How close is too close? – No evidence for optimal fitness at intermediate levels of inbreeding in Drosophila melanogaster

It’s well known that animals can have low fitness if their parents are close relatives such as brothers and sisters. It’s probably less well known that there are also examples of where animals can have low fitness because their parents are too distantly related. Because of these two factors it has been suggested that intermediate relatives (i.e. cousins) might actually produce the highest fitness offspring.

Support for this theory has come from some animals that choose to mate with an intermediate relative such as a cousin. Although inbreeding has already been fairly well researched, many studies have used extremely inbred individuals (i.e. brother-sister matings for multiple generations). There are few studies that look at inbreeding over the range where we might expect to find the highest fitness (i.e. not close or long distance but intermediate).

In this study Stephen looked at how relatedness between parents affects offspring fitness in the fruit fly, Drosophila melanogaster. Fruit flies are a great choice for this kind of study because large numbers of animals of known relatedness can readily be bred in the lab and the fitness of their offspring tested.

Stephen’s research found that the more closely related an individuals parents, the lower their fitness (classic inbreeding depression) and that there was no sign that an intermediate relative produced the highest fitness offspring. It is likely that other factors, such as the inclusive fitness benefits of inbreeding, explain the examples of mate choice for intermediate relatives rather than any direct benefit to offspring.

To read Stephen’s full article click here. Dr Stephen Robinson was an Environmental Advisor/Zoologist with Terrestrial Ecosystems.