miércoles, septiembre 05, 2007

One classic “tradeoffs” of sexual reproduction is the risk of acquiring sexual transmitted diseases. But if we now consider that many microbial and viruses associations survive or have been integrated inside the eukaryotic organism (ie “mutualistic”), this allow us to consider sexual contact as a chance to acquire symbiotic associates. In aphids symbiotic bacteria are inherited maternally, this has been demonstrated to be very conservative in parthenogenetic populations. One way to acquire the symbiont is by ingesting it in diets or by microinjections. Alternatively during sexual reproduction, male-borne symbionts can be acquired by females and subsequently transferred to sexually or parthenogenetic generated offspring. This transference is stable trough generations and can establish inheritable acquired characters such as defense towards natural enemies or resistance to heat.

This is the case of the worldwide distributed pea aphid Acyrthosiphon pisum (Hemiptera: Aphididae). In this insect, besides the main Buchnera aphidicola simbiont (that is required for “normal” development and reproduction), other three bacterial symbionts may be present: Candidatus “Hamiltonella defense”, Candidatus “Serratia symbiotica”, and Candidatus “Regiella insecticola” (all members of Enterobacteriaceae). Different aphid population can present from zero to all of those bacterias. Aphids (in their native habitats) usually have all female parthenogenetic reproduction from spring to autumn, then a single generation of sexually reproducing offprings is born (this is related with the shortening of the photoperiod), as a result of copulation these females will lay overwintering eggs and restart the cycle of parthenogenetic reproduction (Fig.1).

Figure 1: Schema of aphid life cycle using A. pisum as example

During copulation, male insects transfer sperm as well as other substances produced from accessory glands. In this, sexually transmitted viruses and bacteria can be transferred to the female. In the case of the pea aphids symbionts present in the male aphid gonads (Fig. 2) R. insecticola can be transferred to the female by the seminal fluid. This has been tested simulating the sexual reproduction season in laboratory strains with and without the different bacterial symbionts and it was demonstrated that copulation between a male carrier with a non infected female can result in a venereal horizontal transmission of the bacterial symbionts! (Moran & Dunbar, 2006).Thus the female and the following parthenogenetic strain generated acquire the symbiotic association.

Interestingly, the acquired association between the aphid and the different Enterobacteriaceae could generate changes in the organism relation with the environment such as the acquired ability to survive parasitoid wasp infections, fungal pathogens, and expansions in the insect niche thanks to acquired ability to use other host plant or to become resistant to a broader range of temperature.
This show again the relevance of understands the organism as a creative onto-phylogenetic dynamic process of development. Better than to focus only on the phenotypic filter as a motor of evolutionary processes.

Figure 2: Localization of the symbionts within the male reproductive system (from Moran & Dunbar, 2006) by using FISH probe matching the 16S rRNA sequence of R. insecticola (green and bright yellow) and with propidium iodide DNA counterstain (red), (Scale bars, 0.1 mm.).



Moran NA & Dunbar HE (2006) Sexual acquisition of beneficial symbionts in aphids. PNAS, vol. 103, 34, 12803–12806.

8 comentarios:

Sanders dijo...

What can I say... an epigenetic mechanism of inheritence for acquired adaptations. Clear and simple.
Impressive stuff

Anónimo dijo...

There is one thing that I would like to stress, is that by the acquisition of the symbionts the asociation can change its phylogenetic plasticity by means, for example, expanding the host use...


Chico dijo...

Estes exemplos de transferência sexual de simbiontes são ótimos. Mas alguns autores tentam encaixá-los em uma explicação selecionista (e.g. Sterelny et al.). Como o simbionte é transferido apenas para os descendentes, ela seria uma mutação benéfica.
Mas este tipo de explicação não funciona em transferências horizontais e ontogenéticas de simbiontes. Os pogonophoras são um exemplo. Estes anélídeos altamente modificados, sem trato digestivo, adquirem as bactérias quimiossintetizantes das quais dependem para se alimentar dos seus vizinhos, durante a ontogenia.
Não há egoísmo. Todos dividem a "infecção". Por milhares de anos!!!

Sterelny, K., K. C. Smith e M. Dickison. The extended replicator. Biology and Philosophy, v.11, n.3, p.377-403. 1996.

Nussbaumer, A. D., C. R. Fisher e M. Bright. Horizontal endosymbiont transmission in hydrothermal vent tubeworms. Nature, v.441, n.7091, p.345-348. 2006.

Sanders dijo...

Está muy buena la imagen del título..jaja

Sanders dijo...

Si la infección permite ampliar el rango de hospederos, se amplia el rango de psibles interacciones epigenéticas con nuevos hospederos.
Tiene sentido que haya evolución de nuevas especies...Se ha observado que aumente la especiación como resultado de este tipo de infecciones?

Chico dijo...

Com Wolbachia há especiação, porque há incompatibilidade na fertilização.

Cristian dijo...

La especiacion a partir de cambios de hospederos ha sido el ejemplo clasico para demostrar especiacion simpatrida (Bush, 1969). Trabajos recientes han mostrado que, por ejemplo en el caso de las moscas tephritidae, el cambio a un hospedero nuevo, por ejemplo una planta introducida, puede generar especiacion por hibridizacion de las variantes hospedero-especificas!, generando una nueva especie en la planta introducida...

Ahora los continuos ejemplos de simbiosis me hacen pensar que tanto para las relaciones planta-insecto o insecto-bacteria podrian operar con una dinamica en comun, que explique novedad evolutiva...

Cristian dijo...

Aqui va un par de citas, a quien les interese se las mando..

Guy L. Bush (1969) Sympatric Host Race Formation and Speciation in Frugivorous Flies of the Genus Rhagoletis (Diptera, Tephritidae) Evolution, Vol. 23, No. 2. (Jun.,), pp. 237-251.

Dietmar Schwarz, Benjamin M. Matta1 Nicole L. Shakir-Botteri & Bruce A. McPheron (2005) Host shift to an invasive plant triggers rapid animal
hybrid speciation. Nature,436:28.pp 546-549