Frederick William Goetz-Conceptual Framework
October 24, 2015
Background and history of rearing lean and siscowet lake trout lines
History: Lake Superior is the only Laurentian Great Lakes that has self-sustaining
populations of different lake trout forms ("morphotypes" or "ecotypes") including
the lean, siscowet, redfin and humper. These forms differ in morphometry, growth
and lipid levels and may also be separated ecologically on the basis of bathymetry.
The presence of distinct morphotypes or ecotypes within a population are
considered to be evidence of a "resource polymorphism" that may shed light on the
process by which new species arise. Thus, one interpretation of these different
forms is that they represent the divergence of a species in real time. When we
began our research on lake trout morphotypes, the fundamental question that was
still unanswered was whether or not the phenotypic differences observed between
wild lake trout morphotypes had a genetic basis or was the result of the
environment that they inhabited (i.e., environmental plasticity). The answer to this
question set the stage to begin to understand if lake trout morphotypes represent a
species undergoing divergence and it was the beginning of our work on rearing lake
trout morphotypes in captivity.
In the fall of 2006 we obtained fertilized eggs from wild leans and siscowets in Lake
Superior and reared them under identical environmental conditions from embryos
to adults. Those fish are now 8+ years old and have been the platform for several
investigations and are still being used for research on the genetic basis of depth
selection and the sublethal effects of lamprey parasitism (with Cheryl Murphy).
However, their primary and most significant role has been to provide the
initial data that supports a genetic basis for the phenotypic differences
observed between wild leans and siscowets (Goetz et al., 2010). Their other
mission was to supply gametes to produce a second generation of crosses (F2) that
included reciprocal crosses between siscowets and leans (e.g., lean female X
siscowet male and siscowet female X lean male) that could not be produced in the
original F1 lines from the wild. Like their parents, the F2s have already made
several significant contributions to understanding the biology of lake trout
morphotypes. The first is that they have confirmed the genetic basis for the
differences in growth, morphometry and lipid levels seen in the wild between
leans and siscowets (Goetz et al., 2014). While their parents supplied the initial
supporting data, the possibility existed that differences in the first generation were
a result of maternal effects transferred through the eggs from the wild gametes. In
subsequent generations the maternal argument becomes less viable and the
continued differences observed between the F2 leans and siscowets have provided
the most convincing evidence yet for a genetic basis. The final and most conclusive
evidence would be the mapping of phenotypes such as lipid levels and growth to
specific areas of the genome.
The future: Both the F1 (parental) and F2 (progeny) lines continue to be a valuable
resource for other experiments. Using satellite pop-up tag technology, the older F1
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fish are currently being used in a study to determine if depth selection in leans and
siscowets has a genetic basis. The preliminary results of those studies are
astounding; suggesting that going deep in siscowets is a genetic trait but that
vertical migrations from depth to the surface are frequent and cyclic behaviors of
both wild and cultured siscowets. The second year of that study has just been
completed.
The F1 lines have also been used in a past study on lamprey parasitism (Smith et
al., 2015. Differential physiological response to sea lamprey parasitism
between lake trout morphotypes (Salvelinus namaycush) from Lake Superior
Can J. Fish. Aq. Sci. in review and Goetz et al., 2015. RNA-seq reveals
transcriptomic responses to lamprey parasitism in lake trout livers. in
revision) and are part of proposals to continue that work for the future. In
particular those fish will be used to study the sublethal effects of lamprey parasitism
on reproduction, allowing us to include this in modeling the sublethal effects of
lamprey parasitism on lake trout populations.
The F2 lines can have a significant impact in the future in at least two ways. First,
while we know that certain phenotypic differences between leans and siscowets
have a genetic basis, the genes or parts of the genome involved in these differences
are not known. Understanding the regulation of these differences is significant to
understanding differences between leans and siscowets, and could be used in the
future to understand phenotypic differences between other lake trout morphotypes
as well. To do this type of analysis, the F2 line needs to be backcrossed to the
parental line (F1) so that a more diverse range of phenotypes and genotypes can be
obtained for mapping. This means that the F2 line must be maintained for several
years until they become reproductively active. Based on their parents, this should
occur by fall of 2016. At that time they can be crossed and their progeny reared for
a year prior to analysis.
