domingo, julio 27, 2008

Phyloepigenetics IV: Lagartija y nemátodo

Herrel et al 2008. Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource. Proc Natl Acad Sci U S A. Mar 25;105(12):4792-5.


"In 1971 five adult pairs of this species were moved from the small islet of Pod Kopiste (0.09 km2) to the nearby Pod Mrcaru (0.03 km2) by Nevo and coworkers (...). Although the islet of Pod Mrcaru was originally inhabited by another lacertid lizard species (Podarcis melisellensis), repeated visits (twice yearly over the past three years, beginning in 2004) show that this species has become extinct on Pod Mrcaru. Genetic mitochondrial DNA analyses indicate that the lizards currently on Pod Mrcaru are indeed P. sicula and are genetically indistinguishable from lizards from the source population"

"Differences in head size and shape also translate into significant dif ferences in bite force bet ween populations. Our data show that P. sicula lizards consume more plant material on Pod Mrcaru compared with the ancestral population on Pod Kopiste"

"This shift to a predominantly plant-based diet has resulted in the dramatic evolution of intestinal morpholog y. Morphological analysis of preserved specimens shows the presence of cecal valves (Fig. 4) in all individuals, including a hatchling (26.4-mm snout-vent length, umbilical scar present) and a very young juvenile (33.11-mm snout-vent length) examined from Pod Mrcaru."

"The fact that 1% of all currently known species of squamates have cecal valves (13, 14) illustrates the unusual nature of these structures in this population"

"Cecal valves slow down food passage and provide for fermenting chambers, allowing commensal microorganisms to convert cellulose to volatile fatt y acids (15, 16). Indeed, in the lizards f rom Pod Mrcaru, nematodes were common in the hindgut but absent from individuals f rom PodKopiste"

"Because of the larger food base available and the increase in the predict abilit y of the food source, lizard densities on Pod Mrcaru are much greater (..) lizards on Pod Mrcaru do no longer appear to defend territories. Moreover, changes in foraging style (browsing versus active pursuit of mobile prey) and social structure may also have resulted in the dramatic changes in limb proportions and maximal sprint speed previously documented for this population"

"Although the presence of cecal valves and large heads in hatchlings and juveniles suggests a genetic basis for these differences, further studies investigating the potential role of phenotypic plasticity and/or maternal effects in the divergence bet ween populations are needed"

Herrell no discute mucho qué tan relevante puede ser la simbiosis con un nemátodo. Veamos un ejemplo de anfibios

Effects of the nematode Gyrinicola batrachiensis on development, gut morphology, and fermentation in bullfrog tadpoles (Rana catesbeiana): a novel mutualism

Gregory S. Pryor *, Karen A. Bjorndal J. Exp. Zool. 303A:704-712, 2005.


We describe a novel mutualism between bullfrog tadpoles (Rana catesbeiana) and a tadpole-specific gastrointestinal nematode (Gyrinicola batrachiensis). Groups of tadpoles were inoculated with viable or nonviable nematode eggs, and development, morphology, and gut fermentation activity were compared between nematode-infected and uninfected tadpoles. Nematode infection accelerated tadpole development; the mean time to metamorphosis was 16 d shorter and the range of times to metamorphosis was narrower in nematode-infected tadpoles than in uninfected tadpoles. At metamorphosis, infected and uninfected bullfrogs did not differ in body size or condition. Colon width, wet mass of colon contents, and concentrations of most fermentation byproducts (short-chain fatty acids: SCFAs) in the hindgut were greater in infected tadpoles. Furthermore, in vitro fermentation yields for all SCFAs combined were over twice as high in infected tadpoles than in uninfected tadpoles. One explanation for accelerated development in infected tadpoles is the altered hindgut fermentation associated with the nematodes. Energetic contributions of fermentation were estimated to be 20% and 9% of the total daily energy requirement for infected and uninfected tadpoles, respectively. Infection by G. batrachiensis nematodes potentially confers major ecological and evolutionary advantages to R. catesbeiana tadpoles. The mutualism between these species broadens our understanding of the taxonomic diversity and physiological contributions of fermentative gut symbionts and suggests that nematodes inhabiting the gut regions of other ectothermic herbivores might have beneficial effects in those hosts.

Si bien algunos nemátodos son parásitos en reptiles, otros no lo son:

Oecologia. 2006 Dec;150(3):355-61. Epub 2006

O'Grady SP, Dearing MD. Isotopic insight into host-endosymbiont relationships in Liolaemid lizards

