Geographical Variation in Egg Mass and Egg Content in a Passerine Bird

Abstract

Reproductive, phenotypic and life-history traits in many animal and plant taxa show geographic variation, indicating spatial variation in selection regimes. Maternal deposition to avian eggs, such as hormones, antibodies and antioxidants, critically affect development of the offspring, with long-lasting effects on the phenotype and fitness. Little is however known about large-scale geographical patterns of variation in maternal deposition to eggs. We studied geographical variation in egg components of a passerine bird, the pied flycatcher (Ficedula hypoleuca), by collecting samples from 16 populations and measuring egg and yolk mass, albumen lysozyme activity, yolk immunoglobulins, yolk androgens and yolk total carotenoids. We found significant variation among populations in most egg components, but ca. 90% of the variation was among individuals within populations. Population however explained 40% of the variation in carotenoid levels. In contrast to our hypothesis, we found geographical trends only in carotenoids, but not in any of the other egg components. Our results thus suggest high within-population variation and leave little scope for local adaptation and genetic differentiation in deposition of different egg components. The role of these maternally-derived resources in evolutionary change should be further investigated.

Heritable variation in maternally derived yolk androgens, thyroid hormones and immune factors

“…Thus these results provide only weak support for the adaptive covariation hypothesis (Royle et al, 2001;Groothuis et al, 2006). Previous results on phenotypic variation are controversial; in the study species, a phenotypic correlation between androgens and immune factors was found at between-but not within-clutch level , see also Gasparini et al, 2007), but multiple other studies find no support for the hypothesis (Groothuis et al, 2006;Hargitai et al, 2009;Ruuskanen et al, 2011). In selection lines for yolk T and behavior (quails, Okuliarova et al, 2014), IgY or lysozyme levels did not respond to selection similarly as T, suggesting no genetic covariation.…”

“…T4 and T3 concentrations were also analyzed using extractions and radioimmunoassay as described earlier (Ruuskanen et al, 2015) and are expressed as pg/mg. Lysozyme activity was analyzed using a micro-plate modification of a turbidimetric assay, following Ruuskanen et al (2011), where lysozyme breaks down the bacterial cell walls in the suspension which can be seen by the change in turbidity. The results are expressed as lysozyme activity = change in absorbance/min × 1000.…”

“…The results are expressed as lysozyme activity = change in absorbance/min × 1000. Immunoglobulin (IgY) concentrations were determined using enzyme-linked immunoassays, following Ruuskanen et al (2011) and the results are expressed as Uml À1 × 10 5 .…”

“…It has been hypothesized that females balance the costs and benefits of one component with another. For example, higher levels of immune factors could compensate the immunosuppressing effects of high yolk androgen levels (Royle et al, 2001;Groothuis et al, 2006), but the results of the phenotypic correlations are currently contradictory (Groothuis et al, 2006;Gasparini et al, 2007;Hargitai et al, 2009;Ruuskanen et al, 2011;Postma et al, 2014). On the other hand, yolk hormones A4 and T that are phenotypically correlated have been found to show differential effects in offspring: A4, but not T, decreased growth (Muriel et al, 2013) and A4, but not T, was positively correlated with offspring recruitment (Hegyi et al, 2011;Tschirren et al, 2014).…”

Maternal transfer of androgens in eggs is affected by food supplementation but not by predation risk

“…Th e protocol for analyses of yolk T and A4 was as in Ruuskanen et al 2011 (using a modifi ed protocol from Hackl et al 2003, Bertin et al 2008, whereas the analyses of yolk progesterone followed the protocol described in Bertin et al (2008). All the protocols were slightly modifi ed in the amount of water/methanol added depending on the amount of yolk used in the analyses.…”