Many insects harbor microorganisms that can have large effects on their fitness. The pea aphid possesses multiple "accessory symbionts" (PASS, PAR, T, U) found in some aphid populations, as well as an obligate mutualistic symbiont Buchnera. I have found that for some accessory symbionts the within-population frequency varies greatly and is associated with host plant (e.g. for PASS: frequency on alfalfa=85-100%, frequency on clover <10%). Pea aphid collected from New York alfalfa and clover fields has been shown to be host plant specialized and genetically differentiated (Via 1999, Via et al 2000). Thus, one hypothesis is that variation between symbiont communities on different host plants is due to host plant based divergence and the subsequent differentiation of the bacteria. I have examined the level of host plant specificity in pea aphid from California, and find it to be considerably less dramatic. Alternate hypotheses for the variation in symbiont communities are: differential opportunities to acquire particular accessory symbionts on different host plants, and differences in fitness effects of symbionts on different host plants. Chen et al (2000) detected symbiont by host plant interactions that affected fitness of aphids artificially injected with different symbionts, indirectly supporting the latter hypothesis. In the experiment presented I measure the fitness of twenty aphids collected from each of two alfalfa and two clover populations in California on both alfalfa and clover. Quantitative PCR is used to estimate the abundance of each of the five symbionts (4 accessory + Buchnera) within adult individuals of each line. Together, I use these data to evaluate whether aphids from these populations are specialized on their native host plant, whether the presence or quantity of any of these symbionts is associated with fitness on either host plant, and whether there are host plant-by-symbiont interactions.
Species 1: Hemiptera Aphididae Acyrthosiphon pisum (pea aphid)
Keywords: coevolution, local adaptation
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