Great tits inherit egg spots from mom

After examining more than a thousand clutches of eggs, researchers at Oxford University in England say a female great tit inherits her eggshell pattern from her mother’s side of the family.

Just what determines eggshell patterns has perplexed researchers. Cuckoos, for example, have evolved female lineages that mimic egg patterns of other species. The cuckoos swoop into nests of the birds whose eggshells they can fake and leave an egg for the nest owners to rear.

Egg mimicry would be easiest to explain if the gene for spottiness were on the female sex chromosome, so that genes inherited from the father wouldn’t influence it. Research on the solid color of chicken eggs—a slightly different problem—revealed that hens lay brown or white eggs depending on several genes, none of them located on a sex chromosome.

Andrew G. Gosler and his Oxford colleagues have just finished the first extensive analysis of egg-spot inheritance. The great tits they studied don’t mimic other species’ eggs but do exhibit variable egg spottiness.

For 12 years, Gosler himself scored the size of spots, the darkness of their color, and the evenness of their distribution. Now, his team has concluded that neither a female’s age nor health makes much difference in her eggs’ pattern and environmental conditions have only weak effects.

What does matter is heritage on the female side. A tit’s egg spottiness, which is about 80 percent consistent from year to year, strongly resembles her maternal, but not paternal, grandmother’s, say the researchers in the Dec. 22 Proceedings of the Royal Society of London B. That’s the inheritance predicted for a gene on a bird’s female sex chromosome, Gosler notes.