Maintaining Biological Cultures and Measuring Gene Expression in Aphis nerii: A Non-model System for Plant-insect Interactions

This method describes general steps for aphid culturing and steps for the genetic analysis of aphids. These methods can be applied to answer key questions in the evolution and ecology of aphids and the moleculare mechanisms underlying aphid biology. The main advantage of this technique is it can be generally applied to most aphid species and performed at a relatively low cost.

Begin by filling a standard seedling tray with germination mix soil, taking care that the soil reaches the top of the wells. When all of the wells have been filled, make a three centimeter deep hole in the soil of each well and place one seed into each hole. Water the cells until the soil covering the seeds is saturated and place the tray in a green house with daily to every other day watering, to maintain a moderate soil moisture level.

When the seedlings have grown their first set of full leaves fill four inch round pots with potting soil, up to about five centimeters below their rims. Create a whole in the soil deep enough to reach the bottom of the pot and gently scoop the mature seedlings by hand to place them deep within the holes in the new pots. Then, cover the seedlings with soil and water the seedlings daily to every other day to maintain a moderate soil moisture level.

When the plants have grown at least three to four sets of full leaves and are at least ten centimeters tall, manually inspect the plants for any unwanted pests and use a mouth pipette to carefully transfer a single reproducing field collected adult aphid onto the leaf of the first plant to create an iso-clonal line. When all of the aphids have been transferred, securely cover the plants with a custom made cup cage, and place the aphid infested plants in a controlled environment chamber. To maintain the stock populations, use a mouth pipette to safely transfer one to three second or third instar nymphs, and one adult aged aphid, to a new plant on a weekly basis.

And cover each plant with a new cup cage for culture as demonstrated. For DNA extraction from a single adult aphid, place the aphid near the bottom of a sterile 1.5 milliliter microcentrifuge tube containing liquid nitrogen and grind the aphid with a pestle. Next, add 100 microliters of lysis buffer to the microcentrifuge tube and continue to grind the crushed aphid until the sample is visibly disintegrated.

Wash the pestle with an additional 100 microliters of lysis buffer, and incubate the ground aphid at 65 degrees Celsius for 30 minutes. At the end of the incubation add 14 microliters of aemulor potassium acetate to the tube and invert to mix. After 30 minutes on ice collect the cellular debris by centrifugation and transfer the supernatant to a new 1.5 milliliter microcentrifuge tube without disturbing the pellet.

Add 200 microliters of cold 100 percent molecular grade ethanol to the supernatant and invert the tube to mix. After 15 minutes at room temperature, collect the precipitated DNA by centrifugation, and remove the ethanol by pipette. Resuspend the pellet in 200 microliters of cold 70 percent molecular grade ethanol with flicking for a second centrifuge wash.

And resuspend the pellet in 200 microliters of fresh, cold 100 percent molecular grade ethanol. After the third centrifugation, place the tube horizontally on a piece of tissue paper for air drying for five to ten minutes. And resuspend the DNA pellet in 80 microliters of low tris edta.

Then, quantify the resuspended DNA in a spectrophotometer, and store the remaining sample at four degrees Celsius. After crushing up to five adult aphids in liquid nitrogen as demonstrated, transfer the sample to a fume hood and add 800 microliters of guanidinium thiocyanate-phenol-chloroform extraction reagent to the sample tube. Homogenize the sample with a pestle and incubate the homogenate for five minutes at room temperature.

Next, add 160 microliters of chloroform to the sample, followed by 15 seconds of vigorous shaking by hand. Then, incubate the sample for two to three minutes at room temperature and centrifuge to collect the extracted RNA. To precipitate the RNA, transfer the aqueous phase to new RNS free microcentrifuge tube without disturbing the intermediate phase, and add 400 microliters of isopropanol to the RNA.

After a ten minute incubation at negative 20 degrees Celsius, collect the precipitated RNA by centrifugation. Then, wash the sample two times with one milliliter of 75 percent ethanol in diethyl pyrocarbonate treated water, to the sample with slow vortexing and centrifugation to remove any phenol contaminants. After the second wash, mostly air dry the pellet for five to ten minutes on a sterile bench, and dissolve the RNA pellet in 30 microliters of ultra pure water with gentle pipetting before a ten to 15 minute incubation at 55 to 60 degrees Celsius.

Seeds will take approximately two to four weeks, depending on the season, to grow large enough to be repotted. Repotted seedlings will take another two to four weeks to grow to an optimal size for aphid culture. Adult aye-near-ee-eye are distinguished by a darkened cauda, and may be unwinged or winged.

Developing wing pads become visible when nymphs reach the third instar. Single adult aye-near-ee-eye will yield approximately 100 to 200 nanograms per microliter of DNA and 150 to 300 nanograms per microliter of RNA. The relative expression of a candidate gene under different treatment conditions can then be calculated.

When attempting this procedure it's important to not contaminate you're iso-clonal stocks. Also, remember that the quality of your aphids is highly dependent upon the quality of your plants. Following this procedure, other extractions and sequencing techniques can be performed to answer question about the molecular mechanisms underlying aphid biology.