Fertilisation and egg
activation in fish – a new perspective
Aquaflow Technical Leaflet 2003-129
European Network for the Dissemination of Aquaculture
RTD Information (Q5CA-2000-30105) and previously FAIR-3837, URL: http://www.aquaflow.org/
Fertilisation, egg
activation and aquaculture
Despite the continued growth and importance of
aquaculture for worldwide human consumption, very little is still known
about the precise mechanisms associated with fertilisation and egg
activation in fish. Over the last ten years, knowledge of fertilisation and
activation in mammalian eggs has grown enormously. It is vital that lessons
learnt from this field are swiftly applied to address the problems facing
fish fertility.
Egg activation – the role
of calcium
Changes in the concentration of calcium within the
cell constitute one of the most important cell signalling mechanisms.
Calcium also plays a key role during the activation of an egg by
sperm. Activation enables the
fertilised egg to break from its state of arrest in the cycle and start
producing the proteins that will perform vital functions in the embryo. It
has only relatively recently been discovered that sperm activate eggs by
triggering the release of calcium from internal stores within the egg.
The ‘sperm factor’
theory
A rise in intracellular calcium level seems to be the
universal trigger for egg activation. However, the form that the calcium
increase takes at fertilisation varies significantly between species. In sea
urchins, frogs and fish (medaka - Oryzias
latipes), a single, explosive wave of calcium is observed crossing the
egg at fertilisation. In contrast, mammals, some marine worms and sea slugs,
exhibit a series of calcium increases called ‘calcium oscillations’.
Until recently, the most popular theory was that sperm activate eggs through
an interaction between proteins on their outer surface. Recent progress,
however, has provided strong evidence for the existence of a ‘sperm
factor’. This theory suggests that calcium release is caused by a soluble
sperm-specific protein that enters the egg at sperm/egg fusion.
We now know that sperm triggers an explosive rise in egg calcium at
fertilisation in many different animal species (and plants), and that this
characteristic rise is vital to the success of subsequent embryonic
development.
Future perspectives –
potential for aquaculture
Currently, studies of egg activation in fish are
confined to small laboratory species that are of little or no commercial
importance, e.g. zebrafish Zebradanio
rerio and medaka Oryzias latipes.
These limited data suggest that egg activation in both these species is
accompanied by an increase in egg intracellular calcium, though curiously
the mechanism of calcium release appears to be quite different.
Findings are likely to be of great significance to
commercial aquaculture whilst also providing vital comparison with other
animal groups. There is strong evidence to suggest the existence of a
‘sperm factor’ in tilapia sperm. Detailed knowledge of the precise
mechanisms and molecules involved in the activation of fish eggs may pave
the way to improved fertilisation and hatching rates and thus help optimise
the management of cultured species.
For more information:
Kevin Coward, Olivia Hibbitt, John Parrington
Department of Pharmacology
University of Oxford, Mansfield Road,
Oxford OX1 3QT - UK
E-mail: kevin.coward@pharm.ox.ac.uk