Froth, Foam and Bubble....
The Foam Nests of Trinidadian Frogs
Several species of tropical frog lay their eggs in stable foams which
persist until the tadpoles emerge. The function of the foams remains
uncertain but several possibilities exist, such as isolating the developing
eggs from predation, protecting them from infection, dehydration, or sun
damage. Whatever the function of the foams may be, they merit investigation
for two main reasons. First, despite being within easy reach for a range of
organisms which could colonise and destroy the foam and the eggs (bacteria,
fungi, nematodes, etc.), they are observed to remain intact for the several
days required for the completion of development. They may, therefore,
contain novel anti-microbial compounds which could teach us something new
about the mechanisms of action of natural anti-microbial agents. Secondly,
little is known about the composition of the foams and the molecular
mechanisms by which the foams are formed and remain stable for such lengths
of time. Moreover, should the nest foams contain surfactants, then these
are of intrinsic interest because they must be compatible with a very
sensitive developing tissue, and a vertebrate one at that. A simple
detergent, for example, would be membrane-disruptive, and the foam may not
be stable anyway. Aside from satisfying curiosity, the work might lead to
clinically or industrially useful applications.
The object of this project was to collect foam produced by two
species of frog, Physalaemus pustulosus, which lays its foam nest on or
near the surface of small pools of standing water, and Leptodactylus
fuscus, which lays its foam nest in mud burrows immediately adjacent to
pools. Foam was collected, under Roger Downie's specialised guidance, the
eggs removed, and the foam frozen for return to Glasgow for analysis. The
separation was carried out either by hand (laboriously carried
out by Isla and Ross Kennedy - with help from Freya Kennedy, who also played a major role as morale booster) or by gentle centrifugation. Centrifugation
also compresses the foam and provides a liquid which is easier to analyse
than a foam.
A comprehensive analysis of this unusual biological material
requires a team effort.....
Roger Parton (Infection and Immunity) helps in the search
for anti-microbial compounds, Les Fixter, John Kusel and John Beeley
(Biochemistry and Molecular Biology) look for lipids, carbohydrates
and surface-active compounds in the foams, and Alan Cooper (Chemistry),
Malcolm Kennedy (I&I) - assisted by Tennie Videler and Emma Wilson - along with Nick Price in Stirling University,
examine the biophysical properties of proteins purified from the foams. The
work is supported by a grant from the Wellcome Trust under a new
scheme (Sir Henry Wellcome Commemorative "Showcase" Awards) to support unusual research directions.
Our colleagues may think we are crazy even thinking about this
project, but not as crazy as Trinidadians thought we were collecting frog
nests from dirty ditches and ponds in a graveyard! Perhaps they will all be
proved correct.
Malcolm Kennedy
Alan Cooper
Click for some pictures of the Trinidad 1997/8 foam collection expeditions.
Return to Biophysical Chemistry home page.