Editor’s Note – This is the third essay in my series on the
phenomenon of double digit intrapopulation mtDNA sequence variation, which I
have begun calling “mitochondrial superheterogeneity.” The points I plan to advance here will depend
upon my crazy “wildebison” model of 6Apr16, which (in turn) depended on the field
observations I reviewed in my essay of 15Mar16.
This essay was subsequently published as: Dillon, R.T., Jr. (2019c) Mitochondrial superheterogeneity and speciation. Pp 163 - 170 in The Freshwater Gastropods of North America Volume 3, Essays on the Prosobranchs. FWGNA Press, Charleston.
Understood in their proper context, typical mtDNA
sequence data sets yield weak, null models of population relationship, not
directly relevant to the question of speciation [1], but certainly correlative. A mitochondrial gene sequence is a perfectly
fine single character. Given a decent
sample size per population, typical mtDNA sequence data can reasonably be used
to estimate interpopulation genetic divergence, which (especially in
combination with data from other sources) is generally correlated with the
likelihood of speciation [2].
So in the last couple months we have focused on the mtDNA
sequence data collected by Whelan & Strong from five populations of
Leptoxis “ampla” inhabiting tributaries of the Cahaba River, draining south
through Alabama into the Mobile Bay [3].
Whelan & Strong also sampled n = 20 Leptoxis praerosa from a
tributary of the Tennessee River, draining west across the top of Alabama
toward the Mississippi River. Their data
set included many striking examples of mitochondrial superheterogeneity. So while intriguing as an evolutionary
phenomenon, doesn’t the occurrence of up to 20% sequence divergence within conspecific
populations of “ampla” compromise the utility of mtDNA as a tool for
distinguishing “ampla” from praerosa?
No, perhaps just the opposite. But before I develop that surprising point, let’s
back up a few years and review what we actually know about the evolutionary
biology of Leptoxis populations in Alabama.
The Leptoxis fauna of the Mobile Basin was last comprehensively
monographed by Calvin Goodrich in 1922 [4], prior to his 1934 - 1941 awakening to
the Modern Synthesis [5]. The entire pleurocerid
fauna of Alabama was then decimated by a massive program of impoundment and
channelization, leaving behind what may be the single biggest taxonomic mess anywhere
in North American malacology [6].
In 1998, Chuck Lydeard and I published a survey of
genetic variation at 9 allozyme-encoding loci in eight populations representing
all four nominal species of Leptoxis known (at that time) to persist in the
Mobile Basin of Alabama [7], calibrated with three populations of the
widespread and well-studied Leptoxis praerosa of Tennessee. Our Alabama populations included three identified
as Leptoxis ampla from the Cahaba drainage [8], two populations identified as
L. taeniata from tributaries of the Coosa River, and one population of Leptoxis
picta from the main Alabama River way downstream in Monroe County [9]. The allozyme variation among those six
populations was comparable to that observed among our three control populations
of Leptoxis praerosa, and negligible compared to the genetic divergence between
praerosa and the ampla+taeniata+picta group combined. Thus our (9 gene, 11 population, 333
individual) results suggested that Leptoxis ampla (Anthony 1855) is a junior
synonym of L. picta (Conrad 1834) [10].
So returning to the mtDNA sequence data of Whelan &
Strong. It is reassuring to note that,
despite the high incidence of superheterogeneity W&S observed scattered among
the five Cahaba drainage pleurocerid populations of (what should better be
referred to as) Leptoxis picta, their 16S+CO1 gene tree nevertheless depicted the
single population of Leptoxis praerosa they sampled from that Tennessee tributary
as genetically distinct.
The bottom half [11] of Whelan & Strong’s Figure 2
(reproduced above) shows negligible sequence divergence within their
20-individual sample of L. praerosa, but approximately 13% divergence between modal ("clade 9") L. praerosa and modal (“clade 15”) L. picta [12]. Granted, the sequence variance within
populations of L. picta (“ampla”) does indeed range well above 13%. But a mode-to-mode difference of such
magnitude is certainly consistent with an hypothesis of speciation.
And we have thus far given short shrift to the histone H3
sequence data adduced by Whelan & Strong, which is especially interesting
because the gene is nuclear, and tends to evolve quite slowly. Figure 3 of W&S (reproduced below) shows
essentially zero sequence divergence within or among any of the five L. picta
(“ampla”) populations, but 2.0% divergence between picta and praerosa [13].
Levels of genetic divergence in allopatry, no matter how
striking, do not directly address the question reproductive isolation, and
hence can only constitute indirect evidence of speciation. But the nine allozyme-encoding genes of
Dillon & Lydeard, plus the 13% mode-to-mode sequence divergence W&S report for the CO1 gene, plus the 2.0% sequence difference at histone H3,
taken together with whatever morphological evidence may have accumulated [14], certainly
combine to suggest that the specific distinction between L. picta (aka “ampla”)
and L. praerosa is indeed a valid one.
Now begging your indulgence, let me return to Figure 4 of
Whelan & Strong – the third time I have reproduced the figure below in
three months. And let’s focus once again
on the situation at red-star-14 in the lower left corner, showing that the Shades Creek Leptoxis sample
included 16 copies of the modal haplotype and 1 copy of a 12% divergent
haplotype, modal in the Cahaba River at US52. (The red-star numbers in Fig 4 below correspond to the clade numbers in Fig 2 above.) Last month I suggested that cases of mitochondrial superheterogeneity
such as this are the signatures of very rare dispersal events, as though a
wildebeest were airlifted into a herd of bison, and successfully
interbred. Seen in this light, the
results depicted at red-star-14 constitute direct, positive evidence that the
Leptoxis population of Shades Creek is conspecific with the Leptoxis population
of Cahaba-US52.
