NANFA-L-- two interesting articles

Subject: NANFA-L-- two interesting articles
From: Peter Unmack (peter.lists at)
Date: Tue Oct 19 2004 - 20:14:22 CDT

G'day folks

Here are a couple I saw while looking through a table of contents of
Molecular Ecology and relates to some of our recent conversations.
Bottom line is that the first one says that the typical number of loci
used to estimate heterozygosity would be too small too reliably estimate
the degree of inbreeding (meaning that most current studies are inadequate
for assessing inbreeding and bottle necks). The second article compares
fitness and survival of hatchery versus natural trout in Michigan and
relates to some of the other conversations we were having. Hope they are
of interest.

Peter Unmack
Canadian River, Oklahoma
Molecular Ecology
Volume 13 Issue 10 Page 3021 - October 2004
Does heterozygosity estimate inbreeding in real populations?


Many recent studies report that individual heterozygosity at a handful of
apparently neutral microsatellite markers is correlated with key
components of fitness, with most studies invoking inbreeding depression as
the likely underlying mechanism. The implicit assumption is that an
individual's inbreeding coefficient can be estimated reliably using only
10 or so markers, but the validity of this assumption is unclear.
Consequently, we have used individual-based simulations to examine the
conditions under which heterozygosity and inbreeding are likely to be
correlated. Our results indicate that the parameter space in which this
occurs is surprisingly narrow, requiring that inbreeding events are both
frequent and severe, for example, through selfing, strong population
structure and/or high levels of polygyny. Even then, the correlations are
strong only when large numbers of loci (~200) can be deployed to estimate
heterozygosity. With the handful of markers used in most studies,
correlations only become likely under the most extreme scenario we looked
at, namely 20 demes of 20 individuals coupled with strong polygyny. This
finding is supported by the observation that heterozygosity is only weakly
correlated among markers within an individual, even in a dataset
comprising 400 markers typed in diverse human populations, some of which
favour consanguineous marriages. If heterozygosity and inbreeding
coefficient are generally uncorrelated, then heterozygosity-fitness
correlations probably have little to do with inbreeding depression.
Instead, one would need to invoke chance linkage between the markers used
and one or more gene(s) experiencing balancing selection. Unfortunately,
both explanations sit somewhat uncomfortably with current understanding.
If inbreeding is the dominant mechanism, then our simulations indicate
that consanguineous mating would have to be vastly more common than is
predicted for most realistic populations. Conversely, if heterosis
provides the answer, there need to be many more polymorphisms with major
fitness effects and higher levels of linkage disequilibrium than are
generally assumed.
Molecular Ecology
Volume 13 Issue 11 Page 3379 - November 2004
Lower fitness of hatchery and hybrid rainbow trout compared to
naturalized populations in Lake Superior tributaries


We have documented an early life survival advantage by naturalized
populations of anadromous rainbow trout Oncorhynchus mykiss over a more
recently introduced hatchery population and outbreeding depression
resulting from interbreeding between the two strains. We tested the
hypothesis that offspring of naturalized and hatchery trout, and
reciprocal hybrid crosses, survive equally from fry to age 1+ in isolated
reaches of Lake Superior tributary streams in Minnesota. Over the first
summer, offspring of naturalized females had significantly greater
survival than offspring of hatchery females in three of four comparisons
(two streams and 2 years of stocking). Having an entire naturalized
genome, not just a naturalized mother, was important for survival over the
first winter. Naturalized offspring outperformed all others in survival to
age 1+ and hybrids had reduced, but intermediate, survival relative to the
two pure crosses. Averaging over years and streams, survival relative to
naturalized offspring was 0.59 for hybrids with naturalized females, 0.37
for the reciprocal hybrids, and 0.21 for hatchery offspring. Our results
indicate that naturalized rainbow trout are better adapted to the
conditions of Minnesota's tributaries to Lake Superior so that they
outperform the hatchery-propagated strain in the same manner that many
native populations of salmonids outperform hatchery or transplanted fish.
Continued stocking of the hatchery fish may conflict with a management
goal of sustaining the naturalized populations.
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: Fri Dec 31 2004 - 11:27:50 CST