Nitrogen isotopes have been widely used to investigate trophic levels in ecological systems. Isotopic enrichment of 2-5 per thousand occurs with trophic level increases in food webs. Host-parasite relationships deviate from traditional food webs in that parasites are minimally enriched relative to their hosts. Although this host-parasite enrichment pattern has been shown in multiple systems, few studies have used isotopic relationships to examine other potential symbioses. We examined the relationship between two gut-nematodes and their lizard hosts. One species, Physaloptera retusa, is a documented parasite in the stomach, whereas the relationship of the other species, Parapharyngodon riojensis (pinworms), to the host is putatively commensalistic or mutualistic. Based on the established trophic enrichments, we predicted that, relative to host tissue, parasitic nematodes would be minimally enriched (0-1 per thousand), whereas pinworms, either as commensals or mutualists, would be significantly enriched by 2-5 per thousand. We measured the (15)N values of food, digesta, gut tissue, and nematodes of eight lizard species in the family Liolaemidae. Parasitic worms were enriched 1+/-0.2 per thousand relative to host tissue, while the average enrichment value for pinworms relative to gut tissue was 6.7+/-0.2 per thousand. The results support previous findings that isotopic fractionation in a host-parasite system is lower than traditional food webs. Additionally, the larger enrichment of pinworms relative to known parasites suggests that they are not parasitic and may be several trophic levels beyond the host.

Correlating diet and digestive tract specialization: Examples from the lizard family Liolaemidae

Shannon P. O’Gradya, Mariana Morandob, Luciano Avilab and M. Denise Dearinga
Zoology 2005, 108 : 201-210


A range of digestive tract specializations were compared among dietary categories in the family Liolaemidae to test the hypothesis that herbivores require greater gut complexity to process plant matter. Additionally, the hypothesis that herbivory favors the evolution of larger body size was tested. Lastly, the association between diet and hindgut nematodes was explored. Herbivorous liolaemids were larger relative to omnivorous and insectivorous congeners and consequently had larger guts. In addition, small intestine length of herbivorous liolaemids was disproportionately longer than that of congeners. Significant interaction effects between diet and body size among organ dimensions indicate that increases in organ size occur to a greater extent in herbivores than other diet categories. For species with plant matter in their guts, there was a significant positive correlation between the percentage of plant matter consumed and small intestine length. Herbivorous liolaemids examined in this study lacked the gross morphological specializations (cecum and colonic valves) found in herbivores in the families Iguanidae and Agamidae. A significantly greater percentage of herbivorous species had nematodes in their gut. Of the species with nematodes, over 95% of herbivores had nematodes only in the hindgut. Prevalence of nematodes in the hindgut of herbivores was 2× that of omnivores and 4× that of insectivores.

Los dichosos nemátodos se encuentran en todas las especies de reptil que tienen válvulas cecales


Todo este cambio, en sólo 34 años.... es acaso una acumulación por selección direccional de varios genes? Grano fino, o grano grueso?

Cuánto de este cambios fenotípico drástico se debe más bien al efecto inmediato de diferentes condiciones epigenéticas, como la mentada asociación con el nemátodo?

4 comentarios:

oilbird dijo...

me gusta esto de las "Phyloepigenetics"
ayer no mas estuve leyendo este paper en "Evolution and Development":

Temperature-dependent plasticity of segment number in an arthropod
species: the centipede Strigamia maritima

, y creo es un bonito y simple ejemplo de Phyloepigenetics.
en este paper encuentran una especie de centipedo (creo, un cienpies) y ven que el numero de segmento varia directamente con la temperatura durante la embriogenesis. lo interesante es que tambien encuentran un gradiente latitudinal de el numero de segmentos dentro de la especie, donde los que viven mas al sur tienen mas segmento que los de mas al norte. ahora donde entra la Phyloepigenetics es que ademas hay un gradiente latitudinan del numero de segmentos dentro de la familia de estos bichos, Geophilomorpha, en donde especies que viven en lugares mas calidos tienen mas segmentos. es decir la plastisidad del rasgo no solo explica la variabilidad dentro de la especies si no que probablemente explica la variabilidad dentro de el clado. lo mas parsimonioso seria suponer que la especie que dio origen a todos en el clado tenia una respuesta plástica del numero de segmentos a la temperatura y despues se fijo en las distintas especies.(esto por que la mayoría de las especies no tienen plasticidad en el numero de segmentos)
el otro punto que me parece interesante es que en este caso la temperatura afecta directamente la morphogenesis y no algo como el crecimiento. en estos bicho el mayor numero de segmentos no afecta el largo, mas segmentos solo significa segmentos mas cortos.


A. Vargas dijo...

JOSHA de phyloepigenetics puro, chinasky!!!

A. Vargas dijo...

"Moreover, changes in foraging style (browsing versus active pursuit of mobile prey) and social structure may also have resulted in the dramatic changes in limb proportions and maximal sprint speed previously documented for this population"

nematodo cambia dieta, dieta cambia conducta, conducta cambia morfología...

todo ese cambio no requiere mutación genética alguna...phyloepigenetics pura

De hecho en este blog estamos discutiendo casos de phyloepigenetis desde antes de inventarle el nombrecito...

A. Vargas dijo...

Ese paper de los cienpiés está para chuparse los dedos... más encima que el aspecto adaptativo del rasgo de tner más o menos segmentos, sí como si cambia o no la respuesta del medio...como tantos casos, sólo queda encogerse de hombros. Puede proveer un caso excelente de asimilación genética de un rasgo no-adaptativo.