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22001155 SOUTHEASTERN NATURALIST 1V4o(1l.) :1147,3 N–1o9. 61
Morphological Analyses of the Mecardonia acuminata
(Plantaginaceae) Species Complex in the Southeastern USA
Adjoa Richardson Ahedor1,* and Wayne Elisens2
Abstract - The Mecardonia acuminata complex is a classic example of a widespread endemic
species in the southeastern US. Morphological variations in the complex resulted
in the classification of at least three varieties or subspecies for the species by previous
botanists. However, the distributions and diagnostic features of two of the subspecies,
peninsularis and microphylla, are unclear due to shared morphological features and widespread
distribution of the subspecies acuminata. The present study involved examination
and biostatistical analyses of 3 vegetative and 5 reproductive characters that were known to
serve as diagnostic features of 1 or more taxa of the species. Results of the study indicate that
subspecies peninsularis can be distinguished by its ascending peduncle angle of suspension,
diffuse basal branching habit (dendriform at base) of the shoot and small leaves, especially
in the southern ranges of the complex. Subspecies microphylla can be distinguished based
on its short (less than 20 mm) fruit peduncles and divaricate peduncle angle of suspension. Subspecies
acuminata comprises individuals with divaricate peduncle angle of suspension and
long fruit peduncles (>20 mm). Subspecies acuminata was also observed to comprise many
individuals that were intermediates of 2 or 3 subspecies. Discriminant function analyses on
longitude, latitude, and biogeography as well as Wilks’ lambda estimates suggest moderate
to low clinal variation and a much broader historical range distribution of subspecies
peninuslaris. The morphological variations in the species complex may be due to secondary
contact with ongoing integration into subspecies acuminata.
Introduction
Mecardonia acuminata (Walter) Small (Axilflower) is a perennial herbaceous
species that occurs mostly on the Coastal Plains of the southeastern USA, ranging
from Maryland to Florida and eastern Texas, through the Mississippi Embayment
to southeastern Missouri (Fig. 1; Brown et al. 2002, Pennell 1935). It is widespread
along the southern ranges of its distribution but sparsely distributed on the extreme
northern, western, and eastern ranges (Pennell 1922). The species is a wetland indicator
commonly found in roadside ditches, on moist sandy or heavy loam soil that
is acidic or sub-acidic, or along streams in pineland and deciduous woodland (Pennell
1935). It flowers mainly in the summer followed by the formation of fruits that
persist through the fall (Pennell 1935, Rossow 1987), although individuals found
in the peninsular Florida flower throughout the year (Pennell 1935).The flowers of
M. acuminata can be distinguished from those of other species of Mecardonia by
the white corolla with longitudinal purple veins on the posterior side of the throat
1Engineering and Science Division, Rose State College, Midwest City, OK 73110. 2Department
of Microbiology and Plant Biology and Oklahoma Biological Survey, University of
Oklahoma, Norman, OK 73019. *Corresponding author - aahedor@rose.edu.
Manuscript Editor: Joey Shaw
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(Pennell 1935, Rossow 1987, Wunderlin and Hensen 2003). It exhibits some degree
of morphological variation; Pennell (1922) proposed the possibility of 3 to
5 subspecies. In a later classification of the species, Pennell (1935) affirmed the
classification of the species into 3 main subspecies: acuminata, penisularis, and
microphylla (Pennell 1935, Rossow 1987). His classification was based mainly on
leaf shape and peduncle lengths. Rossow (1987) reaffirmed the taxonomy of the
3 subspecies based on the length of leaf and peduncle and branching habits of the
stem. Wunderlin and Hensen (2003) confirmed the occurrence of all 3 subspecies
in Florida; they distinguished the 3 subspecies based on leaf and peduncle length,
sepal width, and branching patterns of the shoot. Based on the combined findings
of previous botanists, subspecies acuminata is widespread and can be found almost
throughout the entire distributional range of the species (Pennell 1935, Rossow
1987). Its leaves are greater than 25 mm (Pennell 1946, Rossow 1987) in length,
peduncles range between 25–35 mm long, and lateral branches arise at a distance
(mid-point) from the base of the stem (Rossow 1987, Wunderlin and Hensen 2003).
Subspecies peninsularis occurs in central to southern Florida (Pennell 1922, 1935;
Rossow 1987; Wunderlin and Hensen 2003). Its northern extent overlaps with the
southern extent of subspecies acuminata in central Florida (Wunderlin and Hensen
2003). It can be distinguished from the 2 other subspecies based on its small leaves
that are less than 25 mm long (Pennell 1935) and its diffuse basal branches (Pennell
1935, Rossow 1987, Wunderlin and Hensen 2003). Its peduncle is similar in length
Figure 1. Map showing distribution range of Mecardonia acuminata in the southeastern
USA and the 5 biogeographic regions within the range. Shaded region shows entire complex
range and range of subspecies acuminata; the dark gray dotted line denotes the boundaries
of each biogeographic region.
