Feedback from the Monarch authorities regarding research project

[Paul Cherubini]

Jim Wiggins wrote:

>We would like to open the proposed study to discussion.

Jim, below is what Dr's Karen Oberhauser (Univ. of Minn) and Sonia Altizer wrote to the 
dplex discussion list in July this year on this topic. They are against your propose transfer 
and release of tagged monarchs accross the Rocky Mountains because the risks involved 
with such as experiment outweigh the proposed benefits:

Dr. Karen Oberhauser wrote (in July 1999 to deplex-list):

Dear All:  Dr. Sonia Altizer has been studying monarch butterflies, genetic
differences between populations, and monarch diseases and parasites for
several years.  She is currently a post-doc at Princeton University, and
has forwarded the below message regarding recent discussion on transfers of monarchs.  I 
have also attached her message for those of you that would
rather read a word document of the same message.

Karen

>From Sonia:

As a scientist who has been studying differences between monarch
butterflies throughout their North American range for several years, I
feel obligated to respond to some recent comments posted on the DPLEX. I
apologize in advance for the length of my reply, but feel it is necessary
in light of the recent misrepresentation and misinformation that has been
posted recently concerning my work and that of my colleagues. Most of my
research has focused on the interactions between monarchs and their
obligate protozoan parasite, Ophryocycstis elektroscirrha.  However, as a
result of this work I have become interested in issues concerning
selection, gene flow, and genetic drift among monarchs in different
geographic regions.

First, I would like to clarify several broad issues related to monarch
population differentiation.  Populations may become genetically
differentiated as a result of two distinct processes: natural selection
and genetic drift.  Selection and drift tend to differentiate populations,
whereas migration tends to homogenize them.  Genetic drift operates within
populations and can cause random changes in allelic frequencies (and loss
of genetic variation) even in the absence of other evolutionary forces.
Drift will operate much more quickly in populations that are small and
isolated, but over long periods of time can be a substantial force in
larger populations as well.  The general rule is that drift can cause
measurable genetic divergence among populations (at selectively neutral
loci) if the effective number of migrants between demes is 1 or fewer per
generation (Hartl and Clark 1997).

In the case of North American monarchs, where populations are large (many
millions of individuals), drift will probably operate slowly at the scale
of entire populations.  One study (Eanes and Koehn, 1979, using allozyme
loci) showed that eastern North American monarchs are highly genetically
variable and local breeding demes become significantly genetically
differentiated during the summer. $âü!" 8
8
has yet compared allelic frequencies of nuclear genes subject to drift
across eastern and western populations.  Using the above rule, if an
average of 4 or 5 monarchs per year successfully migrated across the
continental divide and survived to yield progeny in the different
populations, this may effectively homogenize the populations at neutral
genetic loci.

Another study by Brower and Boyce (1991) examined variation in
mitochondrial DNA sequences in monarchs from eastern and western North
America, Trinidad, and Tobago.  Although the sample sizes were fairly
small, almost no variation in mtDNA was reported in any of these
butterflies.  That all the butterflies were genetically identical suggests
that either (a) the mitochondrial genome is highly conserved and evolving
very slowly (either due to lack of mutation or strong selection against
deviants) or (b) monarchs have undergone a post-Pleistocene bottleneck and
recolonization that predates the divergence of South American and North
American monarchs. The authors felt that more evidence supported the
latter interpretation, as mitochondrial genomes are more susceptible to
variance in female reproductive success and historical reductions in
population size.

The second force that may lead to among-population divergence is natural
selection.  Many studies have shown that in natural populations, selection
can generate and maintain genetic divergence in the face of substantial
gene exchange and over relatively small spatial scales (e.g. Koehn and
Hilbish 1987, Antonovics and Bradshaw 1970, Via 1991).  It is likely that
monarchs in different populations experience different selective pressures
generated by climatic variables, migratory distances, host plant species,
and natural enemies.  If these factors vary among populations and
characters affecting monarch performance are heritable, then the
populations may become locally adapted despite gene flow among locations.
The relative strength of selection and the magnitude of gene flow will
interact to determine to what degree and how quickly populations diverge.

My own interests have focused on coevolutionary interactions between
monarchs and the parasite O. elektroscirrha.  This parasite is transmitted
from adults to larvae via several routes (maternal, paternal, and
horizontal) and can have significant negative effects on monarch fitness
(Altizer and Oberhauser, in press).  Parasite prevalence in 3 North
American populations is very different, with less than 8% of the eastern
migrants infected throughout the past 20 years (with a brief increase in
prevalence during 1977-1983).  In contrast, approximately 30% of monarchs
in western North America are heavily infected, and over 80% of a
non-migratory population in southern Florida was heavily infected in
recent years (Leong et al. 1997, Altizer et al. in press).

