From mothers to eggs – maternal anti-influenza antibody transfer in Mallards

A Mallard nest. Photo by Flickr user nottsexminer, used under a CC BY-SA 2.0 license.

A Mallard nest. Photo by Flickr user nottsexminer, used under a CC BY-SA 2.0 license.

By Jonas Waldenström

I have spent the last two days at home nursing one of my offspring that has been down with a cold. Actually, since this is the oldest daughter, nursing generally involves providing her with unlimited access to her mother’s iPad and all the sandwiches she cares to eat.

The added benefit is that I have had more time to read and think than I usually have, far away from the office turmoil. So while I have time, I thought I could toss in yet another blog post.

A few months ago I served as an external examiner on Jacintha van Dijk’s PhD thesis in the Netherlands. That was an enjoyable experience – because of the quality of the thesis, and the unfamiliar and ancient procedures of a Dutch defense. I got to wear a funny hat and toga, there was a whole lot of ceremonial ‘all rise’, some marching in and out of rooms in predetermined processions, and other strange things that we don’t do in Sweden.

Anyway, most of Dr van Dijk’s papers are now published, and today I reread a story on maternal antibodies against avian influenza virus in Mallards, published in PLOS ONE this November.

For animals, the energy put into rearing offspring is a substantial investment. Generally, the more you invest the better chances the offspring has to reach reproductive age. However, as all things in ecology, energy isn’t endless, and animal needs to trade-off investments in one life history parameter to those of other parameters. For instance, in animals with several breeding seasons, current reproduction needs to be balanced with survival.

The last 15 years, ornithologists have looked into allocations of maternal antibodies between mothers and offspring. When an egg is laid some of the antibodies of the mother can pass over to the yolk, providing the hatching chick a kick-start of antibodies to fight infections. Such maternal antibodies do not last more than a few weeks, but may nevertheless be important in the early stage of a chick’s life. For instance, this is seen in commercially reared chickens, where maternal antibodies against Campylobacter can protect the chicks from colonization up to two weeks. It has been argued that the mother can choose how much antibodies different eggs receive, thereby modifying the future prospects of her offspring.

In this paper, the Dutch team investigated deposition of anti-influenza antibodies in Mallard eggs. They collected eggs from free-living Mallard nests – which is quite an achievement, since the nests are often tucked away and camouflaged. They also investigated eggs from captive ducks, more conveniently situated in a pen just outside the research institute.

Antibody concentrations were determined in both egg yolk and in the blood of the mothers, and they controlled for egg size, embryo sex, egg laying order, and female body condition. However, first they needed to check whether the incubating female indeed was mother to all the eggs in the clutch, since mallards are notorious egg dumpers.

Association between the AIV antibody concentration in egg yolk and female serum from (A) the field study and (B) the captive study. Note: axes are minuslog10-scaled. (From the original publication doi:10.1371/journal.pone.0112595.g001)

Association between the AIV antibody concentration in egg yolk and female serum from (A) the field study and (B) the captive study. Note: axes are minuslog10-scaled.
(From the original publication doi:10.1371/journal.pone.0112595.g001)

Indeed, maternal anti-influenza antibodies were found in Mallard eggs from antibody positive females, similar to earlier studies conducted on gulls. There was a positive correlation between antibody concentrations in the eggs and the concentration in the females, but there was no effect of any of the other investigated factors, including the body condition of the female. One more thing, though: there seemed to be an increasing concentration of antibodies with egg laying order; thus, later laid eggs had higher concentrations of antibodies than the early eggs in the clutch.

It remains to show whether this maternal transfer confers protection against influenza virus infections in young mallards, but it is an interesting finding. If maternal antibodies do protect, that could potentially affect local perpetuation patterns of flu, by temporarily reducing the general number of susceptible animals. Or if the antibodies are specific to only those subtypes that infected the mother (which is likely), it could potentially affect subtype distributions in the population, favoring subtypes that are antigenically different. However, if maternal antibodies wane after a few weeks, these effects, if any, should be transient.

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