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to that of subspecies acuminata. Subspecies microphylla, which is considered rare,
occurs from northern Florida through southern Georgia to southeastern Louisiana
and occurs in sympatry with subspecies acuminata (Pennell 1935, Rossow 1987,
Wunderlin and Hensen 2003). Its leaf size is similar to that of subspecies acuminata,
its peduncles are normally less than 12 mm long (Pennell 1935), and its sepals
are at least 2 mm wide (Wunderlin and Hensen 2003).
Despite the established morphological characteristics of the 3 subspecies, variations
of the characters are obvious in each subspecies and therefore pose problems
to subspecies identification on the field. Furthermore, due to the sympatric association
of subspecies microphylla and acuminata, identification of the former is
ambiguous. Similarities between the 2 are evident particularly in the leaf shape,
size, and branching pattern (habit), which are supposed to be major delimiting features
of subspecies peninsularis and acuminata (Pennell 1922, 1935; Rossow 1987;
Wunderlin and Hensen 2003).
The objectives of this study were to (1) assess the morphological variations of the
species complex and test the current taxonomic circumscription of the 3 subspecies,
(2) assess any clinal variation among characters, and (3) determine the geographical
range distribution of the 3 subspecies and any biogeographic implications.
Field-Site Description
Within the range of the species complex in the southeastern US, there are 5 phylogeographic
regions that can be hypothesized based on climate and topographic
features that could represent physiographic barriers (Soltis et al. 2006). These barriers
are the subtropical climate in southern Florida, the Apalachicola and Mississippi
rivers, and the high elevation of the Fall Line (Soltis et al. 2006). The regions
are (1) Florida Peninsular, (2) Atlantic Coastal Plains, (3) East Gulf Coastal Plains
(between Apalachicola and Mississippi rivers), (4) West Gulf Coastal Plain (west
of the Mississippi river), and (5) northwestern region of the Fall Line (Fig. 1). We
sampled a total of 402 specimens from 13 southeastern states (Appendix 1). Three
hundred and thirty were herbarium specimens obtained from the Missouri Botanical
Garden (MO), the Botanical Research Institute of Texas (BRIT), the University
of Florida (FLAS), the University of Georgia, and the University of Oklahoma
(OKL). The remaining 72 were fresh field specimens collected from 7 states:
Florida, Georgia, Alabama, Mississippi, Tennessee, Louisiana, and Texas.
Methods
We evaluated 8 vegetative and reproductive characters: habit, leaf size,
leaf shape, floral peduncle length, fructiferous peduncle length, peduncle
angle of suspension, fruit length, and sepal width (Table 1). Some of the reproductive
characters were not measured for all specimens due to the varying
reproductive stages of the plants at the time of sampling. Habit, a vegetative
character, was also not recorded for all herbarium specimens since the roots had
been removed from some specimens. We recorded length and shape of leaves
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for all specimens and assessed plant habit to be either dendriform at base, dendriform
at mid-point/region, or intermediate (dendriform at mid-region but also
with reduced basal branching) (Fig. 2). We measured length of leaf from the tip
of the leaf to the base for leaves located in the mid-section of the plant (mature
leaves). Leaf shape for each plant was determined to be linear, elliptic, ovate,
lanceolate, or oblanceolate (Pennell 1935). We made separate measurements of
the length of floral and fructiferous peduncles for plants that had both flowers
and fruits. We measured sepal width at the broadest mid-portion of the outermost
sepal and fruit length from the tip of the fruit to the point of attachment of the
fruit to the peduncle. Leaf size, peduncle length, fructiferous peduncle length,
fruit length, and sepal width were measured in millimeters. We scored peduncle
angle of suspension as 1 = divaricate and 2 = ascending; habits as branching/dendriform
at mid-point = 1, at base = 2, and intermediate = 3; and leaf shapes as 1 =
oblanceolate, 2 = elliptic, 3 = linear, 4 = ovate, and 5 = lanceolate.
Table 1. Variation among quantitative morphological characters in the Mecardonia acuminata complex.
n = samples size, S.D. = standard deviation, F = Fischer’s statistics, ANOVA = one-way analyses
of variance conducted for each taxon. Fisher’s statistic was used to determine if variances are equal.
