Marking today’s publication of the genome of Daphnia pulex in Science, a commentary in
BMC Biology by Diethard Tautz, tackles the issue of why and how this diminutive water flea has many more genes than any other animal genome
sequenced so far. An ongoing process of gene duplication and retention appears  responsible for this, and as many of the recent duplicates show differential
expression in response to different environmental challenges, selection for
specific adaptations to Daphnia’s ever-changing aquatic environment must
have an important role.  However,
arguments for the adaptive value of new genes are insufficient, on their own,
to explain a rate of duplication that is estimated as three times that of other
invertebrates. Tautz proposes that the peculiar population genetics and life
history of Daphnia work hand in glove with selection to account for this dynamic evolution.

Evolutionary theory
tells us that positive selection is more efficient in large populations, but does
not yet formally incorporate additional features shown by Daphnia that
may account for its propensity for the rapid acquisition of
environmentally-responsive new genes. 
Tautz reminds us that Daphnia survives in a boom-bust aquatic
world, at the centre of a nutrient flux in which its population rises to dizzy
heights each spring as algae bloom, only to crash again as predators increase
and food runs out. When times are good it reproduces parthenogenetically,
favoring the rapid growth of the population, and the rapid amplification of
clonal variants best suited to the prevailing conditions. However when
austerity hits, it switches to sexual reproduction, producing eggs that can
survive for decades in the mud floor of its home.  Thus all lakes harbor a genetic reservoir of
resting eggs derived from animals that had a particular advantage at a previous
time, awaiting resurrection when conditions suit them again.  Tautz argues that this unusual reproductive
cycle, itself an adaptation to survival in a fluctuating environment, adds
significantly to the evolutionary dynamics that have led to the large and
ecoresponsive gene repertoire of Daphnia, now established as an
eco-genetical model organism par excellence.

Penelope Austin,

Deputy Editor, BMC Biology

Photo: Paul Hebert