Positive genetic covariance and limited thermal tolerance constrain tropical insect responses to global warming

Tropical ectotherms are particularly vulnerable to global warming because their physiologies are assumed to be adapted to narrow temperature ranges. This study explores three mechanisms potentially constraining thermal adaptation to global warming in tropical insects: 1. tradeoffs in genotypic performance at different temperatures (the jack-of-all-trades hypothesis) 2. positive genetic covariance in performance, with some genotypes performing better than others at viable temperatures (the ‘winner and ‘loser genotypes hypothesis) or 3. limited genetic variation as the potential result of relaxed selection and the loss of genes associated with responses to extreme temperatures (the gene decay hypothesis). We estimated changes in growth and survival rates at multiple temperatures for three tropical rain forest insect herbivores (Cephaloleia rolled-leaf beetles, Chrysomelidae). We reared 2746 individuals in a full-sibling experimental design, at temperatures known to be experienced by this genus of beetles in nature (i.e., 10-35°C). Significant genetic covariance was positive for 16 traits, supporting the ‘winner and ‘loser genotypes hypothesis. Only two traits displayed negative cross-temperature performance correlations. We detected a substantial contribution of genetic variance in traits associated with size and mass (0-44%), but low heritability in plastic traits such as development time (0-6%) or survival (0-4%). Lowland insect populations will most likely decline if current temperatures increase beyond 2°C. It is concerning that local adaption is already lagging behind current temperatures. The consequences of maintaining the current global warming trajectory would be devastating for tropical insects. However, if humans can limit or slow warming, many tropical ectotherms might persist in their current locations, and potentially adapt to warmer temperatures.

Methods
Description of methods used for collection/generation of data:

See manuscript for details: DOI: 10.1111/jeb.13905 

Methods for processing the data: 

-The data in columns 3-26 (Consecutivo-Fecha_medicion) were recorded in lab notebooks and entered 

into Excel spreadsheets, when columns 2 and 27 (sheet and Comentarios.revisar.datos.Abril.2017) were added.

-Data accuracy was checked again by Christina S. Baer and column 39 (Checked_CB) was added.

-The complete dataset for 12 Cephaloleia populations was combined and processed in R.

-hatch, pupate, and emerge were coded based on whether there was a value in Fecha_eclosion,

Fecha_pupado, or Fecha_nace_adulto, respectively.

-dev_time was calculated for individuals who reached adulthood by subtracting Fecha_eclosion fro

Fecha_nace_adulto.

-Asurv_time was calculated for individuals who reached adulthood by subtracting Fecha_nace_adulto

from Fecha_muerte_adulto.

-hatch_yrwk, death_yearwk, and emerge_yrwk were calculated to represent Fecha_eclosion, the death date (one of 

Fecha_muerte_larva, Fecha_muerte_pupa, or Fecha_muerte_adulto), and Fecha_nace_adulto (if 

applicable) as the year and calendar week number as defined by ISO 8601. The calendar weeks 

begin on Monday. If the week containing January 1 has >=4 days in the new year, it is week 1

of the new year; otherwise it is the last week of the previous year.

-ldays was calculated by subtracting the start of the life stage (Fecha_eclosion) from the 

either the transition date to the next life stage (Fecha_pupado) or the death date within the 

stage (Fecha_muerte_larva)

-pdays was calculated by subtracting the start of the life stage (Fecha_pupado) from the either 

the transition date to the next life stage (Fecha_nace_adulto) or the death date within the stage 

(Fecha_muerte_pupa)

-adays was calculated by subtracting the start of the life stage (Fecha_nace_adulto) from the 

death date within the stage (Fecha_muerte_adulto)

-Data for eggs that did not hatch were removed from the raw dataset.

-A dataframe for each species was saved as a .csv spreadsheet