Letters a and b denote subsets of taxa based on characters as obtained from Duncan’s multiple range
test. *** = P ≤ 0.001, ** = P ≤ 0.01, * = P ≤ 0.05
Quantitative character/
Taxon n Mean S.D. F ANOVA
Leaf length (mm)
M. acuminata 406 23.50 7.05 108.95***
ssp. acuminata 346 25.11b 5.77 4.20***
ssp. peninsularis 48 11.29a 2.13 1.74
ssp. microphylla 16 22.53b 8.02 2.69
Floral peduncle length (mm)
M. acuminata 298 17.01 5.40 13.23***
ssp. acuminata 251 17.20b 5.22 2.58**
ssp. peninsularis 40 17.13b 5.89 1.16
ssp. microphylla 10 9.0a 3.62 1.49
Fruiting peduncle length (mm)
M. acuminata 338 22.73 4.94 42.34***
ssp. acuminata 285 23.17b 5.21 2.66**
ssp. peninsularis 41 22.46b 6.12 1.88
ssp. microphylla 16 12.00a 1.50 4.64*
Fruit length (mm)
M. acuminata 331 6.50 0.84 7.56***
ssp. acuminata 280 6.56b 0.87 0.83
ssp. peninsularis 39 6.0a 0.80 0.88
ssp. microphylla 15 6.13ab 0.83 0.63
Sepal width (mm)
M. acuminata 387 1.60 0.36 7.16***
ssp. acuminata 328 1.62a 0.35 2.03**
ssp. peninsularis 47 1.42a 0.32 2.50**
ssp. microphylla 15 1.61a 0.49) 0.59
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The data generated from the measurements and scores were statistically analyzed
using SPSS ver. 15.0 for windows (SPSS, Inc. 2006), and graphs were plotted
using SigmaPlot ver. 10.0 and Excel (Microsoft Corp. 2003). We performed descriptive
statistics for each character and constructed box plots to depict patterns
of variation in characters (Pereira et al. 2007). We conducted multiple analyses of
variance (MANOVA) to determine differences among group means (Woods et al.
2005, Zar 1996) and calculated Wilk’s lambda values (U-statistics) to test the null
hypothesis. F-values (group mean squares/error mean square) with levels of significance
(P-values) were estimated from these analyses. We excluded from each
analysis samples with missing values and also performed Spearman’s rank-order
correlation analysis to determine relationship among pairs of characters. Correlations
were tested for significance with two-tailed tests. We performed multiple
regression analyses to determine clinal variations for each character (Henderson
2005) and calculated regression coefficients for each character, with longitude
and latitude as the independent variables. The curves corresponding to the highest
regression coefficient of 5 relationships (linear, logarithmic, exponential, power,
and quadratic) were chosen (Schmalzel et al. 2004). Regression lines and coefficients
were calculated using the Least-Squares method in Excel (Microsoft Corp.
2003). Since most of the herbarium specimens were not identified to the subspecies
level, we performed the multivariate analysis discriminate function (DF) analyses
to determine if samples grouped into clusters without a priori assumption of the 3
subspecies (Boonkerd et al. 2002, Woods et al. 2005). Individual specimens served
as operational taxonomic units (OTUs), and missing data were excluded from
the analyses (Crawford 2003). In these evaluations, we conducted 3 separate DF
analyses based on latitude, longitude, and the 5 phylogeographic regions that were
predetermined based on physiogeographic barriers in the southeastern US (Soltis
Figure 2. Three types of habit corresponding to branching/dendriform patterns observed in
Mecardonia acuminata species complex.
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et al. 2006). To assess the distinctness of taxa in these DF analyses, we visually
inspected scatter plots for partial or distinct clusters.
The data matrix was then partitioned into subspecies based on known morphological
characteristics (Pennell 1922, 1935; Rossow 1987; Wunderlin and Hensen
2003). Thus, a priori decision of subspecies was imposed in these subsequent
analyses. Univariate analysis of variance (ANOVA) was performed on quantitative
characters and chi-square analyses were performed on qualitative characters
to determine if a statistically significant difference existed among subspecies. We
conducted regression analyses to assess clinal variations in each subspecies and DF
analyses to evaluate subspecies delimitations.
In order to test the taxonomic significance of branching patterns (habit), we further
partitioned the original data matrix into the 3 observed habits: dendriform at
mid-point, dendriform at base, and intermediate. We conducted discriminate function
analyses to determine if the different subspecies would segregate into distinct
or partial clusters based on habit. Diffuse branching at the base (dendriform at base)
is believed to be one of the major diagnostic features of subspecies peninsularis
(Pennell 1922, 1935; Rossow 1987; Wunderlin and Hensen 2003).
Results
Overall patterns of morphological variations across subspecies
Descriptive statistics. All specimens were collected from 25° to 38° North, and
-76° to -96° West. Leaf lengths ranged from 7.00 mm to 45.00 mm with a mean of
23.50 mm. Fruit lengths ranged from 4.00 mm to 9.00 mm with a mean of 6.50 mm.
Sepal widths ranged from 1.00 mm to 2.50 mm with a mean of 1.60 mm (Table 1).
