Papilio joanae

Stanley A. Gorodenski stanlep at extremezone.com
Tue Jan 30 22:22:19 EST 2001


Felix, I find this discussion of Papilio polyxenes and P. joanae very
interesting.  I would like to give some of my views on the subject, and
hopefully get some answers to my questions (as well as corrections to
any misconceptions I may have).  I do not know anything about these
butterflies except from what I have read in LEPS-L.
 
First, I want to go over my understanding of mDNA in these kinds of
studies, to make sure I am on the right base.  Please bare with me. To
start, although mDNA  is part of the genome, it really operates at
another level compared to nuclear DNA.  It does not undergo
recombination, nor is a mitochondria formed by two gametes that were the
product of recombination.  Consequently, its role in adaptive selection
and speciation is highly limited.  Further, because the mitochondria's
primary function is being the provider of cellular energy through
respiration and oxidation, it has a limited range of response to
selective pressures, i.e., the response would be to either increase or
decrease providing the cell energy.  This type of response is pretty far
removed from selection pressures related to most speciation events.
Perhaps during past ice ages such selective pressures (to either
increase or decrease the mitochondria's activity) would have been more
important to the process of speciation.
 
As Patrick Foley pointed out, the so called 'evolution' of mDNA is
primarily mutation, and I would guess that quite a large number of
mutations are possible that do not affect the functioning of the
mitochondria (neutral mutations), or mutations that affect the
functioning very little.  mDNA genomic differences between
non-interbreeding populations of the same species across large distances
would be expected  to differ due to random mutational events in
conjunction with genetic drift, and perhaps as the result of the founder
effect, i.e., a few or one female starts a new population.  These are
sampling effects.  When populations hybridize, perhaps determining the
relative frequencies of the different mDNA genotypes may give some
information as to the extent of  hybridization and introgression.
Knowing the mutation rate can be used to calibrate the evolutionary time
scale of differences between populations.  Other analyses could lead
ultimately to the identification of the more 'primitive', or oldest,
genotype.
 
While this process is going on in the background, recombination and
segregation of nuclear genes are being subjected to adaptive selection
pressures, and other pressures that can result in speciation events.
Because the so called 'evolution' of mDNA is pretty much independent of
these processes, an analysis of mDNA can help establish if two species
are related, and to help determine when speciation events could have
occurred, either on an evolutionary time scale, or relative to other
speciations.
 
Now to P. joanae.  Based on mDNA lineages, you said "This says that one
part of the total genome of P. joanae is a lot more like that of P.
machaon than P. polyxenes."  Based on my very limited understanding of
mDNA analysis, this argues against  P. joanae  being "… the most
primitive ancestor from which polyexenes, machaon, brevicauda, bairdii
all arose."
 
However, you also say "… another part of the genome of  P. joanae, which
is the genes for the black color pattern, is a lot more like P.
polyxenes than most populations of P. machaon."  If these genes came
from P. polyxenes due to introgression, than a difficulty would be
explaining away the differences in their mDNA.  Much has been said here
about Haldane's rule on this topic, but Haldane's rule, from my
understanding, is not a certainty in all hybridizations.  This raises a
question - are all hybrid females sterile when P. joanae females are
crossed with P. polyxenes males, as well as the reverse mating, i.e.,
males with females?  If they are then it would be hard to explain how
polyxenes black color pattern genes introgressed into P. joanae.  The
only possible way it could happen is if F1 hybrid males mated with wild
type P. joanae females which seems a little more improbable, but maybe
not.
 
If Haldane's rule does not apply 100% to either sex combination mating,
then introgression of black color polyxenes genes could have
introgressed into P. joanae populations from a P. joanae female x  P.
polyxenes  male mating.  I feel the mDNA lineages so clearly indicate
that either species did not arise from the other (based on my,
admittedly, very limited knowledge of the subject), that this gives rise
to the next question, which is:  If Haldane's rule does not  apply 100%
in hybridizations between these two species, then would not one expect
to also find introgressions of certain P. joanae genes in P. polyxenes
populations through P. joanae male x P. polyxenes female mating?  Are
there any recognizable joanae genes, or gene complexes, in P. polyxenes
populations that could have been introgressed through hybridization?
 
Finally, since butterflies have wing patterns determined by essentially
the same genetic mechanisms, and especially since P. joanae and P.
polyxenes are so closely related, could not the similarity between the
black color genes of P. joanae to P. polyxenes be the result of the same
genetic solution to similar selective pressures (keep in mind that I
know nothing about the genetics of these butterflies and know only what
I have read in LEPS-L)?
 
