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2017 SOUTHEASTERN NATURALIST 16(3):326–330
The Karyotype of Plestiodon anthracinus (Baird, 1850)
(Sauria: Scincidae): A Step Toward Solving an Enigma
Laurence M. Hardy1, *, Larry R. Raymond1, and Shannon Harris1
Abstract - The cosmopolitan lizard genus Eumeces was first revised in 1936 and consisted
of 15 species-groups comprising a total of 50 species. Nine species in North America
were later recognized as belonging to the genus Plestiodon and all contained the diploid
chromosome number of 26. Modern cladistic techniques indicated that Plestiodon anthracinus
(Coal Skink) was near the ancestral form for the fasciatus group. We employed the
hypotonic citrate method to study chromosomes of 5 Coal Skink specimens from Louisiana
and Arkansas and found them to have a diploid number of 24 (12 macrochromosomes, 12
distinctly smaller chromosomes, all biarmed) and a fundamental number of 48. The diploid
number of 24 is probably derived by some chromosome rearrangements in the evolution of
Plestiodon and of the Plestiodon anthracinus group.
Introduction
Taylor (1936) recognized 15 species-groups within the cosmopolitan genus
Eumeces (= Plestiodon), which he regarded as monophyletic. Within Eumeces,
Taylor (1936) identified the anthracinus group, which contained 3 species: Plestiodon
anthracinus Baird (Coal Skink), P. copei (Taylor), and P. septentrionalis
Baird (Prairie Skink), all endemic to North America. Dixon (1969) removed copei
from the anthracinus group and placed it in the brevirostris group, based on several
morphological characters. Lieb (1985) upheld removing P. copei from the anthracinus
group, but added P. tetragrammus Baird (Four-lined Skink) to P. anthracinus
and P. septentrionalis in this group. Schmitz et al. (2004) reported that the laticeps
species-group includes the obsoletus and anthracinus species-groups in a clade that
comprises laticeps, inexpectatus, fasciatus, obsoletus, and septentrionalis. However,
they determined that “E.” anthracinus is not part of this group, because it is
consistently placed outside of the latter clade and is mostly recovered as a sister
species to “E.” egregius (Schmitz et al. 2004). This view was supported by Brandley
et al. (2012) who placed P. anthracinus as the sole member of the anthracinus
species-group within the fasciatus species series. Plestiodon tetragrammus was included
in the fasciatus species-group (Brandley et al. 2012). Multiple studies have
concluded that Eumeces (sensu lato) is, in fact, polyphyletic (Brandley et al. 2005,
Caputo, et al. 1993, Griffith et al. 2000, Schmitz et al. 2004), with the east Asian/
North American species of “Eumeces” being designated as Plestiodon by Brandley
et al. (2005). Brandley et al. (2012) included P. anthracinus as the sole member of
the anthracinus group in their revised phylogeny and placed the anthracinus group
1Museum of Life Sciences, Louisiana State University in Shreveport, Shreveport,
LA.*Corresponding author - lhardy@lsus.edu.
Manuscript Editor: Kristen Cecala
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2017 Vol. 16, No. 3
as a sister taxon to the fasciatus group (their Clade C5). The mtDNA data (Brandley
et al. 2012) provides strong support for the sister relationship of P. anthracinus with
P. egregius and P. reynoldsi (C4 clade) and the exclusion of P. septentrionalis from
close relationship with P. anthracinus. The cosmopolitan lizard genus Eumeces
(sensu lato) contains at least 25 valid species (according to ITIS 2016), including 8
species referred to Plestiodon as invalid. We follow Brandley et al. (2005:388) and
Brandley et al. (2012:182) for the use of Plestiodon.
Despite the large number of species in the family Scincidae, the karyotypes of
this family are comparatively poorly known (Giovannotti et al. 2009). All of the
species that have been karyotyped share the characteristics of having a relatively
low diploid number, with the first 4 pairs of chromosomes being metacentric and
larger than the remainder of the chromosome complement (e.g., Gionvannotti et al.
2009). We had access to several live specimens of the uncommon Coal Skink in
northwestern Louisiana and Arkansas, which allowed us to compare the karyotype
of Plestiodon anthracinus to other species in the genus. We tested the hypothesis
that the diploid number of chromosomes in P. anthracinus is 26, the known diploid
number reported for other species of Plestiodon (Dowling 1975).
Materials and Methods
We euthanized specimens by an intraperitoneal injection of 10% Nembutal and
processed for analysis of mitotic and meiotic cells sampled from bone marrow from
crushed vertebrae and/or testes. We prepared chromosomes by the hypotonic citrate
method of Patton (1967) and used the modification by Cole and Leavens (1971).
We made an intraperitoneal injection of 10% Velban as a mitotic inhibitor, instead
of colchicine. We examined chromosomes under a Leitz Dialux microscope and
photographed appropriate Giemsa-stained images with a 10.16 cm x 12.70 cm (4” x
5”) black and white, high-contrast film. We prepared the karyotype from a scanned
image (positive) of the 10.16 cm x 12.70 cm (4” x 5”) negative. Chromosome terminology
follows Cole (1970).
