Editor’s Note. This is the second installment of a three-part series on the discovery of a cryptic pleurocerid species in East Tennessee. You might want to back up and read my essay of 13Sept16 if you haven’t already. The present essay was published in the FMCS Newsletter Ellipsaria 18(2): 16 - 18 [pdf] if you’re looking for something citable.
So when last we left our intrepid malacologist, puzzling at
his lab bench in the summer of 2008, he had come around to the realization that
the population of Pleurocera simplex inhabiting Pistol Creek at the Courthouse
Park in Maryville, Tennessee, was actually two reproductively isolated
populations, fats (S6f) and skinnies
(S6s). Which population might be the bona fide P.
simplex of Thomas Say (1825)? And what
might be the identity of the other?
I actually started out with a pretty good hunch. In 2007 John
Robinson and I ran allozyme gels on five populations of P. simplex from up in
Virginia, including a nice sample from Say’s type locality in Saltville [1]. The Saltville population (S5) was fixed for the Oldh100 allele that turned out to be
diagnostic for the Maryville S6f fats. And interestingly enough, the simplex population
John and I sampled from Indian Creek at Kesterson Mill way down at the
southwest tip of Virginia (S1) was nearly
fixed for that Oldh104 allele diagnostic for the Maryville S6s skinnies. I hadn’t noticed
any shell morphological differences between the Saltville simplex and the Kesterson
Mill population at the time, however.
So in August of 2008 [2] I drove up to Virginia for additional
samples from Saltville and from Kesterson Mill, which I returned to
Charleston to run alongside a fresh batch of Maryville samples, to make sure
all the bands matched up. Sure enough, the Maryville S6f fats were indeed genetically
similar to P. simplex collected from its type locality at Saltville S5, not
just at the Oldh locus but over all ten of
the allozyme loci I have found informative for studies of this sort. And the Maryville S6s skinnies were similar
to Kesterson Mill S1.
 |
| Figure 1. See footnote [3] for methodological details. |
I also took the calipers to a bunch of the shells from
Saltville and Kesterson Mill. And sure
enough, exactly the same pattern reappeared that we first noticed at
Maryville. Dividing the total shell
length into body whorl height (B) and apex height (everything else, A), the (N = 37) shells I sampled from
Kesterson Mill showed a significantly greater ratio of A to B than the (N = 40) shells from Saltville. Figure 2 shows that the simple regression of
apex height on body whorl height for the Saltville type population S5 was A =
0.157B + 2.46 (r = 0.36), very significantly different (ANCOVA t = -9.52,
p < 0.0001) from the regression for the Kesterson Mill population S1, A = 0.556B - 0.09 (r = 0.84). Saltville snails are fat, and Kesterson Mill snails are skinny.
 |
| Figure 2. |
I have picked two shells directly off the regression lines to
illustrate the difference between the two species. So since the Maryville S6f fats match that
lower-sloping S5 sample of N = 40 from
Saltville both genetically and morphologically, and since Saltville is the type
locality of Pleurocera simplex (Say 1825), clearly the Maryville S6f fats must be the bona fide simplex [4].
Then what might be the identity of the Maryville S6s
skinnies, matching the higher-sloping S1 population from Kesterson Mill both
genetically and morphologically? Tune in next time!
Notes
[1] Dillon, R. T., Jr. and J. D. Robinson. 2007. The
Goniobasis ("Elimia") of southwest Virginia, I. Population genetic survey. Report to the Virginia Division of Game and
Inland Fisheries. 25 pp. [pdf]
[2] Yes, this was the
same trip I featured last month, in which I resampled Maryville. I’ve been sandbagging my story a little bit,
to make it unfold in a more linear fashion.
To tell you the truth, I got the “hunch” mentioned in this month’s essay
pretty much immediately upon reading my first Maryville simplex gel, and last
month’s story unfolded pretty much simultaneously with this month’s story.
[3] Subsamples of 14 individuals from population S5 and 41 from population S1 were analyzed
electrophoretically together with the 20f
+ 51s individuals from Pistol Creek I featured in last month’s blog post. These data were combined with the data sets
of N = 34 from population S5 and N = 37 from population S1 previously published
by Dillon & Robinson (2007), and with the N = 17f + 13s data I had previously accumulated from Maryville.
Then BIOSYS version 1.7 (Swofford & Selander, 1981) was used to calculate
matrices of Nei's (1978) unbiased genetic identity (below the diagonal in Fig
1) and Cavalli-Sforza and Edwards (1967) chord distances between all pairs of
control populations S1 and S5 and the two cryptic S6 populations co-occurring
in Maryville. Chord distances were used as the basis for the neighbor-joining
tree shown above the diagonal in Figure 1, as calculated using PHYLIP v3.65
program NEIGHBOR (Felsenstein, 2004).
[4] Ultimately I only
used that sample of 20 + 17 = 37 fats as my S6 Maryville control in Dillon
2011. The existence of a second Pleurocera
population at Maryville cryptic under simplex was not mentioned in my 2011
paper at all:
- Dillon, R. T., Jr. 2011. Robust shell phenotype is a local response to stream size in the genus Pleurocera (Rafinesque 1818). Malacologia 53:265-277. [pdf]