Sarah J. Adamowicz, T. Ryan Gregory, Maria Cristina Marinone, and Paul D.N. Hebert
Molecular Ecology 11:1209-1217.
Abstract
It has long been known that polyploid organisms are more prevalent
in
arctic environments. Past explanations for this geographical
trend
have focussed on the role of glacial cycles in generating polyploids
and
the influence of abiotic factors in favouring polyploidy in the
north.
In combination, these mechanisms probably suffice to explain the
observed
geographical cline in ploidy levels in members of the Daphnia pulex
complex in the Holarctic. While only diploid members of the D.
pulex group are found in the temperate regions of North America and
Europe, allozyme and DNA quantification analyses indicate that the
southern
Argentine fauna is dominated by polyploids. Indeed, the present
study
is the first to document the presence of polyploid members of the D.
pulex complex in any temperate climate. The results of
phylogeographic
analyses suggest that this difference in polyploid distribution between
the northern and southern hemispheres is based more on ecological and
historical
contingencies than direct selection for polyploidy. Specifically,
competition with diploid relatives likely limits the lower latitudinal
range of polyploids in the north, but appears not to have occurred in
Argentina.
Because of these differences, the present study provides important
insights
into the diverse factors that determine the distributions and
evolutionary
fates of polyploid organisms.