Floral peduncle lengths ranged from 5.00 mm to 32.00 mm with a mean of 17.01
mm, whereas fructiferous peduncles ranged from 11.00 mm to 38.00 mm with a
mean of 22.70 mm (Table 1, Fig. 3). Leaves were mainly oblanceolate (55.3%)
in shape, followed by elliptic (41.7%), linear (2.5%), ovate (0.2%) or lanceolate
(0.2%). Habits of specimens were observed to be 57.6% dendriform from mid-point,
23.7 % dendriform from base, and 15.6% intermediate (Table 2). Intermediate habit
has not been reported in previous studies, but was observed to be prevalent in some
populations, especially in Tennessee and northern Alabama. Peduncle angle was
divaricated in 65.1% of the specimens and ascending in 32.2% of the specimens.
All characters were found to be statistically significant across latitude (P less than 0.01)
and longitude (P < 0.05) except for sepal width, which was significant only across
longitude at P < 0.1. Estimates of Fisher’s F statistics (F -value) suggest that leaf
length (F = 108.95, P < 0.001) and fructiferous peduncle length (F = 42.34, P less than
0.001) (Table 1) were the two most variable quantitative characters in the species,
whereas leaf shape (χ2 = 479.1, P< 0.001) and habit (χ2 = 131.14, P < 0.001) were
the two most variable qualitative characters.
Clinal variation in characters
Pearson’s correlations and linear regressions. All characters were correlated with
one or more other characters. Eleven of the 28 pairwise character correlations were
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significantly correlated at P ≤ 0.00, P ≤ 0.01, or P ≤ 0.05 (Appendix 2). The strength
of significant associations ranged from |r| = -0.24 (leaf length and peduncle angle) to
0.66 (floral peduncle and fructiferous peduncle). Leaf length and peduncle angle exhibited
significant or moderate associations with both latitude and longitude (Fig. 4).
Fruit length and sepal width were also correlated with latitude, whereas floral and
fructiferous peduncles were correlated with longitude. Moderate to weak clinal
Figure 3. Box plots illustrating variations in five morphological characters in the three
subspecies. Means denoted by vertical bars (|), shaded boxes indicate 50% of variation
ranging from 25th to 75th percentile. Horizontal bars delimit the 10th and 90th percentiles,
dots denote outliers. Numbering on Y-axis: 1 = ssp. acuminata, 2 = ssp. peninsularis, and
3 = ssp. microphylla.
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variations were therefore observed for most characters from south to north (latitude)
and from east to west (longitude). Leaf length exhibited the strongest clinal variation
along latitude (R2 = 0.282, P < 0.001) and longitude (R2 = 0.1, P < 0.001) (Fig. 4).
These analyses indicate low but statistically significant associations between leaf
length and both latitude and longitude.
Multiple analyses of variance (MANOVA). Multivariate ANOVA (MANOVA)
for each character along latitude indicates statistically significant results (P ≤ 0.01)
for 7 out of the 8 characters measured (Table 3). The results indicate similarities
within most characters except for leaf length and peduncle angle. No significant
difference was observed for leaf shape along latitude (Wilk’s Lambda = 0.944,
P = 0.43). No significant effects of latitude were detected for habit, leaf length,
peduncle angle, and sepal width (Table 3).
Canonical discriminate function analyses (DFA) and biogeography. The two
DF analyses on latitude and longitude did not reveal distinct but partial clusters.
The first DF analysis based on latitude revealed partial separation of samples
occurring along latitude 26° to 28° in southern Florida, where most subspecies
peninsularis occur (Fig. 5). Samples of latitude 29° (northern Florida Peninsular)
were intermediate between subspecies peninsularis (pure) and the remaining
M. acuminata cluster, but closer to the latter group. This finding indicates that
plants in latitude 29° are morphologically similar to subspecies acuminata even
Figure 4 (following page). Linear regression analysis of all Mecardonia acuminata specimens
showing latitudinal and longitudinal associations of characters.
Table 2. Variation among qualitative morphological characters in the Mecardonia acuminata complex
(n = sample size). Significance values: * = 0.05, ** = 0.01, *** = 0.001.
% observed dendriform branching at
Charatcer Taxon n Basal Mid-Point Intermediate χ2
Habit M. acuminata 395 23.7 57.6 15.6 121.4***
ssp. acuminata 340 18.2 69.2 18.8 131.14***
ssp. peninsularis 45 71.1 28.9 0.0 8.02*
ssp. microphylla 15 26.7 73.3 0.0 3.27
% observed
Character Taxon n Divaricate Ascending χ2
Peduncle angle M. acuminata 405 61.5 32.0 47.184***
ssp. acuminata 339 72.9 28.0 65.490***
ssp. peninsularis 48 25.0 75.0 12.000***
ssp. microphylla 15 80.0 20.0 5.400*
% observed
Character Taxon n Ovate Elliptic Lanceolate Linear Oblanceolate χ2
Leaf shape M. acuminata 405 0.2 41.7 0.2 0.2 55.3 556.171***
ssp. acuminata 345 0.3 43.2 2.3 2.3 55.9 479.10***
ssp. peninsularis 48 0.0 33.3 0.0 0.0 66.7 5.33*
ssp. microphylla 15 0.0 40.0 0.0 13.3 46.7 2.80
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Figure 5. Two-dimensional scatter plot of canonical discriminate function analysis of
morphological characters evaluated in this study versus latitude. Symbols correspond to
Mecardonia acuminata entire operational taxonomic units (OTUs) from latitudes.
