Thursday, June 25, 2009

Aphid Juice Bar

I did not recognize her. She looked nothing like her mom, not her dad either. During our first introduction she barely looked up as she sucked her little victim dry, its legs a-flailing.

I guess it takes a while to learn manners but soon enough, she'll affix herself to the leaf and begin her metamorphosis - into a ladybug! But, as it turns out, apparently she won't leave behind her predatory predilections then either. And until that day when these larval ladybugs pupate they will remain here, popping-off aphid bonbons.

Aphids are insects that derive their nutriment from the contents of their host plant phloem. Phloem is the plant vascular tissue that transports sugars derived from photosynthesis from the leaves down through the plant and into the roots. Just near the phloem lies the xylem, which is full of water that is being drawn up from the root hairs and out through the leaves by evaporation through tiny openings in the leaf surface - the stomata.

While aphids are efficient harvesters of phloem fluids - sap - they face the dilemma that while sap is available in abundance it is not very rich in an important subset of molecules necessary for life: essential amino acids (these are just a set of building blocks that are all built exactly the same except that they differ from each other by one side chain; when strung together they form polymers called proteins and these are essential for the basic functioning of any cell). This is a similar problem as is faced by any organism that relies heavily on plant based material for its diet (like termites, shipworms, and vegetarians).

And, to every gardeners initial chagrin, the aphids have apparently managed to acquire the essential amino acids afterall - by outsourcing. While the aphids themselves do not have the tools (the enzymes, which are made of proteins!) encoded in their genes they do have a business partner that does: Buchnera aphidicola.

B. aphidicola is a bacteria that has been working with aphids for so long (millions of years) that it no longer has the genes necessary for it to survive on its own in the world. It doesn't matter because without B. aphidicola the aphid reproductive rates plummet and they can't acquire the necessary nutrients from their obligate sap diet... and so the aphid is obliged to maintain them. Every aphid, as she produces her batch of mini-me clones (by parthenogenesis: the development of unfertilized eggs into adult females), transfers bacterial cells to the egg (direct mother-child transfer of microbes is called vertical transmission).

So, aphids pierce the phloem and let the sap flow through them and, in part, out of them (this is the 'honeydew' collected by some ants ). And as the fluid passes through them they sequester the rarer nitrogen compounds and transfer them to their symbiont bacteria for processing. The bacteria have the genes to build essential amino acids needed by the aphid and produce them in response to varying concentrations of compounds in the sap consumed.

Thus it seems the aphid is set to suck every plant dry and proliferate with abandon. But, as I observed, with some horror I admit, there are predators that can search out these patches of aphids and winnow them down from armies to pitiful doomed patches.

How then, does a ladybug know where to put her larvae? Here you must begin to suspect that quiet character in the background - the plant. Not only is the plant offering enticements in the way of nectar in exchange for facilitating its sex (fuzzy pollinators carry pollen from the male floral organs - the stamens - of one plant to the the female floral organs - the carpels - of another plant) it is also sending off puffs of chemicals to alert insect predators to the presence of available prey. Herbivorous prey items causing cellular damage to plant surfaces by chewing on leaves or poking stylets into vascular tissue cause chemical reactions that produce volatile chemicals that can be detected by flying insects and used to localize prey. The plants are calling for help.

Some research has suggested that the plants can use these volatiles not only to facilitate insect recruitment but to induce the production of deterrent chemicals in neighboring trees that have receptors for the volatiles and respond to their binding by expressing genes involved in the pathways that make chemicals distasteful to potential herbivores, it has been called the phenomenon of 'The Talking Trees'.

Some Questions Answered and Some Questions for Later

Are there other neat Aphid Symbiont stories?

Yes, there are!

>>>The presence of another bacteria often associated with aphids, Regiella insecticola, is associated with higher resistance to a fungal insect pathogen. Not only does the presence of the bacteria protect the individual, but its presence in the dead remains of aphids (presumably sucked dry ladybug larvae) protects against fungal colonization and production of spores that might spread out to surrounding aphids. (Scarborough 2005 Science)

>>>Changes in Buchnera aphidicola's genome affect the heat tolerance of their host aphids! Two alleles (versions) of the gene for the promoter of a small heat shock protein (ibpA) are maintained in Buchnera populations. One version has a deletion and has lost its functionality as a heat shock promoter. When it is cooler and the aphids haven't experienced any thermal stress those aphids containing Buchnera with the deletion have higher reproductive rates, but in those populations that do experience a heat shock the survival of aphids containing Buchnera plummets. (Dunbar 2007 PLOS Biology)

Are there other neat higher-order interaction stories?

Of course!

>>>Grasses (Lolium perenne) with a fungal endosymbiont (Neotyphodium lolii) produce alkaloid mycotoxins. Ladybug larvae eating aphids that suck the alkaloid containing sap of these grasses take longer to develop into adults and produce fewer offspring ladybugs when they eventually do mature. (de Sassi Proceedings of the Royal Society B) Why aren't the aphids protected, you ask? Perhaps by adapting to tolerate these compounds these aphids have been able to outcompete other herbivores? Perhaps, given time, the plants will produce a novel compound that suddenly affects these aphids....

On what species of plant are the aphids & ladybugs in my pictures living?

>>> I don't know! Maybe someone can comment me the answer? Thank you!

What is the function of the cornicles - those two spines on the aphid posterior?

>>> This one is for later.

Are ladybug larvae distasteful to birds? They sure stand out on those leaves!

>>> And this one too.

Some Links

Antje Schulte's photo story of Ant herders and a "huge predator who's eating their 'cows'. "
... and her blog here on Blogspot, Four Feet and More

The kids story of how the ladybug became the Massachusetts state insect.

Some Papers

Nitrogen fixation in marine shipworms
(Carpenter 1975 Science)

Fungal plant endosymbionts alter life history and reproductive success of aphid predator
(de Sassi 2006 PRS-B)

Aphid thermal tolerance is governed by a point mutation in bacterial symbionts
(Dunbar 2007 PLOS Biology)

Behavioural side-effects of insecticide resistance in aphids increase their vulnerability to parasitoid attack
(Foster 2007 Animal Behaviour)

Aphid protected from pathogen by endosymbiont
(Scarborough 2005 Science)

A fragile metabolic network adapted for cooperation in the symbitic bacterium Buchnera aphidicola
(Thomas 2009 BMC Systems Biology)

Methyl salicylate, a soybean aphid-induced plant volatile attractive to the predator Coccinella septempunctata
(Zhu 2005 Journal of Chemical Ecology)

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