Prenatal Acoustic Signals Influence Nestling Heat Shock Protein Response to Heat and Heterophil-to-Lymphocyte Ratio in a Desert Bird

Abstract

Heat shock proteins (HSPs) are essential to cellular protection against heat stress. However, the causes of inter-individual variation in HSP regulation remain unclear. This study aimed to test the impact of early-life conditions on the HSP response to heat in zebra finches. In this arid-adapted bird, incubating parents emit "heat-calls" at high temperatures, which adaptively alter offspring's phenotypes. Embryos were exposed to heat-calls or control-calls, and at 13 days post-hatch nestlings were separated into two different experiments to test responses to either chronic nest temperature ("in-nest" experiment) or an acute "heat-challenge". Blood samples were collected to measure levels of heat shock cognate 70, heat shock protein 90α, corticosterone and the heterophil-to-lymphocyte (H/L) ratio. In the in-nest experiment, both HSPs were upregulated in response to increasing nest temperatures only in control-calls nestlings (HSC70: p = 0.010, HSP90α: p = 0.050), which also had a marginally higher H/L ratio overall than heat-call birds (p = 0.066). These results point to a higher heat sensitivity in control-call nestlings. Furthermore, comparing across experiments, only the H/L ratio differed, being higher in heat-challenged than in in-nest nestlings (p = 0.009). Overall, this study shows for the first time that a prenatal acoustic signal of heat affects the nestling HSP response to postnatal temperature.

Keywords: Taeniopygia guttata; acoustic developmental programming; glucocorticoid; leucocyte profile; phenotypic plasticity; prenatal sound; stress proteins.

Figure 1

Experimental design: eggs were collected on the day of lay (embryonic day 0, E0) and artificially incubated until hatching (day 0, D0). Prenatal playback exposure (in blue) to either heat-calls or control-calls occurred in the last 5 days before hatching (E10 to E14), and the postnatal nest temperature (in green) was manipulated daily from D0 to D12 post-hatch. Nestlings’ body masses were measured on D12, and on D13 nestlings were separated into two experiments (in yellow), and a blood sample collected, to test the impact of prenatal acoustic experience on either (i) Experiment 1 (Exp1) in-nest: chronic response to nest temperature, or (ii) Experiment 2 (Exp2) heat-challenge: acute response to a heat-challenge in a temperature-controlled chamber.

Figure 2

Levels of heat shock proteins across nest temperature in in-nest nestlings. Individuals were exposed to prenatal control-calls (black circles) or heat-calls (red triangles) and, upon hatching until day 12, experienced a mean daytime nest temperature (12D-T_nest). At day 13, the nestlings were taken from their nest to collect blood samples. (A) Levels of constitutive heat shock cognate 70 (HSC70; N = 35) and (B) stress-inducible heat shock protein 90α (HSP90α; N = 35) were measured in the red blood cells. Open circles show individual data and regression lines are represented with 95% CIs.

Figure 3

Means (±SE) of the stress biomarkers in in-nest or heat-challenged nestlings. Individuals were exposed to prenatal control-calls (black circles) or heat-calls (red circles) and, at day 13 post-hatch, blood samples were collected from individuals in their nest (in-nest) or following a heat-challenge. The panels show levels of (A) heat shock cognate 70 (HSC70; N = 73), (B) heat shock protein 90α (HSP90α; N = 73), (C) the heterophil-to-lymphocyte (H/L) ratio (N = 44) and (D) corticosterone (CORT; N = 53). Removing the high HSC70 value (A) or the high CORT value (D) did not change the results. Open circles show individual data and solid circles show group means ± SE.

Figure 4

Body mass of heat-challenged nestlings in relation to their (A) heterophil-to-lymphocyte (H/L) ratio (N = 21) and (B) heat shock cognate 70 (HSC70) levels (N = 38). In (B), the p-value is marginal (p = 0.051) when the high HSC70 value is excluded. Regression lines are represented with 95% CIs.