Table 3. Results of multivariate analyses (MANOVA) of Mecardonia acuminata complex showing the
extent to which morphological characters differ across latitude/longitude and among biogeographic
regions and subspecies. Significance values: * < 0.05, ** < 0.01, *** < 0.001. Degrees of freedom for
number of groups per analyses: Latitude = 13, Longitude = 20, Biogeography = 4, Subspecies = 2;
degrees of freedom for total number of samples evaluated = 208.
Leaf Leaf Peduncle Floral Fruit Fruit Sepal
Statistic Habit length shape angle peduncle peduncle length width
Wilks’ lambda
Latitude 0.875 0.636 0.944 0.741 0.876 0.887 0.888 0.865
Longitude 0.830 0.705 0.913 0.788 0.901 0.889 0.885 0.842
Biogeography 0.905 0.716 0.983 0.873 0.952 0.948 0.984 0.953
Subspecies 0.975 0.668 0.998 0.871 0.903 0.790 0.946 0.974
F-Value
Latitude 2.483** 9.919*** 1.021 6.051*** 2.454** 2.215** 2.185** 2.705**
Longitude 2.052** 4.185*** 0.947 2.697*** 1.101 1.215 1.302 1.876**
Biogeography 5.671*** 21.467*** 0.941 7.824*** 2.714* 2.953* 0.873 2.687**
Susbspecies 2.709 53.265*** 0.230 15.832*** 11.431*** 28.494*** 6.129** 2.816
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though they have basal branches and small leaves. The second DF analysis based
on longitude did not reveal any significant clusters, although samples from longitude
-76° to -82°, along the Atlantic Gulf Coast, were slightly separated from
samples from the remaining specimens (Fig. 6). Both latitude and longitude DF
analyses therefore support the occurrence of subspecies peninsularis in southern
Florida, but suggest a much smaller range of distribution than previously reported
(Pennell 1922, 1935; Rossow 1987; Wunderlin and Hensen 2003). The DF analyses
did not reveal a partial segregation of samples occurring along latitude 30° to
31° or longitude -83° to -90°, where subspecies microphylla is known to occur.
This result suggests that subspecies microphylla is morphologically similar to
subspecies acuminata and it is embedded in the range of the latter, confirming the
sympatric distribution of the two subspecies.
Results of DF analysis to test the effect of biogeographic barriers on the distribution
of the species showed a consistent and marked partial separation similar to that
Figure 6. Two-dimensional scatter plot of canonical discriminate function analysis of
morphological characters evaluated in this study versus longitude. Symbols correspond to
Mecardonia acuminata entire OTUs from longitudes.
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obtained for latitude (Fig. 7). Region 1 (southern Florida) was partially separated
from the remaining 4 regions (2–5) which did not show any apparent biogeographic
pattern or separation. The lack of a biogeographic pattern for regions 2–5 is consistent
with species that exhibit high levels of outbreeding and physiological tolerance
to fluctuating environmental conditions (Fritsch and Lucas 2000) .
Tests of subspecies
Canonical discriminate function, univariate, and regression analyses. Discriminate
function analysis to test subspecies delimitation based on previously reported
diagnostic features (Pennell 1922, 1935; Rossow 1987; Wunderlin and Hensen
2003) clearly separated subspecies into 3 distinct clusters (Fig. 8). Seven out of the
8 characters analyzed showed significant variations within each subspecies (Tables
1, 2). Fruit length was less variable in subspecies acuminata and peninsularis
(Fig. 3). Subspecies acuminata demonstrated the most variants for all characters,
with most outliers within the range of the other 2 subspecies. Discriminate function
analyses, conducted to test the reliability of habit in distinguishing subspecies, did
not resolve the data into 3 subgroups (Fig. 9). Diffuse branching at the base habit
Figure 7. Two-dimensional scatter plot of canonical discriminate function analysis of
morphological characters evaluated in this study versus Biogeographic Regions. Group
centroids correspond to Mecardonia acuminata specimens obtained from 1 = Florida Peninsular,
2 = Atlantic Coastal Plains, 3 = East Gulf Coastal Plains, 4 = West Gulf Coastal Plains,
and 5 = Northwestern Region of the Fall Line. Symbols: ×’s represent OTUs of Florida
Peninsular, circles represent OTUs of the Atlantic Coastal Plains, triangles represent OTUs
of the East Gulf Coastal Plains, pentagons represents OTUs of the West Gulf Coastal Plains,
and squares represent OTUs of the Northwestern Region of the Fa ll Line.