And similarly at red-star-12. Here we see that the Leptoxis populations of
Cahaba-Bibb/Shelby and Cahaba-US82 share a unique, rare CO1 haplotype that
Whelan & Strong didn’t discover modally anywhere. This would seem to constitute direct,
positive evidence that the Bibb/Shelby and the US82 Leptoxis populations are
conspecific both with some third (unsampled) population where that particular
haplotype is much more common, and with each other, as well.
So more broadly, I would suggest that rare, superdivergent mtDNA haplotypes be understood as markers of genetic
compatibility with second populations some distance removed. And at red stars 10, 11, and 13, we see three
additional markers of genetic compatibility with three other populations of
Leptoxis that Whelan & Strong did not sample. All of these populations must be conspecific
– the populations of Leptoxis picta in which W&S found the markers, and the
four other populations elsewhere. In an
ideal world, given complete sampling, it seems possible to me that the
phenomenon of mitochondrial superheterogeneity might afford a direct, positive
tool to reconstruct specific relationships among extensive sets of highly
allopatric populations in their entirety.
Well, perhaps you’d like to join us back here in the real
world, Dillon? The pleurocerid fauna of
real Alabama is just a ghost of its former self. And the Leptoxis populations where the
sequences shown at red stars 10, 11, 12 and 13 are modal may well have been extinguished
80 years ago. Or longer? And those mtDNA haplotypes might be genetic
fossils, of populations long gone to glory?
Interesting to think about.
But turning the page.
As I am sure most of my readership will have by now noticed, the mtDNA
survey of Whelan & Strong extended to cover six populations of Pleurocera, sampled
alongside the six populations of Leptoxis upon which we have focused these last
three months. Their Pleurocera results
offer some striking contrasts with their Leptoxis results, one of which is
illustrated in the copy of their Figure 3 I have reproduced above. Why doesn’t the population W&S identified
as “Pleurocera pyrenella” from the Tennessee drainage appear genetically
distinct from the five populations they identified as “Pleurocera prasinata” from
the Mobile Basin? Tune in next time!
Notes
[1] Gene trees and
species trees are not the same thing:
[2] Examples of the correlative relationship between mtDNA
sequence divergence and speciation:
[3] Whelan, N.V. & E. E. Strong (2016) Morphology, molecules and taxonomy: extreme
incongruence in pleurocerids (Gastropoda, Cerithiodea, Pleuroceridae).
Zoologica Scripta 45: 62 – 87. Open Access:
[html]
[4] Goodrich, C. (1922) The Anculosae of the Alabama
River Drainage. Misc. Publ. Univ. Mich Mus. Zool. 7: 1-57.
[5] Goodrich’s (1934 – 1941) “Studies of the Gastropod
Family Pleuroceridae” series may represent the earliest application of the modern
evolutionary synthesis to malacology – I don’t know of any earlier. For more, see my essays:
[6] But we can’t blame
the entire mess on Alabama Power Company and the Corps of Engineers. As was true throughout The South, the state
of Alabama was blanketed with cotton and other row crop agriculture through the
nineteenth century and well into the twentieth.
Bank-to-bank farming dumped tremendous loads of silt into the smaller
rivers and streams, which still chokes most of the pleurocerid habitat, even to
this day.
[7] Dillon, R.T., and C. Lydeard (1998) Divergence among
Mobile Basin populations of the pleurocerid snail genus, Leptoxis, estimated by
allozyme electrophoresis. Malacologia 39: 111-119. [PDF]
[8] One of the three Leptoxis ampla sites of Dillon &
Lydeard matched one of the five Whelan & Strong sites precisely – the
Cahaba River at HWY 52 bridge.
[9] We also included two populations of Leptoxis plicata
from the Black Warrior drainage, not relevant to this particular essay. We did not include the subsequently
rediscovered Leptoxis “foremani/downiei” or Leptoxis “compacta.”
[10] Our good
friends Nathan Whelan and Ellen Strong neglected to cite Dillon & Lydeard
(1998) in their more recent work on the Alabama Leptoxis under review here. I’m sure this was just a simple oversight.
[11] The upper half of Whelan & Strong’s Figure 2
depicted the 16S+CO1 genetic relationships among their six populations of
Pleurocera. We’ll return to those data
next month.
[12] For this 13% estimate I blasted randomly-chosen L.
praerosa CO1 sequence KT164014 against randomly-chosen clade 15 L. picta
sequence KT163938.
[13] To obtain this 2.0% estimate, I blasted randomly-chosen
L. praerosa H3 sequence KT164460 against randomly-chosen L. picta (“ampla”) H3
sequence KT164387, just as in note [12] above.
[14] Whelan &
Strong did not offer any morphological observations on L. praerosa. But it is my subjective impression that there
may be subtle differences in the whorl shape or spire height between praerosa
and picta (“ampla”) – perhaps noticeable only in juveniles, or in adults
inhabiting calmer, more protected waters [15].
[15] And it is a crying shame that the captive-rearing
study published by Whelan and colleagues (2015) in J. Moll. Stud. 81:85-95 was
not controlled for current speed.
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