DOI: 10.5061/dryad.cvdncjt6x
title: Additive genetic effects in interacting species jointly determine the outcome of caterpillar herbivory

description: 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.
description: 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 <em>ad libitum</em> 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
 <em>Medicago sativa</em> 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 <em>L.
 melissa</em> 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
 <em>M. sativa</em> and 157 <em>L. melissa</em> as
 part of a complementary smaller common garden experiment that is described
 below). <em>Medicago</em> <em>sativa</em> 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 <em>L. melissa </em>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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