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(dendriform at base), a character used in identifying subspecies peninsularis was
found to be present among 18.2% of subspecies acuminata and 26.7% of subspecies
microphylla, mostly occurring north of central Florida (Table 2, Fig. 9). The
remaining 6 characters were less variable in each subspecies and therefore were
reliable for delimiting subspecies. Duncan’s multiple range test (Table 1) separated
leaf length into 2 subsets and indicated a statistically significant difference between
subspecies peninsularis and the other 2 subspecies (P = 0.06). Two subsets were
also obtained for both floral and fructiferous peduncle lengths. In each of these 2
characters, subspecies microphylla was separated from the other 2 subspecies. Fruit
length/size of subspecies peninsularis was smallest with a mean of 6.00 mm, and
that of subspecies acuminata was large with a mean of 6.56 mm. The lack of statistically
significant subsets for some characters indicates overlap of ranges among
subspecies (Fig. 3). Linear regression analyses for each subspecies suggest moderate
clinal variations for most characters of subspecies microphylla (Fig. 10). Clinal
variations in leaf and peduncle length were also observed for subspecies peninsularis
(Fig. 11), but no significant clinal variation was observed for any character of
subspecies acuminata (Fig. 12).
Figure 8. Two-dimensional scatter plot of canonical discriminate function analysis of morphological
characters evaluated in this study versus subspecies. Group centroids correspond
to 1 = subspecies acuminata, 2 = subspecies peninsularis, and 3= subspecies microphylla.
Symbols: triangles represent OTUs of subspecies acuminata, circles represent OTUs of
subspecies peninsularis, and squares represent OTUs of subspecies microphylla.
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Discussion
Morphological variations and taxonomic circumscription
Subspecies acuminata. The results confirm the widespread distribution of subspecies
acuminata, even at the lower latitudes (26°) where subspecies peninsularis
is predominant. Subspecies acuminata exhibits a wide range of variation in all
characters examined with no significant clinal variations of characters. Fruit length
was similar in all 3 subspecies, and subspecies acuminata and microphylla share
similar leaf length and sepal width. Subspecies acuminata also shares similar floral
and fructiferous peduncle length with peninsularis. Morphological characteristics
shared between subspecies acuminata and the other subspecies indicates that some
individuals may be either morphological intermediates or hybrid s.
Subspecies peninsularis. Evidence from this study revealed that subspecies
peninsularis can be identified by small leaf length and sepal width and that it has a
Figure 9. Two-dimensional scatter plot of canonical discriminate function analysis of morphological
characters evaluated in this study versus habit. Group centroids correspond to
Mecardonia acuminata specimens with 1 = branching at mid-point, 2 = branching at base, and 3
= intermediate branching. Symbols represent OTUs separated out based on branching pattern.
Figure 10 (following page). Linear regression analyses of subspecies microphylla specimens
showing latitudinal and longitudinal associations of characters.
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Figure 11. Linear regression analyses of subspecies peninsularis specimens showing latitudinal
and longitudinal associations of characters.
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Figure 12. Linear regression analyses of subspecies acuminata specimens showing latitudinal
and longitudinal associations of characters.
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sympatric distribution with the other subspecies north of Florida. It was identified
in the states of Georgia, Tennessee, Maryland, and Texas (Fig. 9, Appendix 1). In
northern Georgia, it was found in the same county (Catoosa) as subspecies microphylla
(Appendix 1). The diagnostic features found in these areas suggest that
subspecies peninsularis may have had a broader distribution range than previously
presumed by botanists. The current southern populations of peninsularis may
represent relicts that were restricted to a glacial refugium in southern Florida
(Pennell 1935). It has been reported that Southern Florida served as a refugium
for many plants and animals during the Pleistocene (Soltis et al. 2006). The small
leaf lengths/sizes of these populations may be due to climatic or ecological factors
and not taxonomy. These climatic or ecological effects are evident in its prolonged
flowering season in southern Florida (Pennell 1935) and in the fact that most members
of subspecies acuminata and microphylla sampled from southern Florida had
leaf lengths ranging from 8.00 mm to 20.00 mm.
Subspecies microphylla. It is apparent from our observations that the major
diagnostic characters of subspecies microphylla are shorter floral and fructiferous
peduncles. Although short floral peduncles were observed in some members of subspecies
acuminata, their corresponding fructiferous peduncles were relatively long
(>20.00 mm). Because of the many shared characteristics of the two subspecies and
their sympatric distribution, identification of subspecies microphylla is problematic.
Nevertheless, DF analyses of all 8 characters combined revealed 3 distinct subspecies,
thereby supporting the taxonomic circumscription of subspecies microphylla.
Figure 13. Map showing distribution of Mecardonia acuminata ssp. peninsularis and M.
acuminata ssp. microphylla within the range distribution of the species complex. Dotted
line denotes western and northern limits of M. acuminata ssp. acuminata.
