletion or silencing of genes or changes in the pattern of gene expression.

Heredity (2007) 99, 364–373; doi:10.1038/sj.hdy.6801031;
published online 11 July 2007

Podrán encontrar el pdf en el grupo yahoo...

miércoles, junio 04, 2008

Onde fica a competição?

Fico pensando como é absurda essa noção de um mundo competitivo com organismos carregando consigo suas preciosas mutações nucleotídicas, parecendo um jogador de futebol americano correndo com sua bola escondida debaixo do braço e batendo cabeça com seus oponentes.

Que isso não é assim em bactérias, que há uma imensa transferencia horizontal de genes, já é bem documentado na literatura. Não se pode aceitar a idéia de que "eu guardo minhas mutações pra mim e venço você".

Agora há mais um exemplo muito interessante e curioso mostrando como essa situação se passa no viver de organismos metacelulares - rotíferos bdeóides, invertebrados aquáticos que podem sobreviver a períodos de dissecação.

Vejam só que curioso o que passa!

Science 320, 1210–1213 (2008)
Many single-celled organisms collect genes from other organisms — a process known as horizontal gene transfer — but multicellular organisms tend not to. Now tiny invertebrates called bdelloid rotifers have been found to take on genetic material from a range of other species, including bacteria, fungi and plants.
Multicellular creatures rarely do this because their germ line is sequestered in the gonads, explain Eugene Gladyshev, Matthew Meselson and Irina Arkhipova at Harvard University in Massachusetts. Bdelloid rotifers are different. They often experience desiccation, potentially opening up their cell membranes to chunks of outsider DNA. This unusual way of injecting diversity into their genomes may help to explain why these rotifers have gone 40 million years without sex.


abraços,
Gustavo