Specimens examined: LSUS 4545, female, Caddo Parish: 4.02 km (2.5 mi) W,
1.61 km (1.0 mi) S Blanchard, 14 September 1980 (LMH 8933); LSUS 5742, male,
Caddo Parish: Walter Jacobs Nature Park (T18N, R15W, Sec 7), 26 March 1981
(LRR 929); LSUS 5743, female, Caddo Parish: Walter Jacobs Nature Park (T18N,
R15W, Sec 7), 12 March 1980 (LRR 712); LSUS 5744, male, Caddo Parish: Walter
Jacobs Nature Park (T18N, R15W, Sec 7), 27 February 1980 (LRR 706); LSUS
8911, male, Arkansas, Polk County, 2.90 km (1.8 mi) S, 2.57 km (1.6 mi) W Big
Fork (Ouachita Mountains Biological Station), 21 July 2003 (LMH 12954).
Results
Our chromosome analysis of 4 specimens (2 males, 2 females) of P. anthracinus
from northwestern Louisiana and 1 specimen (male) from the Ouachita Mountains
of Arkansas indicated a diploid number of 24 chromosomes, including 12
macrochromosomes and 12 microchromosomes. The macrochromosomes include,
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from largest to smallest, 1 metacentric to submetacentric, 1 submetacentric, and
4 metacentrics. The microchromosomes are metacentrics and submetacentrics.
No chromosomes are telocentric and no secondary constrictions or satellites were
observed. Twelve pairs of chromosomes are shown in metaphase (Fig. 1). Both
macrochromosomes and microchromosomes are biarmed, and the fundamental
number is 48. We observed no obvious heteromorphic sex chromosomes.
We counted chromosomes from more than 380 cells, using both mitotic and
meiotic preparations; in all cases, the total diploid number was 20–26. We assume
that the cells containing 20 or 22 chromosomes were incomplete cells because
those numbers were recorded in specimens that also showed the modal number of
24. The 2 cells with 26 chromosomes were part of 384 cells analyzed that included
336 cells containing 24 chromosomes from 1 specimen (LSUS 5744). This result
indicates that the correct diploid number for our sample is 24. In addition, the reduction
in chromosome number from 26 seen in all other species of Plestiodon to
24 in Plestiodon anthracinus is due to a reduction of the microchromosome number
(from 14 to 12) and not a reduction of the macrochromosome numb er.
Discussion
Nakamura (1931a, 1931b) reported a diploid number of 26 chromosomes for
E. latiscutatus, a member of the fasciatus group and, according to Taylor (1936),
the only species from Japan. Talluri (1968) reported a diploid number of 32 chromosomes
for Eumeces algeriensis (see Caputo et al. 1993) a member of the North
African/Southeast Asia clade of Brandley et al. (2005). Deweese and Wright
(1970) reported chromosome data for 6 New World species of Plestiodon (all with
a diploid number of 26), including P. copei from Morelos, Mexico, which is 1 of 3
species of Plestiodon placed by Taylor (1936) in the P. anthracinus species group,
but later removed from this group (Brandley et al. 2012, Dixon 1969, Lieb 1985).
Dowling (1975) listed diploid chromosome numbers for 9 species of Plestiodon, all
Figure 1. Karyotype of an adult male Plestiodon anthracinus (LSUS 5744); 2n = 24; bar =
0.01 mm.
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2017 Vol. 16, No. 3
of which had a diploid number of 26. A review of chromosomal data for Eumeces
included 7 North American species (not P. anthracinus); all of which have a diploid
number of 26 (Caputo et al. 1994). The karyotype of Plestiodon anthracinus differs
from that of P. copei studied by DeWeese and Wright (1970) by having a diploid
number of 24 (only 6 pairs of microchromosomes, not 7 as in P. copei and all other
species of Plestiodon for which karyotypes have been published). Chromosome
change during evolution can be a useful indicator of phylogenetic relationships,
especially when used together with molecular and morphological data (Giovannotti
et al. 2009). However, we do not know the significance of the loss of a microchromosome,
which appears to indicate the loss of genetic material. We do know
that microchromosomes contain genetic material and are probably important to
the genome. For example, a microchromosome is, in some species, one of the sex
chromosomes and, therefore, very important for that species (Cole et al. 1967).
Microchromosomes may be translocated onto a macrochromosome and, therefore,
easily misinterpreted as a chromosome loss; however, it is not lost and the genetic
material is still present and probably functional.
Scincidae have highly conserved karyotypes (Giovannotti et al. 2009) and all
Plestiodon in the North American clade have a diploid number of 26, except for
P. anthracinus. By having a unique karyotype for the genus Plestiodon, based on
present knowledge, P. anthracinus is somewhat of an enigma. Its biogeographic
location would not suggest any particular evolutionary event to explain this, and it
has not previously been associated with any more-derived species group. However,
in the results of Brandley et al. (2011, 2012), P. anthracinus appeared in one of the
more divergent positions of the cladogram, and was nested between the fasciatus
and egregius groups. The most conservative conclusion suggests that the chromosome
rearrangement to a diploid number of 24 in P. anthracinus occurred after the
divergence of P. anthracinus from the fasciatus-species group, and the P. anthracinus
karyotype is independent of all of the North American Plestiodon containing a
diploid number of 26.
Acknowledgments
We thank Charles J. Cole and several anonymous reviewers for advice and constructive
comments on the manuscript.
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