Female blue tit selects male with lots of immune genes

Although spring is in the air, the blue tit keeps a cool head when it comes to choosing a mate and courtship. Elske Schut has discovered that for the female blue tit the number of immune system genes (known as MHC1 genes) her chosen mate possesses is very important. Schut has also demonstrated that blue tits continue to copulate during the laying season, which is uncommon in songbirds. This urge to mate does not seem to be a case of blind passion either. Schut will be awarded a PhD by the University of Groningen on 23 March 2012.

The Major Histocompatibility Complex type 1 (MHC1) is a collection of genes that ensure that the immune system recognizes specific pathogens. MHC1 genes determine, for example, whether a blue tit is immune to malaria. As the MHC1 genes are extremely important for blue tit health, Schut studied whether they play a part in the choice of a mate. A good mate increases the likelihood of a thriving family and healthy offspring.

Equal number of MHC1 genes

MHC1 genes determine which pathogens the immune system of blue tits recognizes. Schut studied (using the Reference Strand Mediated Conformation Analysis technique) whether the MHC1 genes is a criterion for blue tits in their choice of mate, and discovered that there is indeed a correlation between the mates’ number of genes. Schut says, ‘I saw that males and females with lots of MHC1 genes mate together and that males and females with few MHC1 genes also mate together.’ A possible explanation for this is that blue tits prefer a healthy mate with plenty of MHC1 genes. Schut says, ‘And it's hard luck for the blue tits with few MHC1 genes: they have to make do with a mate who also has few of them.’

Large clutches

In the second part of her research Schut looked at the number of sperm cells in the eggs. Blue tits lay an average of eleven eggs during the nesting season, but can lay up to seventeen. Schut says, ‘This is a particularly large clutch in comparison with other species of songbird.’ In order to ensure that as many eggs cells as possible are fertilized, a female blue tit can temporarily store a male’s sperm in a sperm storage organ. Schut says, ‘But as the sperm dies anyway after a while, I did not expect all of the eggs in the large clutches to be fertilized.’

Carry on copulating

In order to find out whether all the eggs were fertilized, Schut carefully opened each second and tenth egg from a clutch and used a microscope to count the number of sperm cells on the membrane around the egg yolk. She saw that all of the eggs hatched, even in the large clutches, and that there were a comparable number of sperm cells in all of the eggs from the same clutch. Schut says, ‘This means that blue tits continue to copulate during the nesting season.’ This is a surprising conclusion because songbirds often mate less or even quit mating once the first eggs have been laid.

Curriculum Vitae

Elske Schut (Groningen, 1983) studied evolutionary biology at the University of Groningen and did her PhD research at the University’s Centre for Evolutionary and Ecological Studies (CEES). She will be awarded a PhD on 23 March 2012. Her thesis is entitled: Sexual selection in the blue tit: The role of the MHC and post-copulatory effects. Her supervisor was Prof. Jan Komdeur. She received grants from the Faculty of Mathematics and Natural Sciences, the Dr J.L. Dobberke Foundation, the Schure-Beijerinck-Popping Fund and the Nicolaas Mulerius Foundation. The research was also financed by GEBACO and INCORE grants awarded to Jan Komdeur.

Further information

Elske Schut, e-mail: elske.schut rug.nl.