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The current investigations revealed that subspecies microphylla is fragmented but
widespread within the range of the complex, particularly in the western portion
(Fig. 13). Our results inferred that the subspecies can be found in at least 10 states,
as opposed to 5 as earlier reported (Pennell 1935, Rossow 1987). Eastern Texas
and western Louisiana, where most microphylla populations were detected, was
once a Pleistocene refugium (Remington 1968, Swenson and Howard 2005). Thus,
the observed distribution of microphylla in the southeastern US suggests a partial
discontinuous distribution separated by the Mississippii River, which is similar to
the distribution of Pinus taeda L. (Loblolly Pine; Al-Rabab’ah and Williams 2002,
Soltis et al. 2006).
Few samples of subspecies microphylla were observed in southern Florida
(Charlotte County) where subspecies peninsularis is prevalent (Fig 10). Therefore,
subspecies microphylla may not be as rare as originally thought, but may be
dispersed within the range of the other 2 subspecies as low-density populations.
Remington (1968) identified 4 major and minor suture-zones in the eastern US
where movement and fragmentation of species ranges were influenced by Pleistocene
glacial retreats and advances. In these suture-zones, species and subspecies
of plants and animals came into contact, interacted, and even hybridized. One of
the major suture-zones, Northern Florida Suture-Zone, extends westwards along
the coast of Alabama and Mississippi in region 1 (Fig. 1). Two minor suture-zones,
Louisiana-East Texas Suture-Zone and Southern Appalachian-Ozark Suture-Zone,
are located in region 3 and region 5. These suture-zones are concordant with most
of the sympatric distribution of subspecies microphylla.
Clinal variations in the species complex
Clinal variations were not observed for characters examined in subspecies
accuminata; however, as latitude increases, leaf length (size) for subspecies
peninsularis and microphylla increases. This latitudinal association with leaf
length has been observed in some endemic southeastern US taxa such as the
Halesia carolina L. (Carolina Silverbell) (Styracaceae) complex (Fritsch and
Lucas 2000). The leaf-length latitudinal association in the 2 subspecies implies
that when they occur north of their core range, they are not distinguishable from
subspecies acuminata due to their larger leaf sizes (except for the short peduncle
lengths in subspecies microphylla; Table 1, Fig. 3). Larger leaves are therefore
not shared taxonomic characters of subspecies acuminata and microphylla alone,
but present in subspecies peninsularis occurring at higher latitudes. Clinal variation
was observed in subspecies microphylla and peninsularis particularly across
latitude. Many characters tend to increase with latitude, which explains why microphylla
is hard to identify in the northern range of subspecies accuminata. With
the exception of its short peduncles, all other characters fall within the range of
subspecies acuminata. Although subspecies peninsularis is not readily identified
in the northern ranges of the species complex, its habit (dendriform at base
or diffuse basal branching) and ascending peduncle angle were observed in some
members of subspecies acuminata.
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Longitudinal clinal variation was minimal for most characters except for floral
and fructiferous peduncle lengths. In subspecies microphylla, there was a significant
increase in floral peduncle length across longitude along with a slight increase
in the fructiferous peduncle. The opposite was observed for subspecies peninsularis
in that its peduncles, particularly floral, decreased across longitude. Thus, at
the more western longitudes of its range, subspecies peninsularis’ peduncles were
similar to those of subspecies acuminata, which obscures its detection regardless
of its relatively small leaves.
Species distributed over one or more climatic belts spanning latitudes often
possess clines for physiological characteristics and their associated vegetative
characters (Stebbins 1950). These clines result from adaptive responses to the environmental
conditions prevailing in the different parts of the species range (Fritsch
and Lucas 2000, Spurr and Barnes 1980). Thus, the partial separation of peninsularis
occurring in southern Florida may be due to the subtropical climatic effect on
leaf size and other physiological characteristics that were not evaluated in this study.
Biogeography of the species complex
The separation of specimens from southern Florida (most of the Florida Peninsular
region) from the rest of the specimens (Figs. 5, 7) confirms the climatic,
biogeographic, or ecological impact of that region on the morphology of the species.
This biogeographic pattern has been observed in a few angiosperms in the
southeastern US including Liriodendron tulipifera (Parks et al. 1994, Sewal et al.
1996). Lack of a biogeographic pattern on the distribution of subspecies acuminata
and microphylla north of the Florida Peninsular region, indicates sympatry and
effective dispersal mechanism(s) of the species irrespective of physiogeographic
barriers. No clear geographic patterns have been found in some plant species occurring
in that region including Liquidambar styraciflua L. (Sweetgum; Soltis et
al. 2006), Prunus (Shaw and Small 2005), and Arabidopsis thaliana (L.) Heynh.
