Plant-insect interactions are common and important in basic and applied biology. Trait and genetic variation can affect the outcome and evolution of these interactions, but the relative contributions of plant and insect genetic variation and how these interact remain unclear and are rarely subject to assessment in the same experimental context. Here we address this knowledge gap using a recent host range expansion onto alfalfa by the Melissa blue butterfly. Common garden rearing experiments and genomic data show that caterpillar performance depends on plant and insect genetic variation, with insect genetics contributing to performance earlier in development and plant genetics later. Our models of performance based on caterpillar genetics retained predictive power when applied to a second common garden. Much of the plant genetic effect could be explained by heritable variation in plant phytochemicals, especially saponins, peptides, and phosphatidyl cholines, providing a possible mechanistic understanding of variation in the species interaction. We find evidence of polygenic, mostly additive effects within and between species, with consistent effects of plant genotype on growth and development across multiple butterfly species. Our results inform theories of plant-insect coevolution and the evolution of diet breadth in herbivorous insects and other host-specific parasites.

Neonate caterpillar was carefully transferred to a Petri dish with a sprig of fresh plant material (a few leaflets) with the stem of the plant tissue wrapped in a damp Kimwipe. We verified each caterpillar was alive and uninjured after transfer. The Petri dish containing the caterpillar was then returned to the incubator. Caterpillars were given fresh leaf tissue ad libitum and were checked daily for survival, pupation and eclosion as adults.

As metrics of performance, we measured 8-day and 14-day caterpillar weight, and weight at pupation using a Mettler Toledo XPE105 analytical microbalance (Mettler Toledo). Weights were recorded to the nearest 0.01 mg, and we took the mean of two independent weight measurements. We then considered the following nine performances metrics: 8-day caterpillar weight (mg), 14-day caterpillar weight (mg), weight at pupation (mg), survival to 8 days (binary), survival to 14 days (binary), survival to pupation (binary), survival to adult (binary), total survival time (integer-valued), and truncated survival time (integer-valued). For truncated survival time, we truncated survival at the maximum number of days required for any of the caterpillars to reach eclosion; this avoids caterpillars that developed slowly but never pupated or eclosed from having the longer survival times than caterpillars that successfully eclosed as adults. 

We isolated DNA from 1064 Medicago sativa plants from the Greenville Experimental Farm common garden (16 of the initial 1080 plants died before they could be used in the experiment) and 922 L. melissa caterpillars (some caterpillars were too small when they died to be recovered and used for DNA extraction), pupae or adults reared on these plants (DNA was also isolated from an additional 172 M. sativa and 157 L. melissa as part of a complementary smaller common garden experiment that is described below). Medicago sativa DNA was isolated from dried leaf tissue by Ag Biotech (Ag Biotech, Monterey CA, USA). We isolated DNA from whole caterpillars, pupa, or the thorax of adult L. melissa butterflies using Qiagen's DNeasy Blood & Tissue kit (Qiagen Inc., MA, USA) in accordance with the manufacturer's recommendation. We generated and sequenced DNA fragment libraries for these samples at the University of Texas Genomic Sequencing and Analysis Facility (Austin, TX, USA).

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