Maternal
reproductive investment can critically influence offspring phenotype, and thus
these maternal effects are expected to be under strong natural selection.
Knowledge on the extent of heritable variation in the physiological mechanisms
underlying maternal effects is however limited. In birds, resource allocation
to eggs is a key mechanism for mothers to affect their offspring and different
components of the egg may or may not be independently adjusted. We studied the
heritability of egg components and their genetic and phenotypic covariation in
great tits (Parus major), using captive-bred full siblings of wild origin. Egg
mass, testosterone (T) and androstenedione (A4) hormone concentrations showed
moderate heritability, in agreement with earlier findings. Interestingly, yolk
triiodothyronine hormone (T3), but not its precursor, thyroxine hormone (T4),
concentration was heritable. An immune factor, albumen lysozyme, showed
moderate heritability, but yolk immunoglobulins (IgY) did not. The genetic
correlation estimates were moderate but statistically nonsignificant; a trend
for a positive genetic correlation was found between A4 and egg mass, T and
lysozyme and IgY and lysozyme, respectively. Interestingly, phenotypic
correlations were found only between A4 and T, and T4 and T3, respectively.
Given that these egg components are associated with fitness-related traits in
the offspring (and mother), and that we show that some components are
heritable, it opens the possibility that natural selection may shape the rate
and direction of phenotypic change via egg composition.