Friday, May 21, 2010

Mrs. Nonaka's garden

Mrs. Nonaka's garden — which she prefers to call her backyard — is a quiet shady oasis in Silicon Valley. Her book club enjoys it for her food and the Chardonnay … and to discuss books; while entrepreneurs like to congregate there for the nonchalant atmosphere and the fusion cuisine served from the grill. Under the trees, the garden is surrounded by rhododendron in various degrees of insolation, so there almost always are some in bloom.

Unfortunately, not all is well with Mrs. Nonaka's rhododendron. When it wakes up from winter, the leaves start becoming increasingly whitish.

The explanation is under the leaves:

There, white critters suck out the sap from the leaves, leaving brownish spots. So, every year she straps the sprayer on her husband's back, who washes down the rhododendron with insecticidal soap.

In the pictures above, the critters have already been washed away, but they are yellow-whitish. They do stand out against the green leaves, but either have no natural predators in Mrs. Nonaka's garden, or they are camouflaged in their predator's visual response, or their color warn (aposematism) chasing them away, as we saw in the post from Sidney.

A propos critters sucking the sap out of leaves, consider the pea aphid Acyrthosiphon pisum shown in the picture below. It comes in a brownish to reddish variant attacked preferentially by lady beetles and a greenish or yellowish variant into which parasitoid wasps deposit eggs.

Acyrthosiphon pisum

According to the classical view on the evolutionary ecology of animal color polymorphism, it can be hypothesized that these opposite predation and parasitism pressures maintain the color variation in natural aphid populations.

But where does the red color come from? As the flamingo painter in the Color Selection video in the 1990 ACM SIGGRAPH Video Review, Mrs. Nonaka knows very well that although chlorophyll is related to carotenoids, the aphids would have to feed on something with red carotenoids, such as torulene. This is because animals cannot make their own carotenoids, forcing color scientists to eat lots of carrots to keep their eyes in top performance.

The riddle has a very non-obvious explanation, that is revealed in a recent Science paper by University of Arizona, Tucson's Nancy Moran and Tyler Jarvik: Lateral Transfer of Genes from Fungi Underlies Carotenoid Production in Aphids.

The recently published draft genome sequence of A. pisum provided a crucial clue to the origin of the aphid carotenoids. Phylogenetic analysis revealed that, strikingly, all the aphid carotenoid synthetic genes clustered with fungal genes with high statistical confidence. Genomic features of these genes strongly suggested that they were transferred from a fungus to an aphid ancestor, and subsequently subjected to duplications in the aphid genome.

So far, this is the first animal of which we know it is able to make its own carotenoids.

By the way, phylogenetic analysis is a tool also used by co-blogger Nathan for his research in color science.

Phylogenetic relations of inferred carotenoid biosynthetic enzymes from the pea aphid genome