The second way that the F1 lines can be used is to understand what occurs when
different morphotypes reproduce with one another in nature. Based on the
occurrences of intermediate looking morphotypes in the wild, we suppose that
crossing occurs between morphotypes. But what exactly will that look like and
what can we expect in regard to traits such as growth and lipid levels? The only
known and characterized intermediates available to answer those questions are the
reciprocal crosses in the F2 lines. These fish represent what a direct cross between
a female siscowet and male lean, and a female lean and male siscowet, would look
like.
Management implications of the rearing of leans and siscowets in captivity
Stocking: Lake trout recovery plans call for the evaluation of native strains of lake
trout as candidates for reintroduction in the Laurentian Great Lakes. This includes
deepwater forms that might inhabit areas of stocked lakes not inhabited by lake
trout. The greatest impact to management that past and future work on the captive
leans and siscowets, is in understanding which traits in different morphotypes could
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be expected to be maintained if these fish are bred in hatcheries for stocking back
into lakes. If traits such as growth, lipid levels, depth selection, etc. do not have a
genetic basis, managers may be wasting their time in breeding other lake trout
forms for stocking. At this point, our results strongly suggest a genetic basis for
some of these phenotypes at least in leans and siscowets but the answer would be
more conclusive if we could map these traits within future crosses of the F2
progeny.
Managing wild stocks: Similarly, we wonder if there is justification for managing
different lake trout morphotypes/ecotypes on an individual basis in Lake Superior.
Clearly, if the phenotypes of different forms have a genetic basis then this would
imply that we should. Thus, recent proposals to commercially harvest siscowets for
lipid in Lake Superior must be looked at carefully given the evidence so far that
suggests these traits are genetic. Results on genome wide scanning in lean and
siscowet crosses would help to confirm this as well as understand if phenotypes in
other morphotypes have a genetic basis that warrant morphotypic management.
Morphotype hybridization: F2 crosses between leans and siscowets are the only
known examples of hybridization between these morphotypes. As discussed earlier,
we feel strongly that this already occurs in the wild and increased densities of
siscowets may promote this in the future. As a result, we should try to understand
what these fish are like phenotypically and if there are differences related to
maternal or paternal effects. It appears so far that there are specific differences.
The F2 lines are the only lines that hold this information and if they are maintained,
can be used in the future to understand the effect of hybridization on phenotype.
Backcrossing of the reciprocal crosses to the parents will give us an eye into the
effects of further introgression.
Understanding and modeling the sublethal effects of lamprey parasitism: While
different stocks of lean lake trout are available for experiments, the F1 and F2 lines
of siscowets and the reciprocal crosses of leans and siscowets are the only ones
available to evaluate the morphotypic, sublethal effects of lamprey parasitism on
reproduction and growth in lake trout. Analyzing these effects will allow us to
include these sublethal effects in models of the population dynamics of lake trout
morphotypes. Further, since the captive lean and siscowet F1 and F2 lines have
been reared identically for their entire life, differences that we observe when
experimenting with them cannot be a result of differences in environmental or
rearing backgrounds.
Goetz, F., Rosauer, D., Sitar, S., Goetz, G., Simchick, C., Roberts, S., Johnson, R., Murphy, C., Bronte, C.,
MacKenzie, S. (2010). A genetic basis for the phenotypic differentiation between siscowet and lean
lake trout (Salvelinus namaycush). Molecular Ecology 19 (Suppl. 1): 176-196.
Goetz, F., A. Jasonowicz, R. Johnson, P. Biga, G. Fischer and S. Sitar (2014). Physiological differences
between lean and siscowet lake trout morphotypes: Are these metabolotypes? Canadian J. Fisheries
and Aquatic Science 71(3): 427-435.
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