(Mouse-ear Cress; Jorgensen and Mauricio 2004). Although the dispersal mechanisms
of M. acuminata species complex were not examined in this study, its
pollen is known to be dispersed by bees (Ahedor 2007). The small seed sizes of
the species (less than 0.50 mm) may be easily dispersed by wind and water (A.R. Ahedor,
pers. observ.).
Physiogeographic barriers in the southeastern US, such as rivers and high
elevations, may not pose barriers to species dispersal. Clustering of peninsularis
specimens from latitude 29° in the Central Florida region (Fig. 5) with northern specimens
(north of latitude 29°) of subspecies microphylla and acuminata suggest these
are probably hybrids of peninsularis and 1 or 2 other subspecies. This region is the
Northern Florida Suture-Zone (Remington 1968) where individuals of subspecies
peninsularis and 1 or 2 other subspecies may have hybridized. These individuals of
subspecies peninsularis exhibit diffuse basal branching or are dendriform at base. It
has been documented that extensive hybridization of species and genera may have
occurred in this Florida Peninsular refugium during the Pleistocene when many taxa
were forced into close proximity (Edwards et al. 2006, Soltis et al. 2006).
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2015 Vol. 14, No. 1
The current widespread distribution of the species in the southeastern US may
be due to secondary contact following the Pleistocene and Quarternary glaciation
events in the eastern US. The lack of vegetative (leaf length) clinal variation and
overlap of characters such as habit, fruit length, and sepal width suggest secondary
contact of previously differentiated entities that are currently mixing freely. Furthermore,
the presence in other subspecies of characters reported as diagnostic of
subspecies peninsularis, the sympatric distribution of subspecies microphylla, and
the identification of a third intermediate habit in the complex suggest hybridization
and backcrossing of subspecies. It is likely that microphylla individuals may be
backcrossing more into acuminata parentals than peninsularis thereby obscuring
their field identification. Interactions of penisularis and acuminata in the Northern
Florida Suture-Zone coupled with ecological factors also resulted in fewer identifiable
peninsularis in those ranges. Therefore, results of this study depict complex
evolutionary processes in the M. acuminata complex that are masked by distinct but
inconsistent morphological features.
Acknowledgments
We thank the following herbaria: Botanical Research Institute of Texas (BRIT), University
of Florida (FLAS), University of Georgia (GA), Missouri Botanic Garden (MO),
Vanderbilt University (VDB), and University of Oklahoma (OK) for specimen loans. We
thank Ms. Amy Buthod for providing assistance with specimen loan requests from herbaria.
We thank Brent Berger and Dwayne Estes for assisting with field collection, Cal Lemke for
assistance in the Greenhouse, and Susan Pittman for assisting with graphics.
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Appendix 1. Table showing states and counties where subspecies peninsularis and microphylla
were identified; * denotes counties where both subspecies were i dentified.
Subspecies peninsularis Subspecies microphylla
State County State County
Florida Broward Florida Calhoun
Charlotte* Charlotte*
Citrus Jackson
Dade Washington
Desoto Alabama Geneva
Dixie Lee
Gilchrist St. Clair
Hardee Mississippi George
Hendry Lamar
Hernando Evangeline
Lousiana Beauregard Grant
Highlands Texas Newton
Indian River Orange
Lake Polk
Lee Red River
Levy San Jacinto
Manatee Titus
Martin Tyler
Marion Virginia King William
Okeechobe North Carolina Catham
Palm Beach Georgia Charlton
Pasco Catoosa*
Pinellas Dekalb
Polk Bradley
Seminole Calhoun
St. Lucie Lafayette
Arkansas Drew Oklahoma McCurtain
Taylor
Volusia
Georgia Catoosa*
Alabama Wilcox
(Maryland) D.C.
Tennessee Rutherford
Texas Ushur
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Appendix 2. Correlation among morphological characters to latitude and longitude in the Mecardonia acuminata complex based on Pearson’s
rank-order correlation analysis. Significance values: * = 0.05, ** = 0.01, *** = 0.001
Leaf Leaf Peduncle Floral Fruit Fruit Sepal
Character Habit length shape angle peduncle length penduncle width Latitude Longitude
Habit 1.000 -0.087 0.122* 0.154** -0.062 -0.030 0.065 0.050 -0.001 0.090
Leaf length 1.000 -0.046 -0.240** -0.010 0.099 0.333** 0.248** 0.531*** 0.307**
Leaf shape 1.000 0.001 0.076 0.022 -0.154** -0.002 0.044 0.037
Peduncle angle 1.000 116.000* 0.069 -0.075 -0.013 -0.219** -0.342**
Floral pedencle 1.000 0.660*** -0.041 -0.132* -0.049 -0.148*
Fruit peduncle 1.000 0.153** 0.018 0.003 -0.186**
Fruit length 1.000 0.259** 0.151** 0.006
Sepal width 1.000 0.196** 0.064
Latitude 1.000 0.249**
Longitude 1.000