One factor that may contribute to these large differences in prevalence is
if monarch resistance or parasite virulence varies among populations.  To
test this hypothesis, I conducted a series of cross-infection experiments
in the laboratory, where I exposed monarchs from different populations to
native and novel parasite strains.  I measured monarch survival to
adulthood, and the parasite loads of emerging adults.  I found that
eastern migratory monarchs were significantly more resistant to parasites
than western monarchs, particularly when exposed to parasites from their
native population.  I also found that parasite strains derived from the
western population caused higher mortality and parasite loads than
parasites from the eastern population.  An interpretation for why eastern
monarchs may be more resistant, and western parasites more virulent, is
offered in my dissertation which I just completed and am currently
revising for publication (Altizer 1998).

Because I found that eastern and western monarchs varied in their parasite
resistance, I decided to determine the genetic basis for divergence in this
trait.  To do this, I conducted an experiment called a line cross
analysis, where I created 15 family lines derived from eastern monarch
parents, 15 family lines derived from western monarch parents (P1 and P2
lines), and 20 hybrid lines derived from crosses between eastern and
western parents (F1 lines). [Anyone interested in the design and analysis
of such an experiment should see Lynch and Walsh, 1997, Chapter 9].  I
divided the progeny of these monarchs (over 2000 in total) among
inoculation treatments where they were exposed to identical doses of
either eastern or western parasites.  I again measured survival and
parasite loads, as well as monarch mass and wingspan.  I found that the
eastern monarchs were again more resistant than western monarchs, and the
western parasites on average were more virulent.  Surprisingly, I also
found that the east-west F1 hybrid offspring were on average more
resistant than either of the pure parental lines.  These results suggest
that genes with both additive and dominant effects are contributing to
between-population divergence in parasite resistance.

There are several reasons why the east-west hybrids may be more resistant
than the parental lines, and my analysis is still underway.  My results
also hint at some genetic incompatabilities between monarch populations
because I also observed an increased frequency of bilateral hermaphrodites
in the east-west hybrid progeny that was not present in the parental
lines.

My results DO NOT imply that we should release hybrid monarchs into
natural populations to increase disease resistance. Parasite resistance is
only one of many characters affecting monarch survival and reproduction.
In addition, the introduction of large numbers of western parasites into
the eastern population may have detrimental consequences, at least on a
small time scale.  Furthermore, we do not know to what degree genetic
mixing of parasites may affect their virulence to monarchs from either
population. And finally, east-west transfers will very likely mask any
degree of natural differentiation at neutral genetic loci presently
occurring in natural populations.

Another line of evidence that indicates eastern and western monarchs may
be differentiated at non-neutral loci is based on comparisons of forewing
length from several thousands of wild-captured and lab-reared monarchs
over the past several years at the University of Minnesota.  Karen
Oberhauser, myself, and several others have found that wild monarchs in
the eastern migratory population are larger than wild-captured monarchs in
western North America and southern Florida.  Although small in magnitude
(on the order of 0.5 mm difference in average forewing length), this
difference is statistically significant.  In addition, my comparisons of
the wingspans of lab-raised monarchs confirm this trend, although the
difference in wingspan among lab-raised monarchs is smaller.  Thus, it
appears likely that both genetic and environmental effects contribute to
size differences in eastern and western monarchs.

Clearly, further work is needed to investigate drift and selection in
North American monarch populations.  As stated by Chip Taylor, analysis of
nuclear genetic markers will help us determine the realized gene flow
between populations. Perhaps more importantly, investigations of traits
potentially under selection are needed to assess the degree of
morphological, physiological, or behavioral divergence between eastern and
western monarchs.  For example, do eastern and western adults or larvae
vary in their preference for different Asclepias species?  Do they differ
in their mating behavior or susceptibility to other natural enemies?  Do
they perform similarly across a range of environmental conditions?  Do
they orient to the same solar or geomagnetic cues, or enter reproductive
diapause under similar conditions?  This is the kind of research that is
needed at present, and no single study can definitively address all of
these issues.

Sighting the anecdotal monarch crossing the east-west continental divide,
or measuring the frequency of tagged adults recovered at overwintering
sites will add little to our understanding of population differentiation.
There are many differentiated populations and species that are sympatric
or hybridize over parts of their range. Unfortunately, the kinds of
studies that are needed may be time-consuming and logistically
challenging, but if based on distinguishable hypotheses and soundly
designed methods, their results should be extremely valuable.

Respectfully,

Sonia M. Altizer
Department of Ecology and Evolutionary Biology
Princeton University
Princeton, NJ USA