By Grant Usick
At first glance the pea aphid looks like your average insect. Little, green, and something your mother curses in the garden. Recent sequencing of its genome (The International Aphid Genomics Consortium 2010) has revealed a very different tale involving the co-evolution of two completely unrelated organisms.
Carotenoids are naturally occurring pigments produced in photosynthesizing plants, algae, bacteria and… aphids? These pigments are responsible for the varying coloration found in organisms on Earth. The most common being the orange carrot that owes its bright color to the carotenoid carotene. For pea aphids, carotenoids display green, a darker red/orange, or a yellowish/white color when environmental conditions aren’t as favorable as carotenoid synthesis is a taxing process.
Pea aphids set themselves apart from the rest of the insects and all other animals, except the two-spotted spider mite, in their ability to produce carotenoids on their own (Altincicek et al. 2012). All the credit cannot be given to the pea aphid alone though. It and all other modern aphids can thank an ancient common ancestor who lived alongside a fungus and received the necessary genes for the synthesis of carotenoids through a process known as horizontal gene transfer. This is defined as the transfer of genes from one individual to another by means other than reproduction, such as conjunction. Conjunction is where genes are transferred between two bacteria cell membranes uncoupled by cell division rather than vertical gene transfer, from mother to daughter cell. Phylogenetic analysis done by Moran and Jarvik (2010) supports this with many similarities between the structure of genes found in modern fungus and the bacterial symbionts of aphids.
Sequencing of the pea aphid’s genome has revealed insight into its many interesting biological features, such as how one genotype can produce winged and non-winged individuals by either sexual or asexual reproduction. The ability to capture energy from the sun was an unexpected addition to that list. The carotenoids form a thin layer underneath the skin of the back of the insect, positioning it perfectly to capture sunlight. True photosynthesis is not achieved, as the fixation of carbon dioxide to produce organic molecules does not occur. It is thought of as a more primitive form of photosynthesis. Research done by Jean Valmalette et al. (2012) found significantly higher levels of ATP (energy currency in organisms) in green colored aphids compared to orange. Pigments were extracted and absorbance tests revealed a striking difference between the level of absorbance of orange and green pigments, showing much higher levels of absorbance in green colored aphids.
This is a peculiar finding that seems to raise a lot more questions than it answers, like why would an insect with a diet rich in carbohydrates waste its energy on expensive carotenoids to capture the more energy from the sun? One theory suggests that the energy captured acts as a store to be later utilized while traveling to a new host plant. There are many questions that remain unanswered, and the little pea aphid continues to surprise.
Altincicek, B., Kovacs, J.L., Gerarda, N.M., (2012). Horizontally transferred fungal carotenoid genes in the two-spotted spider mite Tetranychus urticae. Biol Lett. 2: 253-257.
Moran, N.A.., Jarvik, T., (2010). Lateral transfer of genes from fungi underlies carotenoid production in aphids. Science. 328: 624-627.
The International Aphid Genomics Consortium, (2010). Genome sequence of the pea aphid Acyrthosiphon pisum. PLoS Biol(2): e1000313. Doi:10.1371/journal.pbio.1000313
Valmalette, J.C., Dombrovsky, A., Brat, P., Mertz, C., Capovilla, M., Robichon, A., (2012). Light-induced electron transfer and ATP synthesis in carotene synthesizing insect. Scientific Reports. 2 Article number: 579. doi:10.1038/srep00579