Insect eggs not as defenceless as once believed

Insect eggs are not as defenceless as we may think. They have a protective layer that shields them from outside influences. This discovery was made by biologist Chris Jacobs. The antibacterial substances produced in this layer may be usable as an antibiotic. Dissertation defence on 4 December.

Eggs not defenceless

Up to now, insect eggs have generally been viewed as defenceless. It was thought that their mothers needed to care for them and provide them with substances that kill microbes. That idea originates largely from research on biologists’ favourite model insect: the Drosophila fruit fly. The fruit fly’s eggs are extremely sensitive to infections. Chris Jacobs now shows that the Drosophila is probably an exception. Most insect eggs produce their own battery of antibacterial substances if they are attacked, such as by microbes that cause infections. These substances may also turn out to be good antibiotics. His dissertation co-supervisor Maurijn van der Zee and his research group are going to delve further into this possibility.

Illustratie van de serosa

Illustratie van de serosa

Serosa produces antimicrobial substances

The antimicrobial substances are produced in a protective membrane around the egg: the serosa. This membrane is made by the fertilised egg itself, in the same way that the placenta and the protective membranes are made by the fertilised egg cell in humans. What is special is that in the course of its evolution the fruit fly has lost this membrane, while all other insects still have a serosa.

Super-fast development

Jacobs suspects that this has to do with the speed of development: ‘Fruit fly eggs develop in a single day. They have to if they’re laying your eggs on sources of food that can disappear quickly, like rotten fruit. A serosa and an immune defence system take far too much time and energy. So the Drosophila opts to hatch its larva quickly and to keep the vulnerable egg stage as short as possible. Most insects develop much more slowly, sometimes over whole months, and need good protection.’

Membrane protects eggs from drying out

The red flour beetle (Tribolium castaneum), on which Jacobs conducted his research

The red flour beetle (Tribolium castaneum), on which Jacobs conducted his research

Protection from infections is not the serosa’s only purpose. This membrane also protects the egg from drying out. It secretes a thick cuticle, the same hard layer found on the outside of adult insects. During his dissertation research, Chris Jacobs switched off the gene responsible for producing the serosa and found that eggs no could no longer survive in conditions of low humidity. The serosa is something that insects have innovated and which is not present in their ancestors: shellfish such as lobsters and shrimp. These creatures live in the ocean. This innovation may have provided the conditions that allowed insects to become so incredibly successful on land. Three out of four animal species on Earth are insects.


Turned on its head

Chris conducted his research at the Institute for Biology, in the lab of Dr Maurijn van der Zee. Van der Zee had this to say: ‘Chris’s dissertation research has taught us a lot about how insects evolved. His work also shows that it is unwise to concentrate on just one single model organism. His diverse approach has led to this portion of his work being published in the new online journal eLife. This journal was established by Nobel prize winner Randy Schekman and aspires to become the open access counterpart of Nature and Science. Chris’s work has turned the image we have of a defenceless insect egg completely on his head.’

Doctoral defence: 4 December 2014
Time:  12.30 hrs.
Title: ‘Surviving embryogenesis, the extraembryonic serosa protects the insect egg against desiccation and infection’
Venue:  Academy Building, Leiden
Supervisor: Prof. Herman P. Spaink
Co-supervisor: Dr Maurijn van der Zee.

(3 December 2014)

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Last Modified: 04-12-2014