Stan
 
 
 
Felix Sperling wrote:
>
> Well, I'm glad to see that systematics is getting some serious
> discussion, and Papilio systematics at that.
>
> My name was mentioned by Ron Gatrelle a couple of days ago in the
> context of Papilio name changes, while Norbert Kondla and Mark Walker
> followed it up with intimations about the dark motives and jealously/
> arrogance/ unresponsiveness of the "Powers That Be" who change names.
> I suppose that it might be helpful to mention that I have a job that
> takes 12-16 hours per day of my time, including preparing lectures to
> 450 very demanding Intro Bio students, sitting in committees to help
> along graduate students, planning biodiversity networks, and new
> museums, and otherwise being run ragged in the normal course of one
> day of the life of a professor. So if the slowness of my response
> seems due to the motives above, then I should remind you that I am a
> lepidopterist just like the rest of you - and that there are much
> less nasty explanations for what happens in taxonomy.
>
> Onward to Papilio:
>
> 1. What did my research on Papilio joanae show? In a 1994 paper in
> Evolution, I showed that Papilio joanae and Papilio brevicauda have
> virtually identical mitochondrial (mt) DNA, and that their mtDNA is
> very similar to that of Papilio machaon populations. These mtDNA
> lineages were very different from those of Papilio polyxenes, even
> for P. polyxenes collected near P. joanae in Missouri. This says that
> one  part of the total genome of P. joanae is a lot more like that of
> P. machaon than P. polyxenes. This is particularly interesting
> because another part of the genome of P. joanae, which is the genes
> for the black color pattern, is a lot more like P. polyxenes than
> most populations of P. machaon.
>
> 2. What does this mean? Some other Papilio populations have very
> similar mtDNA to that of P. joanae, including P. brevicauda and some
> peripheral populations that I have interpreted as hybrids with P.
> machaon. All these populations are found in an east-west band across
> the center of North America. One reasonable explanation is that their
> mtDNA is a genetic remnant of formerly more widespread populations of
> P. machaon that lived south of the continental ice sheets during
> glacial times. Assuming that the black color genes of P. joanae did
> come from P. polyxenes, these genes could have been acquired later -
> sometime after the ice receded, and both P. machaon and P. polyxenes
> populations moved northward. P. machaon populations left behind in
> the Missouri Ozark highlands could plausibly have hybridized with
> invading P. polyxenes.
>
> 3. Does this mean that P. joanae is the same species as P. machaon?
> Not necessarily. The phylogeny (= family tree relationships) of one
> gene is not necessarily the same as that of the other genes contained
> by a species. MtDNA is just one small set of totally linked genes and
> the P. joanae mtDNA may be the *only* remnant of the genome of P.
> machaon, while the remainder of the genome of P. joanae is comprised
> of P. polyxenes genes that have introgressed (= swamped out) the rest
> of the old machaon-type genes. Or maybe the black color genes are the
> only genes that have introgressed into P. joanae populations. Or
> maybe the situation is even more complicated. This suggests a rather
> obvious test - look at the relationships of a selection of other
> genes to see what the "average" gene is doing. We've already been
> hard at work on this in my lab, but the work is unpublished and in
> fact the results are still quite ambiguous because we have not found
> enough variation in other genes to get a clear answer. So we are now
> trying other methods and if we are very lucky then I may be able to
> report something interesting in a year or two or three.
>
> 4. What about the name Papilio joanae - should it be sunk? In my
> opinion, it is premature to sink P. joanae on the basis of one just
> one linked gene set, even though those results are very interesting
> and clearly show that there is at least some genetic difference
> between P. joanae and P. polyxenes. Just because the mtDNA of P.
> joanae is so much like that of P. machaon does not mean that it is a
> P. machaon. In another study in Heredity in 1993, I showed that
> Papilio rutulus and P. eurymedon have almost the same mtDNA, yet here
> there is abundant evidence that these two species maintain their
> integrity when they contact each other over a large geographic range.
> This is not to say that I strongly feel that P. joanae is one thing
> or another, only that I prefer to keep the same names that people
> have gotten used to for the last couple of decades. When there is
> clear and strong evidence to the contrary then the name should be
> changed in field guides. If the established name were P. whateverus
> joanae instead, then I would just as strongly be in support of
> keeping that name. And perhaps we never will get crystal clear
> evidence that it is one or the other (nature is a lot more
> wonderfully messy than we try to make it). In that case we might as
> well be spared the confusion of a name change every time someone
> publishes a study with another line of evidence. I have detailed this
> interpretation of P. joanae, and the reasoning behind it, in a book
> chapter that I have in press in the symposium volume from the
> International Butterfly Congress that was held in Colorado in 1998.
> However, the supporting information is all published.
>
> 5. What about subspecies - are they useless? I don't have a problem
> with the idea of naming geographically discrete and diagnosable
> populations as subspecies. In fact I have done it with P. machaon
> pikei, though I only did so after I showed that at least 75% of
> specimens could be identified without knowing where they came from.
> On the other hand, I think that there are an awful lot of subspecies
> names out there that are useless - that is to say that most of the
> specimens cannot be reliably identified without the locality. I would
> challenge anyone who wants to name a subspecies to show that their
> name is not useless in this sense.
>
> Enough for now - I have miles to go before I sleep tonight.
>
>     Felix
>
> Felix Sperling
> Associate Professor - Insect Systematics
> Department of Biological Sciences
> cw405 Biological Sciences Centre
> University of Alberta
> Edmonton, Alberta  T6G 2E9  Canada
>
> fax:   780-492-9234
> phone: 780-492-3991
> email: felix.sperling at ualberta.ca
> http://www.biology.ualberta.ca/sperling/sperling.html
>
>
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