“Disentangling genetic and prenatal maternal effects on offspring size and survival”

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Joel L. Pick, Christina Ebneter, Pascale Hutter, and Barbara Tschirren

Selection experiment reveals that a mother’s investment before birth has long-lasting effects on offspring fitness

Japanese quail (Coturnix japonica) chick at 1 week post-hatching.
(Credit: Dennis Hansen)

The prenatal period is a crucial time for offspring development. Mothers are in a unique position to influence this development, through the resources she invests. In the majority of species, this development occurs in an egg (or seed), allowing us a glimpse at this process. However, although eggs are packed full of nutrients, it is actually incredibly hard to demonstrate that mothers who produce bigger eggs (and so invest more into the development of their offspring) benefit through the increased growth and survival of their offspring. This difficulty arises because offspring not only receive resources from their mother, but they also inherit genes. If the genes that influence offspring growth are the same as those that influenced the size of the egg that the mother produced, then it would appear that the prenatal investment that a mother made influenced an offspring's growth. Furthermore, other components, e.g. mitochondria, are entirely maternally inherited, and these too could influence both growth and egg investment. Therefore isolating the effects of prenatal maternal investment is extremely difficult.

Over the last few years, researchers at the University of Zurich, Switzerland, have been working on a solution to this long-standing problem. Joel Pick and colleagues created selection lines for high and low maternal investment (measured as the size of the eggs) in the Japanese quail, best known as a producer of gourmet eggs. Whilst comparing offspring directly between these lines would not separate maternal from genetic effects, by reciprocally crossing the lines, they could compare the growth and survival of hybrids that had a similar genetic background, but differed in the amount of maternal investment they received during development (i.e. according to their maternal line). Furthermore through matching offspring to the specific egg that they hatched from, Pick and colleagues could further separate the effect of the egg from other maternally inherited components (e.g. mitochondria).

Pick and colleagues could then demonstrate that maternal investment indeed had strong positive effects on offspring growth and survival, that were not caused by correlated genetic effects. Moreover, they could show that these maternal effects were due entirely to maternal egg investment through to independence. At this stage additional to these egg effects, there was an additional effect on offspring size, most likely due to mitochondrial variation, that is previously unreported. Although these distinctions may seem arbitrary, they are extremely important for understanding how both offspring growth and maternal investment evolve, as well as a starting point from which to understand why some mothers invest more than others.

Often the strongest-held beliefs are the most difficult to verify. Here, Pick and colleagues were able to do this, finally demonstrating that bigger is better when it comes to eggs. Read the Article