What’s in an egg: an immune system is based on mom’s background

TLDR: Baby birds have an immune system in the egg that is a function of the variable environment mom lived in.

           Mom’s environment can affect baby – in mammals, the fetus responds to the environment mom is in by responding to her physiology when in utero and to the composition of milk post-birth. In birds, mom’s environment can affect baby by changing the bits and pieces mom puts into the environment of the growing embryo: the egg.

So what does the egg “know” about life from mom? For one, the egg knows about the microbial environment of mom because she puts components of an immune system into the egg. That’s right, when an egg is produced in a female a handy little immune system is deposited along with the embryo! The life history, or “decisions” an animal makes, such as how and where to breed and lay eggs, can affect this egg immune system.

Immune system components are found in the egg white (albumen) and the egg yolk and have been found to be specific to the environment. Just as a human provides antibodies specific to the environment that are transferred to a baby in breastmilk, a bird will produce immune system components specific to the environment and put it into the egg. This may protect the baby bird in its’ new environment after hatch.

In an interesting collaborative study published in Integrative Organismal Biology, C.A. Fassbinder-Orth at Creighton University and colleagues at the U.S. Geological Survey, Novozymes Blair Inc., Millikin University, and Laboratorio de Investigaciones Pesqueras in Puerto Rico detail how life history plays a role in the immune system of birds, contributing to this growing field of “ecoimmunology”.

 

 

IobCowbird1-1
Rob & Ann Simpson/Vireo
Iobbb2
Tom J. Ulrich/ Vireo
IobBB3
G. Lasley/Vireo for National Audubon Society

From top, brown-headed cowbird, shiny cowbird, red-winged blackbird. Or, lazy parent, lazy parent, probably an ok parent (at least they are doing the work!)

Fassbinder-Orth and colleagues studied how the breeding strategy of six species of blackbirds affects the immune system of the egg. Four species of these blackbirds lay eggs and raise their babies in their own nests. The other two species, the brown-headed cowbird and the shiny cowbird, are brood parasites, meaning they lay eggs in the nest of a bird of a different species and depend on that other species (the ‘host’) to raise their babies for them (way to get out of parenting, cowbirds!). In brood parasitic species of this type, the parents will never see their babies. These two species of cowbirds are not picky about who they farm out their babies to – they may lay their eggs in the nests of over 200 different species of birds!

Fassbinder-Orth used this “host-generalist” strategy of these cowbirds to develop a hypothesis: because they are growing up in so many different environments, brood parasite species will be exposed to a wider array of microbes than non-brood parasites. Therefore, the immune system in the egg would be different – evidence of mom passing down their microbial exposure history!

        Indeed, that’s what they found. After collecting eggs from these six species from the wild, they found brood parasites (the cowbirds) to differ in several key measures: immunoglobulins (antibodies) and lysozymes (crucial in fighting off bacterial infection) were higher in the albumen of brood parasite species. Immunoglobulins measured from the yolk, however, were lower in the brood parasites. Three other measures taken were not different between the two kinds of species.

        Fassbinder-Orth points out that this discrepancy is important, particularly when designing future studies. If they had only done a single measure of the immune system in the egg they may have found that brood parasites didn’t differ from non-brood parasites. Instead, we have a more comprehensive picture – brood parasites have distinctly different patterns of immune system component levels in their eggs compared to non-brood parasites, and this is likely a reflection of this system evolving. Mom giving more immunity to eggs going into a highly variable environment gives baby more protection at hatch!

Dr. Darcy Ernst is a faculty member in the biology department at Las Positas College and a visiting scholar at UC Berkeley. Her research interests include reproductive neuroendocrinology, metabolic and stress physiology, and behavior of birds and mammals. Follow her on Twitter @birdphys.

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